Objective:
This Soils Course section will provide explanation of soil processes, along with pictures, examples and testing methods. This section provides information, but it is not a decision-making process. That is provided in the Expert System section. In many cases, links can be used to move the reader between the Expert System evaluation process and the Soils Course information pages.

A History of Soil Science
The development of soil science is a story of identification of the component parts of a complex system, so that adjustments can be made effectively on functioning soils, and so that non-functioning soils can be regenerated and made to work again.

For thousands of years, people have evaluated soils for their ability to grow food, fiber or forage crops. Over 2000 years B.C., the Chinese used a soil map as a basis for taxation rates. Over 1000 years B.C., Homer wrote in the Odyssey about use of manure amendments on fields. Agricultural practices continued for centuries with little knowledge of why the practices were necessary.

In the 1600's A Flemish chemist Jan Baptista von Helmont grew a willow for five years to 164 pounds in an earth-filled tub. When he did not detect any loss in weight from the earth, he concluded that the plant biomass was generated from air and water alone. A short time later, however, John Woodward, in Britain, found that muddy water grew plants better than rain water, helping him to identify the importance of the "fine earth" component. Jethro Tull, in the 1700's, advocated tillage of the fields to break up soil so that more of the fine earth and humus "components" could be adsorbed into the plant. But the nutrient elements in these components remained unknown.

A chemical basis for plant nutrition started in 1834, when a Frenchman, J.B. Boussingault, and a German, Julius von Liebig, were able to show that the amount of "minerals" in manures were related to plant growth responses. This led directly to the concept of fertilizers as nutrient elements. The United States Department of Agriculture was established in 1862, partly to address the problem of decreasing crop yields on land that was not properly cared for. Still, the concept of a "soil" was that of a passive storage medium that fertilizers or organic amendments were added to, rather than of a functioning system of its own.

In the late 1860's, E.W. Hillgard, from Mississippi, and V.V. Doukachaev, in Russia, began to describe that the horizontal layerings in a soil profile were related to climate, vegetation and geology. They began to predict that similar conditions would produce similar soils at other distant locations. These preliminary findings were assembled by C.F. Marbut with the USDA, who used them for the first soil classification system. Hans Jenny (pronounced "yen´ny"), at the University of California, Berkeley, traveled on the Russian steppes and on the North American prairies, added concepts of carbon and nitrogen accumulation to the components that drive soil formation and function. This body of knowledge now allowed field people to predict where different types of soils would occur, based on the knowledge that soils are the combined effects of many processes at the earth's surface. Instead of viewing soils as random occurances on the landscape, the type of soil that formed was now seen to reflect the environment in which it was produced, as a product of the "factors of soil formation."

Soil formation is now understood to be a function of the different effects contributed by climate, organisms and organic matter, topographic relief, parent material and time.

Each effect operates in an interactive fashion with the other effects, producing the wide variety of soils we see across the landscape. This discovery allows us to see that each soil-forming effect works independently. This concept also shows the mechanics of how soils are formed and how they work. The utility of this information for revegetation on harsh sites is that if the soil is not working adequately, an understanding of the various components can allow the deficient component to be more easily identified and fixed.

The Soil Resource Evaluation system was developed with a similar structure. The objective is to define the main soil functions that are required by plants for growth, but that may be missing or deficient on barren, non-vegetated areas. The list of five "factors of soil formation" is modified so that they describe the soil components that plants need to live at any given point in time rather than as a long process of soil formation.

The soil resources needed for plants to grow on sites include slope stability, plant available water, nutrients, biological activity and erosion protection.

When these processes are regenerated on a drastically disturbed site, revegetation is expected grow as well as if the site had not been disturbed.

SRE step 1). Reference Site Evaluation

OBJECTIVES:
The reference site should 1) provide an example of a local site with an acceptable vegetation community and 2) give a working example of the soil conditions needed to support this type of vegetation.

A [reference site] is defined as an example ecosystem that serves as a model for planning a revegetation or restoration project, by providing examples of levels of physical or biological soil attributes that are sufficient for the observed plant growth and diversity at the site (adapted from SER, 2004). These attributes are measured through direct comparison of soil resources at the time of sampling. The trajectory of the soil and plant community regeneration process can be inferred, but not actually known unless the site is resampled at a later time.

The process of selecting a reference site forces planners to reconcile the type of community they imagine attaining on a site with the conditions (slope, aspect, rooting depth, amendments) needed to regenerate that type of plant community on the site.

The analysis of soil resources on reference and impacted sites (Steps 2 through 7) forces soil scientists to evaluate whether the soil tests they use actually represent the conditions that plants require to revegetate the sites.

SRE step 2). Slope Stability and Rooting Depth

2.1.0 Geotechnical stability of reference and impacted slopes (failures)

Dry ravel

2.2.0 Surface erosion stability of reference and impacted slopes (surface erosion)

Although many of these problems will be addressed in Step 7 Erosion control, it is important to verify that the site surface is not moving away as well as the slope structure as a whole. Many slopes that are judged to be "stable" have chronic erosion, and this must be dealt with in conjunction with overall slope treatment, including compaction and rootable volumes.

Visual Clues for Evaluating Erosion Erosion is caused by the impact of raindrops on bare soil, by the force of running water on the soil surface, and by wind. Erosion is a natural process, influenced by climate, soil type, slope, and vegetation type. Loam and silt loam soils are more erodible, than clay or sand dominated soils. Sandy granitic soils, however, are highly erodible. Steep slopes are more erodible than gentle slopes. And well vegetated slopes, especially with a variety of root forms and above ground biomass, are less erodible than areas with sparser vegetation.

Plant cover, surface debris, and biological crusts stabilize the soil, with bare soil between plants being most susceptible to erosion. Certain factors will increase erosion to an unnatural rate. Soil compaction allows for increased runoff. In addition, poorly located or maintained trails funnel water into erosion channels, and sloped campsites lose soil in a downhill flow. Additional factors contributing to erosion include soil surface stability, soil aggregate stability, water infiltration, and organic matter content, all of which can be evaluated against suitable reference communities. Sites with a history of heavy grazing or weed establishment will increase the risk of erosion.

Assess the site compared to reference sites for the following visual indicators of erosion: 1. bare soil Unless the soil has been recently disturbed as by a burrowing animal or tree throw, the soil should either have a crust or have a thin layer of organic duff or an armoring layer of gravel or stones. 2. lag gravels or plants or rocks on a "pedestals" If the surface has many more gravels or stones than the soil profile, the fine soil may have been eroded away, leaving the heavier rocks to "lag" behind. If the process is very slow such as in a desert, the rocks may have a dark oxidized patina, showing that although erosion has occurred, it is not a rapid process. If the soil is actively moving away from the local area by raindrop impact, soil may be protected under bits of wood or rocks and pedestals may form, with the protective object at the top.

3. exposed roots If soil has moved since a tree or shrub grew roots into the soil volume, the exposed roots will show the old soil levels.
4."terracettes" which are level benches of soil deposited behind obstacles
5. an increase in the number, size and connectivity of waterflow patterns (rills) between plants
6. soil deposition at slope changes. Where a steep slope transitions into a shallow slope, the speed of water flow decreases and sediments will deposit in a fan.
7. changes in thickness of topsoil Thick topsoils in depositional positions (swales, lower on slopes) mean soil has been lost from upslope (shoulders, mid-slope positions)
8. exposure of subsoil at the surface Subsoils are marked by higher clay content, redder color, or larger blocky or massive soil structure. On larger scales, these subsoils can form small cliffs, with the softer surface soils and deeper subsoils weathering away and the subsoil protruding prominently.
9. rills, headcutting, and/or downcutting in gullies Rills (only a few centimeters deep) are formed by water movement that has enough energy to suspend and move sediment.
10. reduced plant vigor As topsoil is removed, less moisture and nutrients are available for plant growth and plant size decreases.
11. long unsheltered smooth soil surfaces (wind-blown sites)

Methods for providing rooting depth on slopes.

Adequate rooting depths for plants on harsh sites are critical. Plants require water for moving nutrients, expanding their tissues, cooling leaves, and for cellular processes. Water deficits can quickly reduce growth and can easily become fatal. Tillage of existing slopes, meanwhile, is difficult. Existing slopes with insufficient rooting volume must be treated to increase porosity, but at the same time, the amended volume must be placed on a horizontal or only slightly outsloping (2 º) bench. Otherwise, the porous soil will sooner or later fill with rainwater and its weight will exceed the strength of the soil or the friction at the interface with the underlying slope and the amended portion will fail as a translational slide. Despite these difficulties, a non treated slope can become a chronic source of sediment production, and will eventually destabilize the slope above if it is not stabilized. Some potential methods to till the soil and potentially to incorporate composts are to till with an excavator bucket, rip, spade the slope with tines, construct benches and backfill, dig augered holes, or shatter the underlying material with explosives.

Bucket tillage with an excavator bucket has been done from the top of the slope reaching down, or from the bottom up. In either case, substrate and amendments are lifted and replaced from the same volume. Mixing the amendment may be difficult. The base of the excavated volume should be more or less horizontal so that the tilled substrate volume is replaced on a flat, horizontal surface to avoid lateral sliding when it gets saturated. The slope underneath the tilled material should resemble a series of small level steps.

Ripping behind a crawler tractor is commonly done to loosen the surface soil, but incorporation of surface amendments occurs only to shallow depths, such as 0 to 10 cm (0 to 4 inches)(ref). Deeper ripping is not possible with current equipment suitable for slopes. Ripper shanks often are limited to the top 50 cm (18 inches) and steeper slopes may restrict tillage to vertical passes. This should be avoided because the ripping channels may pipe water rapidly to the bottom of the slope. One suggested option is to create a cross hatch ripping pattern with two passes oriented at 30 º left or right of vertical, thereby avoiding a direct path for water flow.

Spading can be used to open up a moderately compacted slope using tines mounted on a bucket or forklift loader. If the spaded slots are vertical, compost could be raked into the slope. Care must be taken to not generate a uniform blanket of substrate with increased porosity that overlies an inclined slippage plane. When the substrate volume saturates, failure is likely to occur.

Benching and backfilling, or "fill cut" construction has been achieved on a large slope in decomposed granite. The term "fill cut" is meant to be distinct from "cut and fill" in which volumes are moved from cutslopes to engineered fill slopes. The "fill cut" operation refers to a method in which a slope is cut roughly to the final grade, and then a bench is cut across the base of the slope. Then, by sequentially cutting higher benches, while spoiling the material downward and incorporating amendment materials, the final cut slope is created by backfilling the benches starting from the bottom of the slope and moving upward.

The amendments can be bladed across the working bench, mixing them with the substrate, and then spoiled down across the outward face of the bench. This creates a compacted fill for structural stability, but provides an unconsolidated rootable volume across the face for revegetation and sorptive surface hydrology .

In the case of the Buckhorn summit project at SHA 299 0.06, the remaining rock matrix was so weathered and incompetent that continued slumping was a concern. At this site, the benches were cut deeper into the slope and a compacted fill was created in 1.2 m (4 foot) lifts that were set back 30 cm (1 foot) from the final slope grade. Compost and DG were mixed (24 % vol compost / vol DG) and bladed across the slope face to fill the set-back volume. A coir blanket (900 g/m2) was laid across the level bench and draped down and across the loose compost/DG fill. It was anchored with bats across the bottom of the blanket.

Augered holes can be used to develop a rooting volume in a compacted material, but typically, the holes are far too small to sustain a mature plant. The rotating bit polishes the walls of the hole, causing roots to circle around the inside of the hole and not to branch out into the surrounding soil. Many of these projects have failed. Exceptions may occur in which the auger penetrates a thin compacted layer and roots can be led to deeper rootable horizons. Other soils may crack and allow roots to escape the original hole. Predominantly, the plant is stuck in the hole and it becomes stunted and dies. This method is not recommended.

Explosives may be used to fracture the subgrade material without destabilizing the slope. This is a conceptual idea only and has not been tested on this project. A positive aspect would be the ability to treat large slopes without heavy equipment. A liability is that the cracking may not increase rootable volume enough, since only cracks are produced, not fine soil materials. Trained personnel would be required.

SRE step 3). Soil Water Relations (infiltration and water holding capacity

3.1.0 Evaluate soil infiltration rate.

Infiltration - crusts Soil crust : A layer of increased bulk density a few to tens of millimeters in thickness caused by raindrop impact, clay dispersion and clay translocation that blocks pores and decreases infiltration. Becomes brittle when dry. Seals are layers that form when substrates are wet and they reduce infiltration but are not hard when dry.

Infiltration - pores and aggregation
Infiltration is determined by macro-pores. These are large pores created by worms or insects, spaces between soil aggregates, cracks in the soil matrix, and old root channels. When soils are moved, compacted, or traveled on, the large pores are crushed. When infiltration rates are less than rainfall rate, overland flow will commence, initiating surface erosion. Damp or moist soils compact more easily than dry soils because the soil-to-soil contacts are lubricated with water. Soils intended to hold ponded water or for travelways may be intentionally compacted to reduce water content and increase strength when wet. On compacted substrates or constructed fills, pores tend to be small or non-continuous. Incorporation of unscreened yardwaste compost can recreate these larger pores. Use of wood chips increases the porosity of the soil, but the pores are not continuous, so water fills the pores but does not drain. This adds weight without drainage capacity, and probably with reduced tensile strength because of the short length of the chip particles.

Measuring infiltration capacity

Ponded water infiltrometer. The classical method to measure infiltration is to set up rings on the soil surface and to fill them with water. Measuring the drop in water level indicates the infiltration rate. The problem with this method is that many sites will have slopes that are steep enough that water will spill from the low side of the ring before the other side is ponded. This problem is solved by decreasing the size of the ring and fitting it with a valve that controls flow rate (photo below). Now, the infiltration rate is determined by water loss per time (tracked with the vertical graduated cylinder). This method has the advantage that infiltration at various depths down an augered hold can be measured. This device is called a constant head permeameter because the height of water (head) is kept constant at 15 cm above the soil surface to be measured. This mimics the depth of irrigation water a furrow in an agricultural field.

The disadvantage of either the ponded water ring infiltrometer or the constant head permeameter is that when water finds old root channels, burrows, cracks in a dried soils, or cracks due to geotechnical movement, the pressure from the constant head of water creates much more water flow than would be the case with rainfall. Rainfall uniformly soaks the surface but does not accumulate, at least initially (an accumulated volume of water would make it a drainage issue, not an infiltration issue). In order to accurately measure the ability of a soil to infiltrate rainwater, a drop forming rainfall simulator is used on steep, heterogeneous wildlands soils.

Drop forming rainfall simulators are constructed to deliver the amount of water representing a rainfall event, and can be made to give the appropriate drop size and impact velocity. Because Soil Resource Evaluation emphasizes the characteristic of soil infiltration rather than the mechanics of sediment production, all slopes in these studies are covered with straw or mulch following standard erosion control practices. The problems of raindrop splash detachment are disregarded. This allows use of a rainfall simulator that is shorter and easier to transport onto steep slopes.

Increasing infiltration by incorporation of unscreened yardwaste compost

Data from various experiments is compiled in the following table on example infiltration values. Different amounts of compost were amended on various substrate materials (listed in the third line of each heading). Infiltration rates are listed in rates of mm/hr when steady state infiltration occurs.

Example infiltration values attained by different amounts of coarse, unscreened yardwaste compost incorporation (v/v) at various locations in Northern California. An asterisk indicates treatments that were not tested at these locations. Rates are listed in mm/hr at steady state infiltration.

compost incorp% (v/v)	Buckhorn SHA 299 granite	Colusa COL 20 serpentine	Willits MEN101 clay	Willets MEN101 sandstone	Tahoe PLA 89 moraine
non-tilled	*	42	26	42	44
0	34	50	38	52	*
6	34	*	*	*	*
12	46	*	*	*	*
24	60	53	39	51	50

Checking if infiltration rate is adequate for design storm events
Use the infiltration rate (mm/hr) from the example infiltration values table (above) and compare it to a table of return frequency storm intensities (below). Does the soil treatment infiltrate the desired design storm event?

Table of return frequency storm intensities by site location (mm/hr for 15 min interval). Weather data taken from nearby weather stations and from on project sites are used to estimate the volume of precipitation that occurs with different return frequency storms. 48

Return Frequency Interval (yrs)	Buckhorn SHA 299 DG	Colusa COL 20 serp	Willits MEN 101 clay	Willits MEN 101 sand	BlueCyn PLA 89 vol lahar
station location	Whiskytown data	Clear lake data	Willits data	Willits data	Blue Canyon data
2	38	19	28	28	23
5	51	25	38	38	32
10	60	29	44	44	37
25	70	34	52	52	43
50	78	38	57	57
100	85	41	62	62	52

Predicting potential for runoff in different return frequency storms by season.

Using the meteorological data in the Table of return frequency storm intensities, the probability that a site will produce runoff can be predicted depending on the dryness of the soil before the storm (the antecedent water content) and the intensities of the storms during various seasons of the year. The table below is for the Buckhorn Summit site at SHA 200 0.06. Even when the treated slope (24 % v/v) starts to produce runoff, the sediment loss (measured by field rainfall simulator) is much lower than from control (no amendment) plots.

Infiltration, water content and drainage changes resulting from compost amendment on DG substrates.

The decomposed granite slopes at SHA 299 0.06 were instrumented for water content during the 2002-2003 winter season. The green trace shows the water content at 20 cm depth and the blue line shows water content at 40 cm depth in the compost treated substrate. Note that the soil is dry in November, and then wets up with the first rain of the fall. Water content of these surface horizons then declines because the water rapidly drains internally away from the surface. In December, the soil profile has now been moistened throughout and periodic rains continue to re-saturate it. The compost treated substrates consistently retain more rainfall during each rainfall event (sharp peaks) but the amended substrates drain the surface water away faster (indicated by the slope of the declining line to the right of each peak) compared to the unamended plots (red and magenta lines). Internal percolation of water from the shallow horizon to the deeper horizon (blue line at 40 cm) can be seen after each storm because the blue line (40 cm) shows a higher water content than the green line (20 cm). In May, a series of heavy storms occurred in rapid succession. The amended soil rapidly imbibes the rainfall and then rapidly percolates it away from the surface, acting to regenerate the infiltration capacity of the surface horizon.

Note the difference in soil water content between the compost amended substrates (green and blue lines) and the control (non-compost amended) substrates. This water volume is consistently about 10 % of the volumetric water content of the soil (measured at peak contents between the amended and unamended substrates). This surplus water that was infiltrated into the compost amended plots was shed as surface water flow off of the control plots, resulting in erosion and sediment transport. After May, the rains stopped and the soil gradually dried to summer levels. The spike in August was a single summer thunderstorm with intensity greater than 1000 year return frequency.

Notice that the compost did not increase the soil water content as much in dry conditions as it did at saturation. Water in dry soils is held by clay sized particles, not in coarse organic particles like composts. But, since root growth was so much more extensive in the compost treated plots, second year plant growth was nearly 5 times greater on the 24 % plots than the control plots, even when fertilizers were added to compensate for nutrient differences.

[scbhinfiltr2]
The traces on the left half of this graph are the same as in the previous figure, but a second year of data (2003-2004) was added in the right half of the plot. The compost plots still infiltrated rainfall quite well, but the overall water content was lower. This suggests that the compost treatments were becoming less effective for internal drainage or water holding capacity due to settling or to decomposition. The concern is that the effective "service life" of a compost amendment would be quite short. But, by the third year, the plots growing Elymus multisetus (Squirreltail) plants had increased their infiltration rate above even that of a newly tilled plot. As long as plant growth is maintained, the soil appears to be trending to better hydrological properties. This finding suggests that the beneficial effect of compost treatments on soil properties is not permanent, and must be replaced by plant growth in order to maintain adequate infiltration rates.

A soil characteristic that acts to maintain high infiltration rates is the accumulation of stabilized (humified) soil organic matter. This sticky material keeps fine particles from settling in pores, and maintains infiltration at the surface and percolation at depth. The organic matter content of vegetated soils increases and stabilizes over several decades to several centuries. Humic materials, and the constant renewal of biological activity and soil aggregates from plant inputs, must continue to after the compost amendment decomposes, or the site will revert to a barren state. A good example of this process is described by Perry and Amaranthus (1990) involving a Site I (highest class) timber producing site covered with white fir. This site in southern Oregon on decomposed granites was logged by clearcutting in 1968. During replanting, herbicides were sprayed to reduce brush competition. Plant growth continued to decline as several successive replantings were attempted and the soil began to resemble beach sand. Twenty years after supporting a productive forest, all what could survive on the clearcut site was a 30 % cover of cheat grass, scattered patches of fern and an occasional manzanita bush. Desertification of this site was attributed to loss of soil aggregate structure. The objective of regeneration of drastically disturbed substrates essentially to run this scenario in reverse.

Ksat measurement (steady state saturated hydraulic conductivity)
Percolation is the internal drainage rate of a substrate (in mm/hr) in the same way that infiltration indicates the capacity to infiltrate water into the surface of the substrate. The Ksat value is obtained during rainfall simulations by running the simulation for an extended period of time. Initial infiltration values are very high. This occurs when the dry soil rapidly soaks up moisture. Gradually, the pores fill and a lower, constant rate of infiltration occurs. This is variously governed by lateral flow on steeper slopes, or by the deep percolation rate of internal drainage, indicating Ksat has been attained. It is critical that amended slopes have percolation rates that allow excess water to drain freely from the soil on a slope so that positive pore pressure does not cause liquefaction and failure of the porous amended section. Root channels and animal burrows accomplish this on functioning soils, and long shreds of coarse, unscreened yard waste compost accomplish this on freshly constructed, amended plots. Wood chips are often used on field sites because of their ready availability, but their short fiber length is suspected to not create the continuous pores needed for infiltration or percolation.

3.2.0 Plant available water

Water Holding Capacity
Water Holding Capacity (WHC): The amount of water a soil holds at a given matric potential.

Saturated capacity: Soils with all the air space filled with water after flooding. Saturated soils are significant on slopes because they can start to lubricate between soil masses and facilitate geotechnical failures. Saturated paste solutions are made by flooding soils with water and then analyzing the solution that is drained from the soil sample.

Field capacity: (FC) The water content in the soil after no more water drains away due to gravity. The water content at field capacity is assumed to be about -0.001 MPa (-1/10 bar) in coarse, sandy soils and -0.03 MPa (-1/3 bar) in loamy soil. Water holding capacity at field capacity is determined by macro pore content.

Permanent wilting point (PWP): The water content of a soil when crop plants wilt and can no longer take up water. This is generally set to be -1.5 MPa (- 15 bar). Water holding capacity at permanent wilting point is determined by clay content.

Plant available water (PAW): For wildlands soils, the difference between field capacity and the maximum extractable water. For wildlands plants, this is usually more than the 1.5 MPa used for crop plants. Wildlands plants can extract water to 3.0 5.0 or even 7.0 MPa (-30, -50, or - 70 bars) rather than the -1.5 MPa (-15 bars) that are set as a limit for crop plants. Water holding capacity at very dry water contents is determined by clay content and possibly waters of hydration on mineral surfaces and interlayers.

Matric potential: Generally: The force needed to pull water out of soil by the plant, or to drain soil pores by gravity. Technically: The amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water, identical in composition to the soil water, from a pool at the elevation and the external gas pressure of the point under consideration, to the soil water. (Sidle et al., 1985)

[screlcurve]
SHA 299 0.6. Old Faithful, Buckhorn Summit. Soil moisture release curves for DG soils. A Yolo loam agricultural soil is included for reference. The two lowest lines tabulate the water holding capacity for each material calculated from the difference between -0.033 MPa and -1.50 MPa gravimetric water contents. Although the lower three lines have the same mineralogy as the topsoil (open squares), the organic matter in the topsoil retains moisture, and aggregates the minerals such that the water availability for plant growth approaches that of the agricultural soil.

Plant water use target.

The table lists generalized target summer season water use from literature review information and from northern California field data and other arid environments.

Plant available water (PAW) resulting from different treatment depths.

Select a target plant type from the table above. Check the plant available water values from the table below for treatment depths. All treatments use 24 % v/v unscreened yardwaste compost amendments. Note that these treatment depths probably will not provide enough soil moisture for larger shrubs and trees. The underlying substrate must be rootable for a large shrubs or trees to survive on these sites.

Limits to the ability to improve soil water relations with compost amendment.

In soils with greater than 9 % PAW, water availability will not be greatly increased by compost amendment. The clays and organics in the soil are merely displaced by volumes of material (compost) with approximately the same the same PAW characteristics. Removal of coarse rock fragments by screening is a more direct approach to increasing PAW on sites that are not sandy but have high rock content.

Compost incorporation on moderately or poorly structured (aggregated) soils may decrease infiltration because of the tillage needed to incorporate composts.

Compost amendments appear to settle or decompose by about 30 % in the year or two after addition. Regeneration of plant growth (roots, organic matter inputs) is critical for maintaining the increase infiltration that was gained with initial tillage. Generation of soil aggregates requires large amounts of stabilized carbon, but the processes are probably dependent on the soil mineralogy of different sites. Dynamics of soil organic matter carbon are discussed in SRE Step 4. Soil Organic Matter and C and N Pools.

OBJECTIVES:
The site must provide adequate soil organic matter (SOM) for three main functions: 1) coarse organic fragments generate macropores to increase [infiltration], as outlined in Step 3; 2) decomposable carbon supports microbial activity and humified (stabilized) residues, which promote [soil aggregation]; and 3) available [nitrogen] (N) released (mineralized) by decomposition of organic matter supports plant growth and community development.(xxxcopy to ES)

Introduction to soil organic matter and soil aggregation Soil aggregation Application of coarse organic fragments can immediately generate pore structure to increase infiltration and soil organic matter for microbial and plant activity. But, because the compost is more decomposable than natural soil organic matter, it will not provide the same long-term effects. Long-term organic matter inputs and soil aggregation must be provided by plants growing on the site for subsequent years. In this section, the function of soil organic matter in newly constructed substrates is described first, and following that, the role of compost and plants in carrying on these functions will be addressed.

Massive soils are those substrates that will dry to a large, uniform, hard mass. This happens in soils that are compacted, that have any amount of fine soil materials (clays or silts) and that have low organic matter content. Chunks of this massive structure can be broken off and are called "clods". The fracture lines when breaking clods are not anong faces, and break through the clod, not along natural planes. Clods and massive soils can be seen on almost any construction site because excavation equipment moves soil around and compacts it, especially when wet. Massive soils do not infiltrate water well, so they create overland flow, and they do not permit easy rooting from young seedlings, making revegetation difficult. The process of creating water-stable soil aggregates involves developing a smaller breaking pattern in these large clods or massive soil volumes, breaking them into smaller aggregates measuring several millimeters in diameter. Simple grinding and fracturing by force will not create aggregates because the next rain will melt the mass together again.

Aggregated soils are those soils with small regions of strength (millimeters to centimeters across; 1/8 to 1 inch or so) that are bounded by planes of weakness. The strength is provided by several kinds of processes (described below), while the planes of weakness are films and layers of organic matter and clay. When crumbling these aggregates in your hand, they consistently break into smaller and smaller pieces along these natural fracture lines. These fracture lines can be observed by the occurrence of roots pushing along the face and of polished, wax-like dark surfaces. The natural unit of aggregation is called a "ped." Peds can be shaped like a small piece of popcorn or bread crumb (granular structure), or they can be larger and squarish (blocky structure) the size of a matchbox or medicine box. Soil structures formed under vertical compression (snow load or traffic) are flattened and are called "platy." The term "water stable aggregate" is significant because the crumb structures at the surface of a soil must be strong enough so that a large raindrop falling at xx km/hr will not break up the aggregate and disperse the fine particles into pores. While the process of aggregation has been studied on agricultural soils, little is known about the process on wildlands soils, especially as they become less like agricultural soils. This means coarser in texture, less well weathered, with fewer organic coatings and with less organic matter content. Still, some generalities can be made about the process as it occurs in more typical, agricultural soils.

Hierarchy of soil aggregation processes. The source of strength inside an aggregate is derived in agricultural soils from several stages of "construction" or formation, ranging from small scale to large scale (still less than a few millimeters in diameter). At the smallest scale, clay and silt particles are cemented together by oxides (rusts) forming on the mineral surfaces. Adjacent mineral particles are then bridged together by humified soil organic matter. Once formed, these small aggregates are persistent, meaning that this smaller type of (micro-aggregate, < 250 um) structure can last through the soil harvesting, stockpiling and reapplication process. These microaggregates can only get so big before they are not strong enough to adhere together. They break apart in the constant churning within the soil cause by root growth, wet/dry cycles, water flow, and hot and cold temperature cycles. Larger aggregates (macroaggregates, > 250 um) are formed when microbial colonies grow on old roots, dead insects, or bits of compost. These colonies create gels of mucilage that glue the microaggregates together into macroaggregates. They do not last much longer than the microbial activity occurs (several months), and they decrease rapidly with tillage or lack of organic inputs. Macroaggregates are easily disrupted and lost. Fungi, especially mycorrhizal hyphae, are especially good at wiring together these macroaggregates into fairly large structures, thus generating the granular structure several millimeters across that can be seen with the eye in well aggregated soils. Larger aggregates also tend to be reinforced with plant roots, especially the fibrous roots of grasses. (Tisdall and Oades, Water and Oades, Jastro and Miller)

The application of this information to revegetation of harsh sites is that the regeneration of soil function on drastically disturbed substrates is that the process cannot be corrected with a single treatment. Drastically disturbed substrates have some fundamental characteristics that make aggregation difficult. They are typically coarse and have low clay content, they can be relatively unweathered chemically (especially granites and sandy soils), they are typically very low in organic matter content, and they initially have no decomposable substrate to support microbial activity. For all of these reasons, the tendency for disturbed substrates to have poor structure and low infiltration is understandable. Many of these problems can be corrected for at least a few years, however, by application of large volumes of coarse, unscreened yardwaste compost. In some substrates or locations, the use of intensive, agricultural-style plant growth may be able to regenerate the process of soil regeneration, thus reducing the expense of amendment but probably increasing maintenance costs to keep the plants thriving through these difficult first seasons.

Stabilization of soil organic matter (humification)

There are three ways that the decomposable carbon that is added as compost or plant materials becomes stabilized in the soil so that it can persist for long periods of time (many decades to centuries) (Christensen, 1996). Microbial decomposition converts organized, energy rich compounds like plant material (cellulose) into a slowly-degradable mixture of left-over compounds like microbial residues. These various compounds may have chemical resistance to decomposition, or perhaps they are simply too disorganized to allow effective enzymatic decomposition by microbes. This is called chemical recalcitrance. Secondly,chemical stabilization occurs when cations in the soil, mainly positively charged calcium, iron and aluminum ions, bind to the negative charges on partially decomposed organic matter, and bind it into a resistant structure, such as a coating on a mineral surface or as a dense mass of organic matter that cannot be dissolved. Thirdly, the clay particles create aphysical protection that occurs when layers of clay coat the organic matter and keep it from being degraded by microbes, even if it is decomposable. Also, some soil pores are too small to allow microbes to get into and degrade organics that have diffused into these pockets. As mentioned previously, drastically disturbed soils commonly have low biological activity (so low microbial residues and little organic residue to stabilize), they have relatively unweathered mineral surfaces (low oxide content to bind organics through chemical stabilization) and they have low clay levels to shield organics by physical protection. For these reasons, organic matter stabilization and accumulation on drastically disturbed sites is problematic.

Lastly, a component that may occur commonly in natural soils that may be missing from most newly exposed substrates is char from previous burning of plant materials. These char particles are essentially activated carbon, and like the cartridges in water filters, they adsorb large quantities of organics, keeping them from decomposition so they can sorb water and nutrients, and hold them for eventual plant uptake.

4.1.0 Evaluate soil organic carbon (C) pools.

Coarse organic fragments Natural soils have extensive root channels, animal burrows and macropores between soil aggregates that allow water to infiltrate into the surface until a restrictive layer is reached. The moisture then flows laterally downslope within the soil. If flow increases or pore volume decreases, the soil will saturate, and moisture will seep to the surface. If pressures become positive, the soil may liquefy. The stability of the slope is a function of continuous pore space for water flow and of the soil's tensile strength and ability to hold the moist soil against gravitational pull.

Disturbed substrates, especially constructed fill slopes, tend to be uniform masses of substrate with low pore space. The result is a low infiltration rate and a diversion of rainfall to overland flow. Incorporation of coarse, unscreened yardwaste compost has been shown to regenerating these drainage pores (Curtis et al., 2004). A compost amendment approaching 24 % on a volumetric basis (8 % by dry weight) was required to regenerate the infiltration of a vegetated reference site. Other studies also suggest a volume of incorporation of approximately 25 % (v/v) to positively impact soil structure (Brandt and Hendrickson, 1991).

The particle size of the compost is important. Although the studies addressed composts applied as surface amendments, not soil incorporations, evaluations at the Texas Transportation Institute found that although compost and shredded brush were effective as erosion control, fine compost applications (< 7 mm; 1/4 in minus) had greater sediment loss (Landphair and McFalls, 2000). For this reason, application of normal commercial compost (< 19 mm; < ¾ inch) would be expected to provide much lower benefit for infiltration and percolation.

The unscreened yard waste composts used in this study were produced by the City of Redding municipal composting facility. The initial yardwaste material was shredded by a tub grinder with no additional screening other than an initial pass through a 75 x 125 mm (3 x 5 inch) grate. Although the two materials were not tested against each other directly, the longer fiber length (individual shreds to 150 mm, 6 inches) of unscreened, shredded tub ground material is expected to be more beneficial than the chipped wood materials that are commonly available. The longer shreds add more continuous pore space and greater tensile strength to the amended substrate volume than the chips. An option in soils that are compacted but do not need additional nitrogen would be to use the coarse materials that are screened during production of "fine" compost for gardening use. Coarse wood fragments may persist in the soil for an estimated 3 to 5 years (Tietjen and Hart, 1969) before decomposition reduces their effectiveness.

Fine (decomposable) carbon

Much of the compost volume is made up of medium to fine shreds with visible cell wall structure. The finest of these particles resemble small shreds or fibers. The cell wall materials are mainly cellulose, which are steadily degraded into sugars by microbes. The application of compost to soils increases root growth and plant biomass production in ways that are not duplicated by fertilizer and water addition. Although the exact reason for this is not known, some aspect of microbial activity is suspected.

Approximately xx % of the soil organic carbon in a natural soil is thought to be decomposable (Stevenson, 19xx). In a rough calculation, this would be comparable to xx kg/ha compost per year, but the decomposition rate of the finer and coarser fragments would have to be accounted for. The 24 % volumetric addition of coarse yard waste compost to the top 300 mm provides about 4 % total organic C. This is high relative to natural soils, but much of this C will decompose away within a few years. More field work needs to be done on the practical aspects of compost decomposition in different situations.

Numerous examples of excellent plant growth response following large amendment with unscreened yard waste compost are provided in the Case Studies section. Plant growth is regenerated even on substrates that had been barren for decades. So, in spite of the lack of complete information on organic matter performance on disturbed sites, this application method is effective in many situations. While the efficiencies of compost utilization can undoubtedly be improved, especially in the first season or two of use, compost application is works to regenerate plant growth and soil function.

4.2.0 Evaluate soil organic nitrogen (N) pools.

Nitrogen pools
Soil nitrogen (N) can be categorized into three conceptual pools. The largest is the stable organic matter pool that contains N in humified organic forms. These materials decompose only very slowly, with half lives of hundreds to thousands of years. These stabilized N pools amount to between one and several thousand kg total N/ha. A smaller pool, but one that is much more active and dynamic is the mineralizable pool. This material consists of decomposable organics, mainly of microbial and plant origin. This pool may only be a tenth of the size of the stable organic matter N pool, perhaps several hundred kg N/ha. A similar amount of N is contained in plant shoots, roots and decomposing litter. The plant pools are not actually in the soil, but are necessary to continuously feed the soil. The smallest soil N pool is the extractable N pool (ammonium and nitrate) of 10 kg N/ha or less. Soil N pools in a foothill woodland environment were studied in northern California by Jackson et al., (1988). The pie charts show the amounts of the soil N pools for a California wildlands soil.

[scN5.jpg]
The left diagram shows the large, stabilized pool of soil organic matter N. The small wedges in the left circle are expanded in the right figure. Here, the size of the mineralizable, mainly microbial, pool can be seen, along with the various amounts of N in liiving and dead shoots and roots and plant litter. The smallest wedge is the amount of extractable N (ammonium or nitrate). This is the amount typically reported in soil analyses. It is important in agricultural soils because it is much larger and is the direct way that crop plants take up N. In wild lands soils, it is much smaller, and is extremely variable due to weather, soil temperature, plant growth stage, etc. The extractable N pool has low correlation to the amount of plant cover growing on a site (Claassen and Hogan, 2002).

Nitrogen availability Nitrogen availability coming out of the mineralizable pool in a soil depends on the biological cycles of decomposition of organic matter (and, in some locations, on the atmospheric deposition of nitrogen from the atmosphere) and on the rate of removal of nitrogen from the soil by plant uptake, microbial uptake, leaching or gaseous losses.

If the organic matter being decomposed has an excess N content (dried leaves, manures), typical microbial activity will use the C for energy and the excess N will be released to the soil (mineralized). If the organic matter has less N, or a great deal of C (such as wood or straw), any available N will be taken up into microbial biomass (immobilized) to increase the microbial population. Typically, ratios of C:N of greater than 25 suggest that microbial immobilization of any available N will occur and that little will be available for plant growth. C:N ratios of less than approximately 18 indicate that more N will be mineralized than will be used by microbes, and the excess will be available for plants uptake.

Microbial decomposition of plant inputs, whether from composts or plants growing on site, depends on the ratio of available carbon to available nitrogen in the organic amendment. Clean wood chips have a great deal of carbon but very little nitrogen, making microbial decomposition slow. This is good for persistence of the coarse wood fragments needed for macropores for infiltration, but it does not provide the microbial residues that are the source of the humified organic matter. MIcrobial growth depends on both decomposable organic material and available nutrients. Fresh leaves have enough excess nutrients that they are actually given off (mineralized) by the microbed during decomposition. In general, if soils have a ratio of total C to total N (C:N ratio) that is greater than 25, mineral N (ammonium or nitrate) will not be produced in net excess, while if the ratio is less than 15 to 120, excess mineral N will be produced. But, these are based on total elemental analysis of the substrate, not all of which the microbes will perceive as "available." Development of improved "available" C:N ratios will be much more useful for analyzing the N uptake or release from organic amendments than the current total C:N analysis methods.

Some common materials and their C:N ratios are: Alfalfa hay 13:1 Seaweed 19:1 Rotted manure 20:1; Leaves 40-80:1; Oat straw 24:1; Wheat straw 80:1 ; Paper 170:1; Sawdust 400:1

In undisturbed soils, the average proportions of stabilized soil organic matter and fresh plant litter and microbial residues makes a general rule of N mineralization possible. Worldwide, approximately 1 to 3 % of the total N that a soil contains will become "available" for plant growth each year. This makes it possible to do an easy total C and N budget for a soil (including all depths, not just the surface) and to estimate the N release. When the site is impacted by toxic chemicals, anaerobic conditions, fresh organic amendments, atmospheric deposition, or topsoil removal, this general rule is less likely to apply.

Nitrogen fertilization Recommendations for nitrogen (N) fertilization vary from "never" to "usually". If ambient soils exist, even if they are mixed (excavation and refilling of a pipeline, for example) the mechanical process of soil handling will probably result in an increase in the available N, and no further N should be added to reduce the risk of weedy growth. On the other hand, deep excavations into native rock material probably means that little or no N will be available for plant growth. In these cases, additional N is required. Further more, areas near road ways may get annual doses of N from the atmosphere (from car exhaust) of over 30 kg / ha / yr. Depending on the residual soils, the proximity to traffic, and the growth rate of the plants, dosages from zero added N, to 20 to 70 kgN/ha are justified. The higher rates would only apply if there were near agricultural conditions, with good weed control and adequate water. Typically, rates of 20 to 40 kgN/ha would be used.

A close observation of the plant growth will tell a great deal about the plant's perception of N fertility. When all, or perhaps all but the newest leaves become yellowish, the plant is stripping N from older tissues to build new ones. With age, all older leaves may yellow, but with young growth, yellowish coloration is often an indication of N deficiency, or perhaps sulfur (S) deficiency.

Plant indicators of nitrogen deficiency

[scN4]
These Sudan grass plants show a range of N content, from sufficiency (plants in rear) to N deficiency and growth reduction (center front). In a few cases, root growth is low enough that the plant can't get phosphorus (P), creating the reddish coloration on some leaf tips.

[scNconif]
These Douglas fir plants from Washington state show a range of N content symptoms. The tree at left grew next to a nitrogen fixing alder tree and has sufficient N. The tree at right grew about 10 m (30 feet) away on poorer soil and is N deficient. (Zasoski, pers. comm)

[scN1]
Recent hydroseed treatment along Hwy 101. The bands are interpreted to indicate that the center area got sprayed three times because it is at the edge of three staging locations of application, the lower right section that stops at the rocks, the left field application, and the top band. Repeated application gave the junction adequate N, while the broader areas tend to be underfertilized.

[scN6]
All pots in this experiment received the same total amount of N. The pot at left is a biosolids compost. The three center pots are all yard waste composts. The right set of pots is unamended decomposed granite. In the second treatment from the right, the compost actually withdrew available N from the decomposed granite, reducing plant growth compared to the unamended treatment. Eventually, all this N will again become available for mineralization, but the first season's growth may be poor. These differences in N availability cannot be predicted at this time but are expected to be related to different types and amounts of organic materials in the compost as well as feedstock and curing time.

[scn7]
These data are from a Caltrans state wide survey on compost materials available for erosion control. The top two plots of dark symbols are topsoils from sedimentary (COL 20 8.0) or granitic locations (ELD 89 1.8). Top two plots with open symbols are biosolids yardwaste co-composts bottom four open circles are yard waste composts. All were loaded at the same rates of total N (equivalent to 500 kg total N/ha), so the differences in N availability with time represents the mineralization rate of each compost type. If the application rate were doubled to 1000 kg total N/ha, the curves at the right hand side of the graph (YWC 2,3,4) would be similar to the granite topsoil. The left hand side of these same curves, however would not match the N release that is typical of the topsoil from this location. Amendment of the compost may correct the initial release pattern. Co-composts with biosolids release much greater amounts of N.

SRE step 5). Soil Fertility (non-N nutrients)

OBJECTIVES:
The site must provide adequate soil chemcial conditions and nutrient availability to support revegetation on the site.

5.1.0 Evaluate soil chemical conditions.

Soil pH
Soil pH is a measure of the intensity of the acidity or alkalinity of the soil. It is not the measure of the amount of acidity, which would be measured by buffering capacity. The intensity of pH determines whether plants can grow on the site, and it increases or decreases the availability of different nutrients. The amount of acidity determines the number of tons of lime needed to correct acidity. The amount of alkalinity determines the amount of acidifying amendments like elemental sulfur that are needed. Gypsum does not change soil pH. Alkaline and acid soils reduce the availability of phosphorus.

Cations
Positively charged ions in the soil. Calcium (Ca²⁺) and magnesium (Mg²⁺) have two positive charges; potassium (K⁺) and sodium (Na⁺) have one postive charge.

Anions
Negatively charged ions in the soil. Phosphorus exists as an ion compound with between one and three negative charges (PO₄^3-, HPO₄^2-; H₂PO₄^-). Sulfur exists as a sulfate ion with two negative charges (SO₄^2-).

Cation Exchange Capacity (CEC)
Cation exchange capacity is the amount of negative charge scattered across the soil surface that ionically (magnetically) holds the positive charge of various nutrient cations (calcium, magnesium, potassium, ammonium, some micronutrients). It is measured in cmol charge/kg soil and should be 10 or more. Because sandier soils with low CEC may not hold many nutrient cations, the cation content of the soil is expressed as a percent of the nutrient compared to the whole CEC. For balanced nutrient uptake, look at the % of CEC for a particular cation nutrient.

Salinity (EC)
The electrical conductivity of a site (EC) indicates the salt content. This is expressed in deciSiemens per meter (dS/m) but the main thing is to look for values less than 2 or for salt tolerant plants if it is higher. Saline soils start at 4 dS/m. Composts can be fairly salty, mainly because of nutrient ions like K and SO₄^2-. This is not as bad as having salt caused by sodium chloride (NaCl) that disperses soil and plugs pores.

Treating Saline Sites
If a soil is naturally saline, then in theory the plants native to it are already adapted to saline conditions. It is possible, however, to build up salts in the soil. This can be a product of irrigation with saline water, excessive use of fertilizers, or even hunters or herdsmen placing out salt blocks. Deer have been observed grazing at the road edge, which may be because of saltier soils or only because of fresh herbage in the drainage ditch. If a salty white crust is observed, then your site may be saline, but winter rains can wash the crust deeper into the soil and seedlings will still suffer.

Two issues arise with saline soils. The initial challenge is to discourage stock or wildlife from pawing and eating salty soil in order to reestablish native vegetation. In addition, if the salinity level is very high, the soil may actually be toxic to native plants. The best way to determine this is to analyze the soil for a "saturated paste" electrical conductivity. If 1:2 or 1:5 soil:solution ratios are used for salinity analysis, the indicated values will be lower than those experienced by the plant root.

Salinity can be reduced by flushing soils with copious quantities of water, or by treating soils with gypsum prior to flushing. The calcium with its double charge is effective at displacing the sodium with its single charge. Wildlife or stock disturbance can be reduced by the above methods, or by physically blocking access to the salty area such as building an enclosure, placing lots of barriers (like logs) on the ground, and so on. If human activities have resulted in poor soil drainage, then improving drainage should also be considered.

5.2.0 Evaluate soil non-N nutrient availability.

Soil Macronutrients and Micronutrients

There are three macronutrients, nitrogen (N), phosphorus (P), and potassium (K), which are found in significant quantities in most soils. The macronutrients are most likely to be missing if the topsoil layer has been removed.

Nitrogen (treated in the previous section) is supplied from the atmosphere, or from decayed plant and animal matter. Phosphorus and potassium are supplied by the decomposition of parent rock material or from decayed plant and animal matter. Fertilization generally replaces some combination of the three macronutrients.

Micronutrients, while still important to the growth of plants, are in the soil in trace amounts and deficiencies are less common. Calcium (Ca), magnesium (Mg), and sulfur (S) sometimes require supplementation.

The respective amounts of the macronutrients in fertilizers are expressed as percentages of elemental content of nitrogen (N), phosphorus pentoxide (P₂O₅), and potassium oxide (K₂O). So, for example, a "20-10-5" formulation is 20% nitrogen, 10% P₂O₅, and 5% K₂O. The percentage of elemental phosphorus in P₂O₅ is calculated as the % P₂O₅ X 0.43. The percentage of elemental potassium is calculated as % K₂O x 0.83. The fertilizer formulation is often expressed more simply as N-P-K. For instance, someone may say "The N-P-K is 16-48-0."

Phosphorus (P)
A lack of phosphorus is the second most common soil deficiency, especially on arid lands. Phosphorus is lost off the site through erosion because it sorbs to fine mineral particles or in the organic matter fraction. In contrast to nitrogen, phosphorus is very immobile and insoluble in the soil. The over-application of phosphorus does not have toxic affects to plants, but guard against transport of sediment-bound P to local watersheds, especially on newly constructed sites. Due to its immobility phosphorus is not prone to leaching and must be incorporated into the root zone of the plant.

Phosphorus stimulates root growth, which is very important for seedlings. It also promotes maturity including seed production. As such, phosphorus counterbalances the reverse effect of nitrogen, which stimulates shoot growth. Phosphorus also strengthens stems, aids in the absorption of other nutrients, and increases disease resistance. Plants deficient in phosphorus exhibit a general stunted appearance and the seedlings may have purplish foliage (Redente 1993, Hingston 1982).

Synthetic phosphorus is present in many compound fertilizers including ammonium nitrate-phosphate (23-23-0) and ammonium phosphate (11-48-0). If only phosphorus is needed, triple superphosphate (0-46-0) is the common choice; this formulation also contains sulfur and calcium (Redente 1993, Hingston 1982).

Organic amendments containing phosphorus include rock phosphate, basic slag, bone meal, bloodmeal, cottonseed meal, and activated sludge. These are all applied at three to four times the rate of triple superphosphate (Rodale 1961). Steamed bone meal and pelletized rock phosphate (with a ligno-sulphate binder) are reputed to be the best source of organic phosphorus (Integrated Fertility Management 1995). Preparing a site by tilling in legumes as a green manure will also add phosphorus (Rodale 1961).

In clayey soils, the soluble phosphorus in fertilizer will eventually bind with clay particles, thus becoming temporarily unavailable to plants. Legumes have a high phosphorus demand, which, if in ample supply, will stimulate nitrogen fixation.

Measurement of P availability in soils is done by extracting the loosely held nutrient by an extracting solution. For P, acid soils are extracted by an acid extractant, such as "weak Bray" or "double acid" extracts. For neutral and alkaline soils, the "bicarbonate" (Olsen) extract is used. Soil test reports must specify which extract was used. Available P levels should be over 10 for Bray acid extracts and over 5 ppm for bicarbonate extracts.

Potassium (K)
Potassium, also called "potash", is less likely to be deficient compared to nitrogen or phosphorus, but can be limited on coarse, sandier soils. It is more mobile than phosphorus, but leaching is generally not a concern unless soils are sandy or prone to flooding. Potassium is most effective if incorporated into the root zone. Yard waste composts supply abundant potassium.

Potassium promotes photosynthesis, root development, vigor, growth, and maturation of flowers, fruits, and seeds. Potassium counterbalances excess nitrogen. Potassium also increases winter hardiness in legumes (Hingston 1982, Redente 1993). Signs of potassium deficiencies in plants can include dull bluish green leaves with yellowing between veins, which then progresses to either browning leaf tips, spots or patches of discoloration, or on older leaves, the appearance of scorched looking rolled edges.

Synthetic potassium is commonly available as potassium chloride, also called muriate of potash (0-0-60). Potassium is also available in compound fertilizers.

Organic amendments used to increase potassium in the soil include kelp, manure, compost, granite dust, greensand, basalt rock, wood ash, and hay. Kelp is especially high in potassium, and is also rich with trace elements. Leaves, canola meal, and bonemeal have lesser amounts of potassium (Rodale 1961, Integrated Fertility Management 1995).

Measurement of all nutrient cations occurs by displacement by some salt solution (often neutral ammonium acetate) and measurement of the proportions of K, Ca, Mg, and Na. These amounts are expressed both as parts per million (in units of ppm, ug/g, or mg/kg) or as percent of the total number of cations exchanged or CEC.

Potassium should account for more than 1 % of the CEC or greater than 100 ppm.

Calcium (Ca)
Calcium is a component of lime, and may be lacking from soils that have become acidified such as certain foothill or sedimentary soils or mining spoils. Serpentine soils are naturally low in calcium, but deep, developed serpentine soils contain adequate amounts, although they are low in proportion to magnesium.

Calcium aids in cell wall formation and shoot growth. Calcium in the form of bonemeal has long been used as a supplement with bulb plants to replace nutrient reserves. It decreases the uptake of iron, aluminum, and manganese which can be present in toxic levels on acid soils.

Calcium deficiencies show as several symptoms. Terminal buds or root tips may fail to develop, and leaves may be distorted by either appearing rolled forward along the margins or appearing rolled backwards towards the under surface. The edges of the leaves may show yellow bands or the appearance of brown scorching. Root necrosis (death) caused by lack of calcium is a mechanisms by which non-serpentine adapted ecotypes do not survive on these substrates.

Calcium is applied most commonly as various forms of processed lime. Processed limestone fertilizers include burned lime, quicklime (very caustic), lime oxide, or lime hydroxide.

Ground calcium carbonate, also called limestone flour, is a good choice if magnesium is not lacking as it is safer and releases more slowly. Ground dolomite lime (contains both calcium and magnesium carbonates) should only be used if magnesium needs to be supplemented as well. If it would be undesirable to alter pH, gypsum makes an excellent calcium amendment as it contains 23% available calcium. Gypsum also contains 18% sulfur. Gypsum is a common amendment on clayey soils where it loosens the cohesion of the clay particles, making the soil more workable.

Calcium in extracts should account for more than 20 % of the Mg level, and at least 200 ppm. Usually this is never a problem except on serpentine substrates. In this case, a trained botanist or plant expert will help identify serpentine tolerant materials.

Magnesium
Magnesium occurs naturally in limestone formations and is a predominant cation in serpentine soils and substrates.

Magnesium functions in the formation of chlorophyll, aids in the assimilation of phosphorus, and regulates respiration. Magnesium deficiencies appear as interveinal discoloration on older leaves or yellow leaves with brilliant tints which then drop. With grasses a deficiency is evidenced by dwarfed growth and yellow stripes between veins.

Magnesium in extractions should be at least 10 % of the CEC and at least 100 ppm.

Sulfur
In nature sulfur is mineralized from organic matter. Along roadways, vehicle exhaust is a significant source.

Sulfur stimulates root growth, chlorophyll production, seed production, and the formation of root nodules on legumes. Legumes have a higher sulfur demand than grasses. Sulfur is converted into proteins and amino acids by the plant.

Sulfur deficiencies observed in plants include leaves turning light green then yellow, plants are small and spindly, and seed maturation is delayed. In legumes, nodule formation is reduced. Sulfur deficiency looks like nitrogen deficiency, but with nitrogen deficiency, the most recent leaves are often somewhat greener, while in sulfur deficiency, the newest leaves are the most chlorotic (yellow).

Synthetic sulfur is found in compounded fertilizers, or as elemental sulfur, which is a byproduct of the petroleum industry. Very basic soils are treated with the addition of elemental sulfur, which then slowly oxidizes to form sulfuric acid.

Organic soil amendments include pelletized sulfur which is used if the objective is to acidify soils. If increasing the acidity is not desired, gypsum or K-mag can be used instead. K-mag is a mineral containing 27% sulfur, 22% potassium, and 11% magnesium.

Sulfur availability is determined in water extracts, but these are difficult to establish target values for. Sample on the reference site as well as the impacted site and compare values.

Sodium is not a nutrient but may be an issue on salty sites.

Application of Fertilizer or Soil Amendments
With most restoration projects, fertilizer is added at the same time as seeding- generally in the fall. This timing, however, is for convenience and cost-savings. The risk is that the fertilizer will "burn" the germinating plants, since chemical fertilizers tend to be soluble. The better approach is to fertilize the seedlings once true leaves have appeared on the plants. The added benefit to this approach is that the nitrogen will be available right when it is needed rather than leaching quickly through the soil during the wet winter when plants are growing slowly.

Fertilizer can be broadcast with a spreader. Fertilizer should not be placed directly into planting holes unless it is slow release.

SRE step 6). Soil Biology

OBJECTIVES:
The site must support adequate biological activity for nutrient decomposition and cycling, nutrient uptake and to generate soil aggregation.

Decomposer (saprophytic) microbes.

Addition of inoculants, stimulators, activators, or enhancers for decomposer microorganisms are unnecessary. If microbes are not active, suitable substrates are evidently missing and should be provided for them to grow on. Microbes constantly are blown around on dust in the wind and carried by animals. Keeping microbes out of a field site is impossible, so inoculants are unnecessary. Feeding them is necessary.

Mycorrhizal Fungi

Ectomycorrhizae- This group of mycorrhizal fungi is associated with many tree and shrub species. The prefix "ecto" refers to the fungal hyphae wrapping a web-like structure around plant roots and colonizing the spaces between the cells without penetrating the interior of the cells. Nutrients are then absorbed through the plant's root cell walls. Some plant roots can even be linked together by ectomycorrhizal hyphae, acting like a plumbing system allowing nutrients to move from plant to plant. Young seedling plants may be supported by this hyphal network until they are large enough to have a larger carbon flow through photosynthesis.

Douglas fir (Pseudotsuga menziesii) associates with up to 2,000 different species of mycorrhizal fungi throughout its distribution in North America (Trappe 1977). Ectomycorrhizal fungi tend to be generalists, meaning that one species of fungi will interact with many different species of plants.

The spores of ectomycorrhizae can be airborne and can travel readily through the soil. Due to this trait, ectomycorrhizal host plant are unlikely to require inoculation.

Arbuscular Mycorrhizae (AM) - This group is found associated with grasses and forbs as well as some shrub and tree species. The prefix "arbo" refers to the vesicles branching in a treelike pattern. The hyphae of arbuscular mycorrhizae penetrate cell wall of the cortex (an outer layer of root tissue) where they interface directly with the cell membrane of the plant. Vesicular arbuscular mycorrhizae have large soil-borne spores that only migrate short distances through the soil. They are more likely to need inoculation on areas that have not had plant growth previously.

In arid and semi-arid lands, ninety percent or more of the vascular plant species depend on VAM species. The VAM fungi are a less diverse group than the ectomycorrhizae, but are still generalists. Sagebrush (Artemesia), for example, has only four species of fungus that interact with it, but these same fungi will interact with other species as well (Moldenke and others 1994). Western Red Cedar (Thujaplicata), Ceanothus species, and Rubus species (blackberries and salmonberry) are also known VAM plants.

Ericoid Mycorrhizae- This group is specific to the heath family (Ericaceae) which includes many diverse North American plants such as heather, huckleberry, azalea, rhododendron, Labrador tea, and salal. If you are working with a heath family plant species, it will be important to collect inoculum from the same species.

Arbutoid Mycorrhizae- This group is specific mostly to mediterranean species, but also interacts with Arbutus (madrone) and Arctostaphylos (manzanita and kinnikinnick) which may include one of your restoration species.

Orchidaceous Mycorrhizae- As the name suggests, this type of mycorrhizal fungi is specific to the orchid family. The hyphae form a spiraling structure in root cells called coils. This group is mentioned here because of the possibility that a mitigation project may involve orchid species.

How to Inoculate Plants with Mycorrhizal Fungi

Mycorrhizal propagules are the composite of spores, hyphae, and plant root fragments containing mycorrhizal fungi. The technique for collecting mycorrhizal inoculant is quite simple; dig into the root zone of the type of plant species (or group) that you will be using for restoration. Remove some soil, including small pieces of roots. This soil is your inoculant. If local topsoil or organic matter is being used, the appropriate fungal spores are also likely to be present.

There are two junctures at which you might inoculate, either off-site in the greenhouse or on-site when planting. Using both methods will enhance success. Inoculation is most successful during the seedling stage; an attempt to inoculate a more mature nursery grown plant during outplanting is less likely to succeed.

Nursery inoculated plant stock will thrive better once outplanted than plants that are not inoculated. While it is beneficial to inoculate in the nursery, the nursery growing conditions will alter the soil flora due to the growing conditions and chemical use (such as fertilizer) being different than the native plant community. Some nurseries may not want to use this technique because of the risk of introducing soil pathogens to the nursery. Other nurseries are capable of isolating different plant populations in their facility.

The second method is to inoculate plants on site. Ideally, if you are topsoiling, you will already be introducing mycorrhizal fungi (and soil bacteria). If topsoiling is not feasible, you can spread a thin layer (about 1-2 teaspoon's worth) into the root zone of each planting hole. If direct seeding, it is best to inoculate as soon as the seedling plants have emerged. The other option is to inoculate just before seeding, but, as noted above, the delay between seeding and seedling emergence may result in a die off of fungal species. With this option the soil is prepared, a thin layer of inoculant is spread over the soil surface, seed is sprinkled on top of the inoculant so that roots push through the inoculant upon emergence, then a thin layer of soil is spread on top of the seeds (depending on the species). Adding woody materials on the planted site will benefit fungi by making a reserve of moisture available, as will mulching on top of the soil.

Reintroducing Soil Mycorrhizal Fungi and Bacteria

Even though 80% of the earth's vascular plants depend on mycorrhizal symbionts, many plant species do not. These include the genera Saxifraga (saxifrage), Juncus (rushes), Carex (sedges), and plants in the Brassicaceae (mustard) family and Caryophyllaceae (pink) family. These plants are often the first colonizers of naturally disturbed areas.

Most of our weedy introduced species also live mycorrhizae-free lifestyles. This is a key reason why they are able to rapidly invade disturbed areas when the local native plants can not.

Soil Nitrogen-fixing Bacteria
Nitrogen-fixing plants have different types of bacteria depending on the type of plant. The legume or pea family (Fabaceae) is the most well-known group of nitrogen-fixing plants and is colonized by a bacteria called Rhizobium. Various plants form symbioses with different genera of bacteria. An easy way to get the appropriate symbiont is to take soil from under growing plants of the same species and transfer small volumes (a few tablespoons full) to the planting holes of new plantings.

If a plant is a known nitrogen-fixer, and nodules can be identified on the root, inoculation can be done from these nodules. A slurry can be made by blending the roots with water to break open the nodules and release the bacteria. This slurry can be used to irrigate and inoculuate the new plants.

Soil Biotic Crusts
Some soils, especially soils lacking a litter or duff layer, form visible microbiotic crusts. These crusts are comprised of bacteria, fungi, lichen, and mosses. The crusts are easy to spot as the soil is often textured into tiny pinnacles and buttes several centimeters high. Crusts are easily pulverized underfoot. These soils are commonly spotted in arid lands as well as subalpine or alpine areas. If intact, these organisms both serve to hold the soil in place.

The restoration of soil crusts is an emerging science. It is best not to disturb these crusts in the first place as full recovery can take hundreds of years. However, if the damage has already been done (or is otherwise unavoidable) it may help to stockpile and reinoculate the restoration site.

Once the site has been stabilized, backfilled, and planted, the final step is to broadcast the pulverized crust back over the restoration work. The ideal is to replace the crust at a 1:1 ratio, however it is rare to have this much crust material available. A 1:10 or 1:20 ratio is more commonly used. The maximum depth of a respread pulverized crust is one inch.

If salvaged soil must be stored prior to reuse, the top inch of crust should be removed and stored separate from the remaining 3-8 inches of topsoil. Salvaged crusts can be stored in an active or dormant state. To remain active, the crust must be able to photosynthesize and receive moisture; this is accomplished by spreading soil to only an inch or two thick. Dormant storage is accomplished by storing dry soils more thickly away from moisture and sunlight; this can be accomplished by using buckets with lids. For dormant storage, soils must be dry when stored (Belnap and Furman 1997).

Biological or microbiotic crusts are also referred to as cryptogamic, cryptobiotic, and microphytic depending on the type of organisms present. Biological crusts are recognized by the distinct presence of living organisms or their byproducts, creating a surface crust of soil particles bound together by organic materials. In otherwise disturbed areas, look for biological soil crusts in fenced areas, low use areas under shrubs, or between closely spaced rocks.

Biological crusts vary tremendously in thickness, texture, percent cover, color, and species present. For example, cyanobacteria, dominant in arid soils, forms pinnacles up to 15 centimeters high. Other soils may have evident moss or lichen growing on the soil surface (NRCS 2001b, NRCS 1997). A common pattern is for a moss or lichen crust to form a mini-terrace, creating benches 5 to 10 cm wide that are flatter than the slope angle. Because they are stable when wet, they trap sediment and persist during rains.

OBJECTIVES:
The site must be adequately stable against surface erosion and desiccation that plants can germinate, grow and cover the site before erosion resumes.

Overall site drainage
The site should be protected from run-on from slopes above the site. Diversion ditches and armored drains shield the new slope from the volumes of water that move downslope in the soil above the site. These volumes of water are harmless in existing soils with well aggregated structure and good root strength, but they are distructive when they hit the unconsolidated and unaggregated disturbed substrates.

Drainage within the slope can be improved by breaking the slope into shorter segments with wattles or diversion ditches. This, however, avoids solving the problem that a slope needs infiltration and percolation capacity. At the Buckhorn project (SHA 299 0.06) the coir fabric layers formed multiple seepage zones that wicked the water to the surface and allowed it to flow downward across the face of the slope without erosion. These horizontal layers of coir blanket, along with willow poles layed horizontally on each lift, provided a safety valve for water within the unconsolidated, compost amended surface of the slope.

Surface (rain drop impact) protection.
Conventional erosion control methods are effective for this component. Straw is cheap and effective. In some areas, pine needles are available, and form erosion and wind-resistant layers as they interlock. Long needle lengths are important, and care should be taken to not break up long (ponderosa pine type) needles. High technology, bonded fiber matrixes and expensive blankets are often used, but straw remains a standard for effectiveness.

Mulch effects
Protection against rain drop impact takes only a thin layer of straw or hydromulch. Thicker layers of mulch protect against frost heave in cold, high elevation areas and against heat and dessication in hot, dry areas. The thresholds of mulch thickness have not been tested in California.

Thick mulches also have some negative aspects. Small rains may wet the mulch but not the soil. Also, the mulch that keeps the soil from freezing in mid winter, may also keep the soil too cool to germinate seeds in the spring. Despite these potential drawbacks, the protection of a soil against a hard freeze to significant depths (30 cm, in Lake Tahoe on unmulched fill) allows roots of perennials to grow and prepare for the short growing season when the spring arrives.