Soil Rehabilitation Guidebook

[Soil Rehabilitation Guidebook Table of Contents]

Tools and techniques

This section provides information on rehabilitation methods used to restore water, soil and vegetation, along with some ideas for managing cattle on rehabilitated areas.

Water

Drainage and erosion control2

Diversion of water on slopes is a common cause of slope failure. Problems resulting from drainage alteration are compounded when timber harvesting leads to higher soil water content and reduced root strength. Procedures for good water management and erosion control during construction and use of permanent access structures are outlined in the Forest Road Engineering Guidebook. For procedures on deactivating permanent access structures, see Carr (1980, 1985) and Chatwin et al. (1994).

Drainage alterations such as increased surface water flow, interrupted below-ground flow, or restricted drainage also lead to reduced forest productivity on disturbed areas. Soil rehabilitation projects must ensure that naturally occurring above- and below-ground drainage patterns are restored and maintained. Consider implementing the following measures to restore drainage and prevent erosion:

Soil

Topsoil handling

"Topsoil" is defined here as the uppermost soil layer, usually including the top 2025 cm of mineral soil, where the bulk of the rooting zone is located. The topsoil and the forest floor contain large reserves of plant nutrients and the organisms that influence soil nutrient cycling. The best way to establish productive nutrient cycles on rehabilitated sites is to conserve and respread the topsoil and the forest floor.

Guidelines to ensure successful topsoil handling

Other points to remember

Tillage

In many cases, the productivity of disturbed soils is limited by their physical properties. The upper mineral soil horizons of productive forest soils are characterized by an open structure in which large pores allow excess water to drain away. This allows the soil to warm more quickly in the spring. In addition, oxygen flow to respiring plant roots is enhanced in soils with open structure. The depth of surface soil with a favorable physical condition is often a good indicator of expected site productivity. Tillage is used primarily to decompact the soil and re-establish soil porosity, allowing plant roots to penetrate deeper into the soil.

Determining tillage depth

It has been commonly accepted that the deeper the tillage the better, up to some practical physical limit of the equipment usually around 50 cm. Increasing the rooting depth has been considered advantageous even though the natural forest may have grown on shallower soils. Deep tillage has the possible benefit of breaking up impermeable layers such as dense Bt horizons, and increasing rooting depth beyond that of the original soil. Increased rooting depth may also mitigate some of the adverse effects of the initial disturbance on surface soils.

Deep tillage can have a number of negative effects, such as increased costs, added stress on equipment, and transfer of unfavorable subsoils or rocks to the surface. Some types of equipment capable of reaching 50 cm depths (e.g., crawler tractors with ripper teeth) are not very effective for decompaction. The usual effect of tilling with rock rippers is to create large clods, leave deep furrows, and bring rocks to the surface. Furthermore, unless the ripping is done repeatedly, much of the area remains in a compact condition. Shallower tillage or the use of more appropriate equipment is usually more cost-effective and may provide superior results. Examples of more appropriate tilling implements include winged subsoilers designed specifically for this purpose and hoes with long ripping tines. When determining the appropriate decompaction depth, evaluate:

Maintaining soil structure after tillage

Good tillage loosens the soil, breaks massive soil materials into small clods and encourages the formation of more porous aggregates. The long-term stability of aggregates and pore spaces depends on soil texture, soil organic matter and soil biological activity.

In coarse-textured soils, tillage alone may be sufficient to restore soil structure and maintain productivity, provided organic matter and nutrients have not been lost. Good tillage of medium- and fine-textured subsoils can create a reasonable soil structure, but the effect will be short-lived unless aggregate stability is also restored. Without organic matter and biological activity, aggregates created by tillage will fall apart when the soil gets wet, the large pores will disappear because of structural collapse, and the smaller pores will become clogged with silt and clay particles.

Tillage and soil moisture

The goal of tillage is to shatter the soil. Tillage will be ineffective and may degrade rather than improve soil structure if it is conducted when the soil contains enough moisture to make it plastic, in which case it will tend to mold rather than shatter when a force is applied to it. The actual water content at which a soil will shatter varies by soil texture.

Moisture content needs to be determined prior to tilling medium- and fine-textured soils. Ideally, they should be friable, which means they should be dry enough to crumble when worked (rather than smear), but not so dry that they turn to powder. If the soil is too wet, tillage equipment can mold the soil into large chunks that do not improve soil structure. Wet soil can be puddled, resulting in destruction of soil aggregates and their associated pores. If a medium- or fine-textured soil is too dry, the equipment will have difficulty penetrating to sufficient depths, and may pulverize the soil rather than shatter it. Pulverized soil will quickly recompact when it gets wet.

Ensure that rehabilitation plans involving tillage are scheduled only for soils that can be expected to reach an appropriate moisture content in most years. Plans must allow for early start-up if appropriate soil moisture conditions occur early, and for delays if conditions are not appropriate. Local experience with site conditions, pre-work soil moisture monitoring, and post-treatment evaluation of tillage effectiveness are essential for developing good tillage prescriptions.

Equipment used for decompaction

Winged subsoiler

Winged subsoilers can be effective and efficient tools for decompacting soils in rehabilitation work if they are used by an experienced operator under suitable soil conditions. The winged subsoiler, with the wings set at the proper angle, lifts the soil and then allows it to fall back in place as the implement passes, resulting in shatter without burying the forest floor or topsoil or bringing unfavorable subsoils to the surface. A good subsoiling operation can leave vegetation sufficiently intact that it will continue to grow.

On the better models of winged subsoilers, the wings can be adjusted to match soil conditions, particularly if the soil is dry, high in clay content or extremely dense. When adjusted properly, the implement lifts the soil slightly above it without causing extensive smearing or compaction below the wings. The trench created around the shanks should be narrow, and the soil should not be pulled up into furrows. Furrows indicate that the subsoiler is not penetrating deep enough, or that the angle of the wings is set too steep.

The shank spacing can usually be adjusted to accommodate landing sizes or different road and trail widths. Because the entire soil layer is lifted and dropped, soil between the wings is shattered, and adequate tillage can be achieved in one pass. Even though the wings are independently mounted to prevent hang-ups, the subsoiler is ineffective on sites with large amounts of large rocks (>50%) or buried logs.

The winged subsoiler is the most effective implement for decompacting large areas with relatively uniform conditions.

Excavator

Hydraulic excavators are more flexible tools than winged subsoilers in many situations.

Excavators are readily available in all parts of British Columbia, and a number of attachments are available to achieve different soil rehabilitation objectives including, mixing, mounding, tilling, manipulating slash, spreading mulches, etc. A conventional bucket is suitable for a limited amount of tillage; soil with the correct moisture content can be lifted and dropped to achieve good shatter. A mounding rake or similar toothed implement is probably more efficient for decompaction. When equipped with a thumb, an excavator easily handles coarse woody debris that needs to be moved. Various other attachments are available for excavators, including mounders, mulchers and rototillers. Excavators are well suited for the following types of operations:

Other equipment

Several different types of mechanical site preparation equipment are available that could be used for soil rehabilitation. Most implements, however, are designed to create favorable microsites for planting, rather than to decompact extensive areas in a homogeneous fashion. Experience with the use of specific site preparation implements for soil rehabilitation is limited, but many possibilities exist for applying mounding, scalping, disc trenching, mixing or ripping/plowing equipment to the job. For example:

In all cases where the use of site preparation equipment is proposed, the rehabilitation plan must include a clear statement of the productivity objectives, the proposed methods for restoring productivity and sufficient detail to allow the district manager to evaluate the likelihood that the proposed rehabilitation work will achieve stated objectives and serve as a basis for assessing whether the rehabilitation has been adequately performed.

More information on site preparation equipment is available in the Forest Practices Code Site Preparation Guidebook.

Soil amendments, fertilizers and mulches

Soil amendments are materials that are mixed into the soil to restore soil organic matter, long-term nutrient status or soil structure. Chemical fertilizers provide an efficient means of improving short-term nutrient status. Mulches protect the soil from erosion, conserve moisture and moderate soil temperature. Except for chemical fertilizers, soil amendments are bulky and expensive to transport, so local availability is a key factor determining their suitability for various uses.

Organic soil amendments

Logging residues

The most readily available organic materials are usually the residues of logging: branches, tops and unmerchantable stems. Accumulations of these materials adjacent to roads and landings are often burned, providing ash that could serve as a soil amendment.

Fine branches, tops and especially foliage contain significant quantities of nutrients, and can be mixed directly as a soil amendment to improve soil physical properties, enhance nutrient status and increase mineral soil organic matter content. This type of treatment may be particularly useful where topsoil and the forest floor were not conserved before construction, or on cold, dense subsoils that are deficient in organic matter. Depending on the size and shape of the materials, it may be useful to chip fine residues where equipment is available.

Other materials

Chemical fertilizers

A single large application of chemical fertilizer is usually insufficient to restore the nutrient capital of a degraded soil. If soil organic matter has been displaced or destroyed, and if only limited vegetation cover is present, most of the nutrients added in a large application may be lost from the site. Instead, fertilization should be used primarily to enhance the early establishment and growth of vegetation, which will restore soil structure and organic matter content. Modest repeat applications may be needed until the internal nutrient cycle of the site is re-established and can meet the needs of the vegetation, but a site would not be considered adequately rehabilitated if the survival of the vegetative cover depends on continued fertilization.

While natural forests in British Columbia commonly respond to nitrogen fertilization and only rarely to phosphorus, potassium or sulfur, any of these nutrients may be deficient in disturbed and rehabilitated soils. Soil tests can be obtained from commercial laboratories to help determine fertilizer requirements for grasses and legumes, but recommendations based on fertilizer response trials are not available for most tree species and forest soils in the province. In any case, such recommendations are based on annual growth requirements rather than on long-term requirements. Fertilizer tends to be a small portion of total rehabilitation costs, so if nutrient deficiencies are anticipated, complete formulations are usually used at rates that approach safe maximums.

Maximum fertilizer rates are set to reduce the risks of (1) damaging vegetation from over-fertilization and (2) losing fertilizer through runoff or leaching. Damage to young seedlings has been reported at application rates around 100 kg N/ha. The risk of fertilizer damage increases greatly with decreasing moisture and increasing temperature, so that higher fertilization rates can be used without damaging seedlings in climates with higher precipitation. However, in wet environments, large amounts of fertilizer can be lost from recently disturbed sites that are low in organic matter and have limited vegetation cover.

Three general formulations can be considered for rehabilitation work:

High analysis granular fertilizers are preferred because of their lower transport and handling costs. A complete fertilizer with approximately equal concentrations of the macronutrients (such as 19-18-18, containing 19% N, 18% P2O5, and 18% K2O) is desirable because of the low fertility of severely disturbed forest soils. Many possible formulations are available, and for most situations it is difficult to identify a clear advantage for any one recipe. Cost and availability will usually dictate the best formulation to use.

Application rates for initial fertilization will usually range between 30 and 100 kg N/ha, depending on the severity of nutrient depletion at the site, the risk of runoff, the amount and composition of seeded cover, and the rehabilitation objectives. If 19-18-18 was used, the range of application rates would be 160 to 525 kg of fertilizer product per hectare. Application rates in the lower part of the range (3050 kg N/ha) are suitable for erosion control situations where legumes make up a large proportion of the seed mix. For largely grass mixtures where short-term erosion control is the primary objective, use application rates in the middle of the range (5080 kg N/ha). To restore soil structure and long-term productivity for tree growth, application rates in the higher part of the range (up to 100 kg N/ha) are appropriate, but be aware that high rates of fertilization can lead to loss of fertilizer due to leaching. Repeat applications at lower rates are probably better than one large application, but transport and setup costs are usually lower for a single large application. Consult with an expert in your area for more information on typical fertilizer application rates.

Fertilizers can be broadcast on the surface, included in a hydroseeding slurry, or incorporated if shallow mixing (<20 cm) is part of the rehabilitation plan.

Fertilizer is usually applied at the time of seeding, ideally immediately after the seedbed is prepared. Higher losses of seed and fertilizer occur after the freshly prepared surface has been subjected to rainfall. Where vegetation is already established, apply fertilizer when growth is most rapid. In interior British Columbia, fertilizer can also be applied in the fall or winter, though higher losses may result. Fall and winter applications are not recommended in coastal British Columbia because of high leaching losses.

Other points to remember:

Mulches

Mulches are non-living materials spread over the soil surface to reduce erosion and aid plant establishment by conserving moisture and moderating soil temperatures. Several types of mulches can be used, including relatively thick layers of organic material, manufactured mulch mats of various types, and thin layers of mulch primarily applied during hydroseeding.

Thick mulches: Materials suitable for thick mulches include logging residues (either fine slash or chipped debris), forest floor material, straw or hay. As a rough guideline, 510 cm should be a sufficient depth for most sites. Decomposition occurs slowly because the mulched layer dries out repeatedly, but the materials will eventually contribute to the restoration of soil organic matter.

Thick mulches imitate the ecological functions of a forest floor. They are recommended primarily for drought-prone sites, but may also be appropriate where soils are wet or cold as long as trees are planted on elevated microsites or the mulch provides a suitable rooting medium for seedlings. The mulch will keep fine-textured soils moist and soft and tree roots may explore the interface between the mulch and mineral soil. Thick mulches will prevent the growth of grass and many weedy species. They are best used in combination with planted shrubs and trees.

Manufactured mulch mats: Various types of manufactured mulch mats are available, including plastic and fibre matting or netting materials. Some of these products can also aid in germination and vegetation establishment. Mats must be in close contact with the soil surface to be effective, and this may limit their suitability for mechanical slope protection of forest roads where slopes often have rough surfaces. Their ability to trap sediment and biomass is useful in building soil, improving surface soil conditions and restoring soil organic matter. Because of cost, the use of manufactured mats is limited to small, critical areas such as ditch lining and bridge crossings.

Thin mulches: Thin mulches are useful to aid the germination and establishment of grasses and legumes on drought-prone sites, highly erodible soils, unconsolidated (sandy) surface soils, and slopes with southerly or westerly exposures. The mulches can be applied over the top of seed to protect it from desiccation and wind, water or gravity movement. Some types of light mulches need a tackifier applied to or with them to prevent them from blowing or washing away.

The most common mulching technique for use in combination with grasses and legumes is wood fibre applied with a hydraulic seeder with mechanical agitation. Products consist of ground wood fibres mixed with a green dye to improve visibility during application. Although it can be combined with seed and fertilizer, this type of mulch is best applied in a second pass over the top of seeded areas. In this way, seed is in contact with soil and covered by mulch. Recommended rates are 10002000 kg/ha. Application at these rates takes 6 to 12 loads with a seeder for every load of seed application. It is important to note, however, that while rates at the upper end of this range improve erosion control, they may also reduce plant establishment (Berglund, 1978). See Appendix 4 for application instructions.

Thin straw mulches offer excellent soil protection. Straw can be applied by hand, or with a straw blower (available through farm equipment suppliers). A blower speeds application and ensures more even coverage. Recommended rates range from a minimum of 2000 kg/ha to over 5000 kg/ha. For maximum effectiveness, a tackifier should be sprayed over the surface straw to hold it in place. Mechanical means of anchoring straw, such as disking, rolling, or covering it with netting, are generally impractical for forest soil rehabilitation.


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