Soil Management

Water storage in soil is essential because plants use water from soil between precipitation or irrigation events. Adequate water storage in soil is important not only in dry regions (arid, semiarid, and subhumid) but also in humid regions where short-term droughts at critical growth stages can greatly reduce crop yields. Because of its importance, water storage in soil and subsequent efficient use of that water for crop production have long been studied by researchers.

In hot, dry climates, water storage is improved by minimizing soil evaporation, decreasing runoff losses, limiting deep percolation, and reducing transpiration by weeds.

Direct evaporation from the soil is a net loss to crop production and is a substantial component of the total water use of crops grown in arid and semiarid environments. In Mediterranean climatic regions, water loss by direct evaporation from the soil during the cropping season may represent 30% to 50% of the annual rainfall. Recent estimates of soil water evaporation under a lupin crop was 1 to 1.6 mm day-1 when total crop evapotranspiration was 1.8 to 3.4 mm day-1 [137]. Evaporation from soil is reduced by weed control, tillage, and leaving the stubble or other crop residue in the fields during the dry or fallow season.

Materials such as crop residues, plastic films, petroleum-based products, gravel, and soil itself have been studied widely as potential mulches for decreasing evaporation. However, the effect of mulches on evaporation is difficult to establish because of the interacting influences of mulches on soil water infiltration, distribution, and subsequent evaporation.

Adding external materials on the soil surface is generally, but not always, restricted to ornamentals, vegetables, or other high-value crops. The most economical mulch for large-area application is plant residue (standing or flattened). Residues reduce surface temperatures and shade the soil from solar radiation, decreasing the rate of soil evaporation.

Mulching is most effective early in the growing season, before a crop canopy forms and evaporation exceeds transpiration. Theoretical analyses and field experiments show that surface residues are of greatest value for water conservation during the winter rainy season and early spring when the soil surface is often wet.

Mulch effectiveness for decreasing evaporation increases as mulch thickness increases. Because material density largely influences mulch thickness, low-density materials, such as wheat straw, more effectively decrease evaporation than sorghum stubble or cotton stalks, which are more dense. Because the water-content difference between mulched and bare soil is mainly near the soil surface, especially soon after water additions, mulches are very useful for seedling establishment.

Tillage is needed primarily on heavy soils that may crack during fallow and lose water by evaporation through the cracks. Sands and other light-textured soils that do not crack are self-mulching and do not need tillage.

Surfactants may decrease evaporation by decreasing capillary rise of water to the surface. Surfactants probably cause decreased surface tension at the solid-liquid interface, thereby decreasing capillary flow of water and causing the formation of a dry diffusion barrier. Seemingly, the use of surfactants for making more efficient use of water has limited potential. Although evaporation is decreased, total water use by crops is not affected and yields are decreased [138].

Water infiltration and runoff control are virtually inseparable. Reducing surface runoff and increasing infiltration of rainfall is probably the most important way in which the agronomic practices affect the total water supply of the soil. Runoff will occur whenever the surface detention is full and rainfall rate exceeds soil infiltrability. This situation is favored by: poor soil tilth and structure management, small surface holding, and unstable surface soil structure with a tendency to crust.

Infiltration is influenced by soil conditions at the surface, in the tillage layer, and within the profile. Adequate surface residues or increased surface roughness (e.g., microbasins, tied ridges) and aggregation to increase surface retention and infiltration, and stabilization of surface soil structure to rainfall impact may reduce runoff losses out of a field and also help in retaining a uniform soil water distribution.

Avery effective alternative solution is to mechanically create a soil surface microrelief so as to increase surface storage after the soil has been brought to the tilth desired for crop production. The effectiveness of a roughened surface depends, among other factors, on the intensity and amount of precipitation, and on the stability of the surface soil. In ridge-and-furrow cultivated row crops, this technique is variously called basin tillage, furrow damming, or tied ridges, and consists of constructing small dams in the furrow at intervals of approximately 1-5 m.

The use of tillage to increase the infiltration of rainfall into the soil in the short term may have long-term negative effects. If loosening of the upper soil to increase infiltration comes at the expense of compaction below the tillage zone, this can disable infiltration to lower storage zones and induce a perched water table above the tillage pan. This, in turn, can increase the chance of waterlogging, thereby increasing runoff and decreasing infiltration.

Tillage near the contour associated with ridge terraces has long been used to control runoff on relatively steep land in humid regions. Under contour tillage, surface roughness is least along the contours, and largest along the slope of the land. In water-deficit farming areas, contour furrowing for row crops is an effective runoff control and water conservation practice.

Conservation tillage is one of the best means to conserve water under adverse supply conditions by using tillage practices as the principal management tool. The term conservation tillage means any tillage sequence that minimizes soil and water losses; thus, every region, crop, and climate will require a somewhat different set of practices. The objectives are to achieve a soil surface with high infiltrability and adequate holding storage that will retain advantageous properties over extended time periods and will provide a favorable seedbed and rooting medium for agricultural crops.

Modern conservation tillage and herbicidal practices conserve crop residue on the surface, thereby leading to better soil water conservation and cooler soil temperatures during springtime compared with more soil-disturbing, conventional tillage practices. In humid or subhumid regions, cooler, wetter soils can lead to slower crop development, and reduced crop growth under some circumstances. However, in the water-limited environments of dryland cropping regions, no tillage or other residue-conserving practices will increase small-grain growth and yield as well as conventional tillage practices.

Residues intercept rainfall, absorbing and dissipating impact energy. This transfer of energy reduces degradation of soil aggregates at the surface, which in turn reduces the sealing of the surface against infiltration.

Increased residue on the soil surface also has been associated with increased organic-matter content of soils or altered distribution in the profile, thereby affecting infiltration. Little information exists to separate the effects of surface residues on rainfall capture from the effects of organic matter on infiltration.

A commonly used method is stubble mulch (SM) tillage, in which a sweep or blade undercuts the soil surface to control weeds and prepare a seedbed, yet retains most residues on the surface. The depth of tillage usually ranges between 7 and 10 cm. Each operation with a properly operated SM implement reduces surface residues only about 10% to 15%. Hence, adequate surface residues usually can be retained by using SM tillage.

One traditional method to increase storage capacity has been to increase the volume of soil suitable for rooting. When impeding layers or horizons (fragipans, handpans, and plowpans) are present, subsoiling or deep tillage can improve infiltration and crop rooting, although its effectiveness in conserving soil water for a succeeding crop is limited. The effects of deep tillage are often transitory because of the inherently poor structure of the soil or the dispersive nature of the subsoil. In such cases, deep tillage needs to be supplemented with organic matter or chemical treatments for aggregates.

A second method acts by increasing the capacity to store water in a unit of soil volume. Addition of organic matter or other soil amendments should increase the available water-holding capacity. The effects of organic-matter management on soil properties encompass almost all parameters of the soil environment. Microbial activity and growth are stimulated, resulting not only in larger pools of labile nitrogen, but also improved soil physical conditions that maintain porosity and promote aeration and favorable water regimes. Organic substances that contribute to soil aggregation are derived from plant materials, either after alteration by soil animals, bacteria, and fungi, or directly from the plants themselves.

Earthworms improve soil structure, which increases infiltration. While feeding on organic materials and burrowing in soils, earthworms secrete gelatinous substances that coat and stabilize soil aggregates. Water-stable aggregates result also from water-insoluble gummy substances secreted by bacteria, fungi, and actinomycetes. Earthworm activity and intensive soil tillage are not very compatible. Hence, little earthworm activity occurs in many intensively cultivated soils [138]. For maximum earthworm activity, no tillage is desirable.

Plants directly influence soil aggregation through exudates from roots, leaves, and stems. Additional influences are a result of weathering and decaying plant materials, which bind soil particles together; plant canopies, which protect surface aggregates against breakdown due to raindrop impact; and root action in soil, which promotes aggregate formation.

In addition to the direct osmotic effect and possible toxicity of specific ions, soil salinity may have a deleterious effect on physical properties such as infiltrability, water holding and aeration, especially if the soil is rich in exchangeable sodium. The surface soil is more dispersible by raindrops, and crusting and runoff will follow. Reclamation of salt-affected soils under these water-deficient dryland conditions is very difficult because removal of the excess salts by leaching is an essential part of the reclamation process. Gypsum, which often is applied to displace adsorbed sodium from the soil exchange complex, is itself a contributor of soluble salts. Unless it can be leached from the soil in the last stage of reclamation, it thus may further depress yields. Even if temporary relief is obtained, it may not be economical because of the cost of the amendments and their application.

A soil conditioner's effectiveness often is related to its ability to promote floccula-tion. Polymers induce flocculation (or coagulation) of dispersed clay particles by electrostatic absorption of polymer molecules on clay particles, which helps to compensate for the clay surface charge, bridging soil particles together. Anionic polymers are effective flocculants, especially in the presence of polyvalent cations. When applied to soil, these substances result in larger aggregates. Although not economical, except possibly for some high-value crops, the substances improve aggregation, which increases water holding and infiltration in the soil profile and decrease runoff.

Mixing fine-textured materials from outside sources or from larger depths within the profile with sandy surface horizons can increase the water-holding capacity of sandy surface soils. Adding materials from outside sources may be practical in limited areas, but not in large areas because of the large amount of material needed and the expenses involved in transporting the materials. In contrast, mixing finer materials from deeper in the profile with coarser materials near the surface is within the reality of practice. The water-holding capacity of the entire profile may not be increased, but soil near the surface should hold more water, thus improving seedling establishment and early growth [138].

Compaction of the subsoil and placement of relatively impervious materials at depth are methods used to reduce percolation. In sandy soils, the problem of deep drainage is especially acute because of the limited water-storage capacity. Thus, incorporation of clay or organic colloids or residues into the profile may increase its water-storage volume. However, the cost of these agronomic practices would be prohibitive in the context of dryland farming.

Land-farming practices are used widely to control runoff from excess rainfall. Precipitation amount and distribution strongly influence the type of terrace used in a particular region. In higher-precipitation regions, excess water is channeled from fields by graded terraces. Such terraces drain the water at non-erosive velocities into grassy waterways or other grassy areas. When designed for water conservation, terraces often are leveled and may have closed ends (level terraces). The conservation-bench terrace is a practice that has been used successfully in dryland and irrigated farming to reduce runoff and retain precipitation.

Although microwatersheds and vertical mulches serve two distinct purposes, they often are used in a combination system. Microwatersheds increase runoff from a portion of the field and concentrate the water on a relatively small area to increase depth of water penetration. Vertical mulching, by providing a residue-filled soil slot open to the surface, results in rapid channeling of water into soil.

In areas where natural rainfall is insufficient for dryland farming, nonarable areas or hillsides may be used for water harvesting to support an economically valuable crop. Water harvesting conveys runoff to cultivated fields.

Microcatchment water harvesting is a low-cost method of collecting surface runoff in the area (A) and storing it in the root zone of an adjacent infiltration basin (B) to cover the crop water requirement. The crop may be a single tree, bush, or annual crops. This method is also applicable in areas with a high rainfall but low soil permeability. To design an optimum ratio between A and B, the impact of climatic conditions and soil physical properties on the water dynamics must be considered. The design should aim at sufficient available water in an average year: Deep percolation losses during a wet year then must be accepted as well as some shortage during a dry year [139].

In a runoff-farming system, crops are grown in widely spaced strips or rows on the contour, where the areas between the strips or rows are treated to enhance runoff from rainfall. Depending on rainfall distribution, the crops in a runoff-farming system should be able to survive extended periods without rain. Crop Management

WUE depends on the relative rates of assimilation and evapotranspiration as influenced by crop genotype and environment. Those traits are subject to optimization through variations in cropping practice and through special treatment that might be applied to the crop-plant community. The diversity of crop-management techniques involving water management is great (Table 5.19), but, in this section, the cropping practices of

Table 5.19. Crop Management Techniques for drought and water-stress conditions

Crop Management Techniques Benefits Effectiveness

Drought risk management

Change of crop patterns, replacing sensitive Limits the effects of droughts High crops with tolerant ones (eventually decreasing irrigation surface)

Choice of drought-tolerant crops over highly

Limits drought requirements

High

productive varieties

Use of short-cycle varieties

Low water requirements

High

Early seeding

Avoids terminal stress

High

Early cutting of forage crops

Avoids degradation of stressed crops

High

Grazing drought-damaged fields

Alternative use, livestock support

High

Supplemental irrigation of rainfed crops

Avoids stress at critical stages

High

Management for controlling effects of water stress

Use of appropriate soil management techniques

Increases available soil water

High

Adaptation of crop patterns to environmental

Coping with water-stressed

High

constraints and resource conservation

environment

Use of fallow cropping in rainfed systems

Increases soil moisture

Controversial

Use of mixed cropping and intercropping,

Better use of resources

Low

namely for forages

Increased plant spacing of perennials and

High individual explorable

Limited/high

some row crops

soil volume

Cultivation techniques

Minimizing tillage

Avoids Es

High

Adequate seed placement

Prevents rapid drying of soil layers

High

around the seed

Preemergence weed control

Alleviates competition for water,

High

avoids herbicide effects on

stressed crop plants

Reduced and delayed fertilization

Favors deep rooting, adaptation to

Variable

crop responses under water stress

Dry-soil land preparation and seeding of

Saves water

High

paddy rice

Antitranspirants

Reduces plant transpiration

Controversial

Reflectants (increasing albedo)

Decreases energy available for

Limited/high

transpiration

Windbreaks

Decreases energy available for

Limited

evaporation

Growth regulators

Improves responses of physiological

Promising

processes to water stress

greatest interest are discussed. The review of crop-management techniques in Table 5.19 distinguishes the techniques or decisions that are associated with a given risk from those adapted to water-stressed environments that can be adopted easily for drought control

Decreasing water loss by soil water evaporation provides the potential for improved crop production on the same rainfall input. Selection of cultivars with early growth and/or earlier dates of planting to increase early growth are mechanisms for reducing soil water evaporation and increasing crop water use.

Crop rotations, including grasses and legumes, reduce the impact of raindrops and surface aggregate breakdown, crusting, runoff, and erosion, and soil water evaporation while improving soil water storage. Improved soil structural stability is attributed to the positive influence of root systems in increasing infiltration. If legumes are used as cover crops, they can provide nitrogen through fixation for subsequent crops. Cover crops prevent leaching of nitrogen, potassium and possibly other nutrients by incorporating them into their biomass. Cover crops add organic matter to the soil and they can cause an increase in microbial activity by providing a readily available carbon source, which subsequently increases aggregation.

Advances in cropping systems productivity can be accomplished by developing and using crop rotations that either are more timely in utilizing available water or are able to conserve water. However, few reports exist regarding the effect of crop rotation on efficient water use. The one exception is the cereal-fallow rotation used in dry areas.

In cropping systems, the term fallow is used to describe land that is resting, that is, not being cropped. The inclusion of a weed-free fallow is used widely in semiarid regions to conserve soil water storage and increase the water available for the next crop. Fallow also maintains a change in crop succession to minimize carryover of pests, diseases, and weeds, and plays an important role in the mineralization of nitrogen.

As with many other techniques, the use of a fallow period has been questioned. Because fallows inevitably lose water by evaporation from the surface and drainage below the root zone, their efficiency may be low and varies from year to year.

It is obvious than each crop rotation and fallow combination needs to be tested over a long period to evaluate its sustainability. Information pertinent to a given location does not seem to be simply transferable to other locations without additional prolonged testing before adoption.

Weeds pose severe problems in many fields in dry areas. They can dramatically decrease the WUE of crop production, particularly in crops that compete less vigorously with weeds. During late spring and early summer, weeds compete vigorously for available moisture and, under conditions of water deficit, this leads to yield losses. Weeds can be controlled by tillage, herbicides, and crop rotations.

Antitranspirants have been used to control transpiration at the leaf-air interface. These materials may induce stomatal closure, cover the mesophyll surface with a thin, monomolecular film, or cover the leaf surface with a water-impervious film. However, an-titranspirants have potential detrimental effects on net assimilation of photosynthates and evapotranspirational cooling. Moreover, the cost of applying antitranspirants at present disqualifies their use in dryland farming.

Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

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