Agronomic measures for soil conservation are based on the protective effect of plant cover as one of the most effective tools for controlling soil erosion.
However, considering the different densities and characteristics of plants in both natural and agricultural ecosystems, differences in their ability to control erosion can be considerable. Annual field crops, for instance, are the least effective in protecting the soil and cause the most serious erosion problems. This is due to the high percentage of bare ground exposed under these systems, especially in the early stages of crop growth and during the preparation of the seedbed.
Annual field crops must be combined with protection-effective crops, such as multian-nual or perennial forage crops. The frequency with which row crops are grown depends upon the severity of erosion. A high rate of soil loss under a row crop is acceptable when it is counteracted by low rates under the other crops so that, averaged over the total rotation period, the annual erosion rate remains low.
Row crops represent a planting system that currently is used because it facilitates mechanized farming operations. Row crops are planted in wide or narrow rows, depending on plant architecture and the economic characteristics of production. Soil erosion problems are associated mainly with wide-row crops such as maize, sugar beets potatoes, sunflower, sorghum, and soybeans because of the large amount of bare soil left per unit area and because they are tilled and subjected to clean weeding during the crop production cycle. Crops such as wheat, oats, and barley are more protection effective in relation to soil erosion.
All crop types are particularly subject to erosion risk during the period from sowing to complete canopy development. The extent of the risk depends on the seasonal distribution of the erosive rainfall events in relation to the absence of protective cover during the first stages of crop growth. For instance, in Mediterranean areas, erosive rainfall events are concentrated during the fall, which corresponds to the period when the seedbed usually is prepared for sowing winter grains. Another case is that of the corn belt in the United States, where the erosive summer rainfall events also coincide with bare seedbeds or fields characterized by the first stages of maize or soybean growth.
Suitable crops for use in rotation are legumes and grasses. These provide good ground cover, help to maintain or even improve the organic status of the soil, thereby contributing to soil fertility, and enable the development of a more stable aggregate structure. In temperate climates, this process is slow but the effects last for several years of cropping. Hudson  found that, under tropical conditions, this process is fast; however, the effects are often sufficiently long-lasting to reduce erosion and increase yield during the first year of row-crop cultivation, and they rarely extend into the second year. Therefore, two continuous years of planting with a row crop should be avoided.
Cover crops are grown as a conservation measure as annual crops or as perennial stands of natural species or sown perennial grasses, legumes, and/or mixtures. They are grown either during the off-season period or as ground protection under trees. Leguminous crops, grown as winter catch crops, often are plowed into the soil as green manure. In other situations the crop is harvested as forage or used for seed production.
Different climates, soils, and agricultural situations demand different species for annual cover crops. Typical winter cover crops, where winter vegetation is possible, include rye, oats, Italian ryegrass, hairy vetch, crimson clover, sweet clover, lupins, and winter peas. In milder climates, common vetch, field beans, Egyptian clover, and alsike clover also are commonly used. In warmer climates, Crotalaria, Lespedeza, and Dolichos can be utilized conveniently. Finally, in tropical climates, Puerariaphaseoloides, Calopogo-nium mucunoides, and Centrosema pubescens are better adapted.
Perennial cover crops under cultivated trees may be a very good measure to consistently reduce erosion. In tropical areas, under oil palms, a reduction of erosion from 20 t ■ ha-1 ■ year-1 on bare soil to 0.051 ■ ha-1 ■ year-1 with a cover crop has been reported . In the Apennine area of Italy, soil loss from a vineyard was reduced from 951 ■ ha-1 ■ year-1 to 0.051 ■ ha-1 ■ year-1 by a perennial cover crop .
One major disadvantage of cover crops, especially perennial grass sod in orchards, is that, although they can increase the carrying capacity of the soil for machinery traffic, they may compete for available moisture and nutrients with the main crop.
In some situations, strip-crop cultivation, which follows the contours (Fig. 4.35), may increase storage and reduce competition for available soil water and nutrients and/or reduce soil erosion because the grass-sod strips act as traps for runoff and sediments [11, 39]. Sediment deposition behind the grass-sod strip produces a gradual buildup of soil over time and leads to the formation of bench-type terraces, reducing the overall gradient of the slope.
Strip cropping is especially suited for well-drained soils. In clay soils, the reduction in runoff velocity, combined with a low infiltration rate, may produce ponding and waterlogging.
Grass-sod contour strips are not required on slopes below 5%. On slopes of about 9%, the grass strip retards runoff by increasing the infiltration rate. The control of erosion by deposition of sediment on the grass strips may be effective on slopes between 9% and 15%. Grass stripping may be insufficient as a single measure of erosion control on
23 Permanent grass - ~ - Strip boundary
23 Permanent grass - ~ - Strip boundary
Figure 4.35. Contour strip-cropping design for (a) a 5-year crop rotation and (b) the use of buffer strips. The contour lines are in meters above an arbitrary datum. Note the use ofa grass waterway to evacuate excess runoff from the strip-cropped area and the inclusion of rocky areas that cannot be farmed within the buffer strips. Source: .
slopes above about 15% . However, these limits for slopes largely vary with soil characteristics: texture, structure, and stability of aggregates.
Plants utilized for buffer-strip formation must be well adapted to the specific location in which they are used. Normally, they are perennial grasses and/or legumes that have deep-rooted systems. The species must be perennial, quick to establish, and able to survive flooding and/or drought periods .
Mulching is another agrotechnical practice that has been widely subjected to experimentation in relation to soil protection against erosion, particularly in the United States. It consists of covering the soil surface with green and/or dry plant material such as grass or crop residues (straw, maize stalk, standing or chopped stubble, residual leaves and stems) after harvesting by combine machinery.
The mulch cover protects the soil against raindrop impact and reduces the velocity of runoff. From the soil conservation point of view, mulch simulates the effects of plant cover and may be used as a substitute for a cover crop, especially in dry areas.
When the slope gradient increases, it may be a problem to keep the mulch spread uniformly on the soil surface, given its tendency to be transported by wind and overland flow.
There is consistent experimental evidence supporting the very important role of mulch in decreasing soil erosion. In fact, the rate of soil loss decreases exponentially with the percentage of soil surface covered by mulch [41-44].
This can be expressed as a mulch factor (MF), defined as the ratio of soil loss from a soil covered with mulch to that from a soil without mulch . The MF is similar to the residue cover (RC) subfactor used in the estimation of the C factor in the USLE (see also Section 4.3 of this chapter).
Optimal protection requires that an average of 70%-75% of the soil surface is covered by mulch. A reduction of mulch cover below this threshold may decrease the protective value of the cover against erosion. On the other hand, considerable increases in the mulch cover above this value may reduce plant growth.
The control of erosion by mulching gives some problems in relation to farming operations. Tillage implements may become clogged with mulch residue. Weed and pest control, which require special care, become more difficult, and sowing and/or planting under a mulch residue is sometimes unsuccessful and can be done only using specifically designed machinery for sod-seeding under mulch. However, where problems of mulching in relation to crop performance can be overcome, it remains a very useful technique in reducing soil erosion.
Small rock fragments on the soil surface may offer very good protection against erosion. Special attention should be paid to the fact that, although organic mulching decomposes reasonably quickly on the soil surface or when embedded in the soil, rock fragments become a permanent soil feature. When the amount of rock material is above a certain threshold (about 20%), the volume of soil available to roots and the nutrients available to plants are reduced.
Although mulching with small rock fragments is of limited value in arable land, making tillage and seedbed preparation more difficult, it is a traditional soil cover in long-duration tree crops in semiarid areas. The rock cover on the soil surface does not interfere significantly with root development in the soil beneath the rock layer. Instead, it has several favorable effects, not only protecting the soil against erosion, but also enhancing water collection from dew and water concentration in the tree root zone, besides keeping weeds under control.
Considering briefly other kinds of land exploitation (for instance pastures and/or forests), the management system of the vegetation cover is also very important in relation to erosion risks.
Rotational grazing (e.g., moving livestock from one plot to another in turn) is, generally speaking, the best system because it allows the grass to recover and thereby maintains a continuous protective cover against erosion.
Grazing must be carefully managed, by adjusting the stocking density to the available biomass of the pasture. In fact, whereas overgrazing leads to a deterioration of the vegetation cover and topsoil structure and to an increase in erosion risk, undergrazing results in the loss of nutritious grasses and legumes in favor of thorny weeds and shrubs of low palatability.
Pasture burning is one of the most commonly employed methods for the removal of undesirable species and for favoring regrowth of palatable species. Burning modifies the density, stature, and composition of bush species within a plant community but kills only a few species outright. Most brush species sprout vigorously again after fire to the detriment of grasses. Grasses, however, fill the gaps between the brushes and reach their maximum development within a year after burning. In any case, the increase of bare ground due to burning enhances the erosion process in the period before the vegetation cover is reestablished. Moreover, it has been demonstrated that, in the long run, continuous cycles of uncontrolled burning and natural vegetation recovery cause progressive deterioration of the composition of the vegetation and topsoil.
Controlled burning on a rotational basis may be more profitably used to remove undesirable species, especially in temperate areas. In the Mediterranean area, on the other hand, uncontrolled pasture burning is always dangerous, not only in relation to pasture deterioration and soil erosion, but also in relation to its potential to burn forests and settlements.
The use of mechanical brush clearing for pasture renewal is much safer than burning in the semiarid and Mediterranean areas. Such a technique has, in fact, given interesting results in the renewal of pastures in Sardinia (Italy). Leaving the chopped plants on the surface of the pasture as a mulch after mechanical brush clearing is also a very effective system of soil protection against erosion.
In forest management for the exploitation of timber resources, the principle of rotation also can be conveniently applied. The commercial exploitation by patch cutting on a rotational basis is safer than the complete logging, or clear-cutting, of a forest area. The elimination of forest undergrowth is especially dangerous in relation to the acceleration of soil erosion.
Erosion rates are generally higher in the period immediately after logging but decrease consistently in subsequent years with the regrowth of natural vegetation.
Generally, reforestation is a good protection measure against erosion but it takes an average of 7-12 years or more to become effective. However, reforestation plays a detrimen tal role in soil protection related to the extent of soil disturbance that sometimes occurs during the planting of new trees. Planting on undisturbed land is the most protection-effective practice because of the quick regrowth of natural grass and bush vegetation. However, the planted tree growth may be reduced in such conditions in comparison to the growth of trees planted on tilled soil, because of the competition with natural grasses and bushes.
Forest fires represent an additional hazard for increased soil erosion, not only in terms of the elimination of the vegetation cover, but also in terms of the increase in the hydrophobicity of the soil that occurs (depending on burning temperature). The latter effect produces higher runoff generation and increases soil erodibility .
Agroforestry is a system of land management based on the alternation of arable crops with reforested areas and/or with pasture and rangeland. Agroforestry may assume different geometries but generally has a consistent effect on protection of soil from erosion. Rows, bands, or plots of trees producing wood for fire or quality wood for construction alternate with arable fields for food production and/or with pasture or rangeland.
The system is especially important in tropical countries but it is also part of set-aside policies in temperate areas for the production of fruits and valuable timber.
Alternation of reforested areas and pastureland also is suggested for optimal animal husbandry on rangeland in semiarid areas, which allows rotational grazing of the pas-tureland during the winter season and the grazing of the woodland during the summer. The implementation of forage shrubs (e.g., Atriplex halymus) in inland Sicily (Italy) was a measure that resulted on both a higher production of forage available for the grazing of sheep and a very efficient measure for soil protection against erosion.
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