Soil Management

Reconsolidation

A naturally consolidated soil state is that found in an undisturbed topsoil of forest, savannah, prairie, or steppe. In such soils, an environmentally dependent natural plant community is established and a natural balance between the soil-forming processes and soil erosion is achieved. At the same time, a semistationary equilibrium between organic-matter production by plants, and its turnover and humification in the soil by biological activity, has produced a certain stable soil structure. All other factors being equal, natural vegetation cover and the structural conditions of the topsoil maintain soil erosion at a certain stable level, which allows soil development and plant dynamics that are characteristic for the site in question.

Reconsolidation, on the other hand, is the reduction of soil disturbance caused by cultivation practices. Generally speaking, the reconsolidation process is characterized by compaction and rearrangement of the soil particles, after a period during which the soil is loosened by plowing or other tillage practices, into aggregates. Reconsolidation processes are characterized by the type of vegetation involved and the length of time from initiation to new plowing (sod turning).

A grass sod protects the soil and reduces the effects of raindrop impact and scouring by overland flow. Furthermore, the improvement of soil structure and infiltration will decrease runoff generation with the overall effect of a very consistent abatement of soil erosion.

In natural reconsolidation, the vegetation cover depends on the types of weed seeds that the soil contains. This cover affords variable protection for soil erosion, related to the prevailing environmental conditions and to the proceeding cultivation history. In general, soil protection will be scarce in the first year of reconsolidation, especially on marginally arable land. In fact, natural vegetation recovery on abandoned fields often is represented by weeds of modest growth that offer sparse cover to the soil, especially

Figure 4.34. Runoff and soil loss responses as related to lucerne ley in the first, second, and third years. Source: [33].

in extreme climatic conditions. Where climatic and soil characteristics allow the quick development of a good natural vegetation cover, soil protection against erosion will be much better and will reach a stationary equilibrium after a few years.

The reconsolidation process may be aided greatly by sowing a mixture of good grasses that are well adapted to the local environment. The use of an artificial mixture of well-adapted perennial forage grasses and legumes in land reconsolidation is defined as "steered reconsolidation."

Reconsolidation may represent a permanent change in the state of the soil or simply a period of repose for the soil between successive phases of arable cultivation, as in agricultural rotations with perennial forage crops. Although perennial forage crops, such as lucerne and clover leys, can provide considerable soil protection in the years after the first one (Fig. 4.34) [33], they are somewhat less protective than close permanent grassland.

Reconsolidation by forage crops used in rotational agriculture is sometimes called "rotational reconsolidation." This term covers systems ranging from the shifting cultivation of the tropics to the fallow agriculture of the semiarid zones and to the dynamic rotations of arable crops and perennial forage species in temperate areas.

In agricultural systems based on trees, a "grass-sodding" type of steered reconsolidation is being adopted increasingly as a very promising solution to erosion control or for ameliorating soil carrying capacity for machinery traffic. Where competition problems for water and nutrient uptake arise between cultivated trees and grasses, grass-sod management that follows the contours has proved to be a valuable measure for erosion control. Grass-sod management in tree cultivation can be done in several ways:

• by grazing the phytomass using different species of animals,

• by cutting the phytomass for hay production or for covering the soil with mulch,

• by using herbicides for killing and/or drying the aboveground vegetation.

In grass-sod management, it is particularly important to avoid the formation of up- and downslope tractor tracks because such tracks may become preferential pathways for concentrated runoff and erosion.

Organic Matter

Organic matter represents the most important source of energy for soil biological activity. The different products of organic-matter decomposition are very important in promoting the formation of stable aggregates from soil primary particles, by flocculation and cementation phenomena. Moreover, increases in the colloidal organic fraction assist in the slow release and plant uptake of nutrients.

Organic-matter release and turnover from perennial herbaceous plants in the topsoil during reconsolidation provide a basic source of soil humus, thereby playing a very important role for the amelioration of soil structure.

Organic matter can be added to the soil in different forms as green manure, straw, stubble, industrial residues, sludges, or animal manure. This may have already undergone some form of fermentation, as in the case of fermented urban garbage and farmyard manure (FYM).

The addition of raw organic material that contributes to the soil in building up the humus content is expressed in the isohumic factor, as shown in Table 4.14 for some raw organic materials [34].

When green manures (often leguminous crops) are added to soil, they may be subject to a high degree of fermentation and rapid mineralization. Low amounts of humic substances (low isohumic factor) probably will be produced in these cases. The possible increase in soil structural stability will be of short duration and the main effect of the addition of such material is to increase the medium-term supply of nutrients in the soil.

Straw, on the other hand, decomposes more slowly because it is richer in cellulose and lignin. The rate of decomposition depends, to a large extent, on the availability of soil nitrogen sources for the nutrition of cellulolytic microorganisms. The C/N ratio dynamics of organic matter in the soil are mediated primarily by microorganisms. The higher the C/N ratio, the more stable the humic acids binding the soil particles in structural crumbs. Low C/N ratios may favor microorganisms producing humic acids that have little or no effect on soil structural stability. The isohumic factor of this material is high and the buildup of humic acids increases soil structural stability to a higher degree than that achieved with green manure.

Table 4.14. Examples of isohumic factors for several organic materials

Organic Material Isohumic Factor

Table 4.14. Examples of isohumic factors for several organic materials

Organic Material Isohumic Factor

Plant foliage

0.20

Green manure

0.25

Cereal straw

0.30

Roots of crops

0.35

FYM

0.50

Deciduous tree

litter

0.60

Coniferous tree

litter

0.65

Peat moss

The influence of the organic matter on soil structure is enhanced by the presence of base minerals in the soil. Such minerals, in fact, favor the flocculation of colloidal clay and humus to form compounds that comprise soil aggregates.

Tillage Practice5

Tillage is a fundamental soil management technique in agricultural systems to provide a suitable seedbed for plant germination and growth, helping to control weeds and assisting the infiltration of fertilizers and pesticides into the soil.

Tillage loosens compacted soil, regenerates porosity, and increases permeability, thereby generally improving soil physical conditions for plant growth, especially where intrinsic soil physical characteristics are particularly adverse. For instance, the formation of a temporary mechanical structure in a compacted clay soil is a prerequisite condition for the cultivation of annual crops. There are, in fact, a number of different techniques for tilling the soil. All of them, however, represent some degree of soil disturbance which, although necessary for agricultural exploitation of the land, often produce an increase in potential erodibility of the soil.

The most common system of preparing the soil for cultivation is by plowing the soil to loosen it and to turn over weeds, previous crop residuals, organic matter, and remaining pesticides and chemical fertilizers in the plow layer. Subsequent seedbed preparation consists of arrowing the soil with disk or dent cultivators, to eliminate any weed seedlings that may have sprouted in the meantime and to produce a mechanical structure suitable for sowing the crop.

The conventional system of tilling the soil, recognized in principle since the origins of agriculture, has been subjected to many improvements over time in terms of the implements used, the mode of execution, and the kind of power employed to operate the implements. The average plow-layer depth ranges from 100 mm to 1,000 mm. Deep plow layers are common in clay soils where plowing also has the function of improving infiltration.

Different implements may be used to plow the soil. Examples include the moldboard plow, the disk, the chisel, and the rotovator. Sometimes, the work is done using a combination of implements as, for instance, in double-layer plowing. This allows subsoiling and an inversion of the surface layer (to imbed fertilizers, weeds, and crop residues) to be achieved simultaneously.

Plowing generally produces a more or less rough cloddy surface, depending on soil characteristics and moisture content. A clod height of 400-600 mm may be produced in clay soils plowed to a depth of 400-500 mm in dry tilth. In loamy soils, on the other hand, a clod height of 120-160 mm is observed more commonly.

Secondary cultivation consistently reduces the roughness of the soil surface in the same way as raindrop impact and wind action. Seedbed formation through secondary cultivation, however, generates a finer mechanical aggregate distribution in sandy-loamy soils than in clay soils.

Rougher freshly plowed soils are less susceptible to erosion. Cogo [35], in fact, has demonstrated that soil loss by water erosion will decrease with increasing roughness.

In conclusion, it is generally accepted that, all other factors being equal, a traditional seedbed preparation is the most susceptible condition for soil erosion. For this reason, tilled bare fallow is taken as the standard condition with which other cover and management situations are compared [36].

In some circumstances, tillage practices such as plowing and seedbed preparation may be less susceptible to soil erosion when practiced on the contour; see also "Contouring" in Section 4.4.1 of this chapter.

Conservation Tillage

Many studies and experiments have been carried out recently to evaluate the effects of innovative tillage systems in reducing soil erosion while maintaining adequate soil conditions for optimal plant growth and crop yield.

Different kinds of conservation tillage practices have been devised, each one more or less soil specific but also dependent on how well weeds, pests, and diseases are controlled in the cultivation system under consideration (Table 4.15).

In all cases, the basic principle of conservation tillage is to reduce tillage operations, thereby avoiding soil disturbance as much as possible and maintaining the most efficient cover of vegetation and residues throughout the crop cycle in order to reduce soil erosion but provide appropriate crop germination.

Table 4.15. Tillage practices used for soil conservation

Practice

Description

Conventional

Standard practice of plowing with disc or moldboard plow, one or more disc harrowings, a spike-tooth harrowing, and surface planting.

No tillage

Soil undisturbed prior to planting, which takes place in a narrow, 2.5- to 7.5-cm-wide seedbed; crop residue covers of 50%—100% retained on surface; weed control by herbicides.

Strip tillage

Soil undisturbed prior to planting, which is done in narrow strips using rotary tiller or in-row chisel, plow-plant, wheeltrack planting, or listing; intervening areas of soil untilled; weed control by herbicides and cultivation.

Mulch tillage

Soil surface disturbed by tillage prior to planting using chisels, field cultivators, discs, or sweeps; at least 30% residue cover left on surface as a protective mulch; weed control by herbicides and cultivation.

Reduced or minimum tillage

Any other tillage practice that retains at least 30% residue cover.

Generally, the better-drained and well-structured coarse- and medium-textured soils, with average organic-matter content respond better to reduced tillage. On the other hand, reduced tillage operations are unsuccessful on poorly drained soils unless the crop is planted on a ridge.

All conservation tillage practices are characterized by the amount of crop residues left on the surface of the soil at the time of greatest erosion risk. In any case, it is difficult to isolate the role of tillage in controlling erosion from that of the residues left on the surface or incorporated into the topsoil as mulch. The tillage system also may be integrated with special cover crops, represented by leguminous or grass species, which mainly are used to provide a cover to the soil and/or to protect the seedling of the main crop against adverse climatic conditions during the first stages of growth.

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