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VII. Runoff management and erosion control

A. Preventive measures
B. control measures: engineering structures

High surface runoff and accelerated soil erosion are major degradative problems on newly cleared land. Immediately implementing appropriate erosion management techniques is therefore, essential. Accelerated erosion can be most severe in the very first season after land clearing (Lal, 1981a). Soil erosion is caused by raindrop impact surface sealing, and crust formation leading to high runoff rate and amount, high runoff velocity on long and undulating slopes, and low soil strength of structurally weak soils with high moisture content due to frequent rains. Effective erosion management lies in:

(a) preventing or minimizing the raindrop impact, through mulching and canopy cover;

(b) maintaining favorable soil structure for reducing crusting; (c) managing soil surface to enhance infiltration rate;

(d) reducing slope length to minimize runoff build-up; and

(e) disposing excess runoff safely through protected waterways and graded channels.

Based on these principles, erosion control measures are grouped into two broad categories: erosion preventive techniques and erosion control measures (Fig. 12).

Fig. 12 Technological options for erosion management on newly cleared land

A. Preventive measures

1. Mulch farming
2. Conservation tillage
3. Strip cropping
4. Contour farming
5. Cover crops
6. Vegetative hedges or strips

Preventive measures are based on the principle of soil management that minimizes the raindrop impacts enhances and maintains favorable soil structure, minimizes surface crust, favors a high infiltration rate, and reduces runoff rate and amount. The most commonly recommended soil management techniques for erosion prevention are cover crops (Plate 16), residue mulch (Plate 17), and conservation tillage (Plate 18). Relevant crop management techniques for erosion control involve those that provide continuous ground cover (e.g., mixed cropping (Plate 19), strip cropping (Plate 20)), and improved systems of crop management (e.g., new varieties, good-quality seed, early planting, balanced fertilizer use, pest control measures, and other agronomic techniques of good farming that ensure optimum crop stand, vigorous growth, and high yields).

1. Mulch farming

Mulch is a layer of crop residue placed on the soil surface. There are different types of mulches, depending on the source and method of mulch procurement and application (Fig. 13). The technological methods of mulch farming differ on the basis of whether mulch is brought in or produced in situ. Although a wide range of materials is used as mulch, the most practical and feasible material is the residue from a previously grown crop. The beneficial effects of mulching are attributed to:

(i) physical effects that minimize raindrop impact, improve rainfall acceptance through enhancement of soil structure and reduce runoff and erosion;

(ii) biological effects that increase the activity and species diversity of soil flora and fauna, notably earthworms increased biomass carbon, and improved crop growth; and

(iii) chemical effects that alter nutrient status and influence crop growth.

Fig. 13 Different types of mulch materials and associated agricultural practices

A principal benefit of mulch farming is reduction in runoff and soil erosion. The data in Table 13 from slopes of 1% to 15%, show that a mulch rate of 4 Mg/ha effectively reduced runoff even on a 15%, slope. The data in Table 14 show the negative value of the exponent of mulch rate (M), indicating that runoff and soil erosion decrease exponentially with an increase in mulch rate even without a crop cover. In addition to increasing the infiltration rate, mulch also decreases soil evaporation and conserves water in the root zone. The major limitations lie in the large quantities of residues required (usually 4 Mg/ha/yr) for regular and frequent applications economic uses of mulch (as fodder, fuels or construction material) and the additional cost of labor involved in mulch procurement and application. Consequently, mulch farming is likely to be feasible on a small scale for high-value commercial crops.

Table 13 Effect of mulch rate on runoff on different slopes

* Rainfall first season 510 mm second season 249 mm.
(Lal, 1976)

A tillage system that ensures a maximum retention of crop residue on the soil surface is called mulch tillage or stubble mulch farming. It is defined as a method of seedbed preparation that involves leaving crop residues or other mulching materials on or near the surface. When a grain crop is seeded through the mulch of a chemically killed cover crop, it is called sod seeding. If the cover is only suppressed temporarily or not killed, the system is called live mulch. A cover crop, usually a legume, is grown specifically to break the cropping cycle, to produce mulch material, to improve soil structures and to enhance soil organic matter content.

Table 14 Mulch rate effects on runoff and soil loss for an Alfisol on different slope gradients

M = Mulch rate in Mg/ha
R -- Mean annual runoff (mm)
(Lal. 1976)

2. Conservation tillage

Soil structure is extremely prone to intense tropical rains and harsh climate. Mechanical soil disturbance and exposure to climatic elements are major factors that degrade soil structure and accelerate soil erosion. Plowing and soil turnover lead to soil exposure and increase susceptibility to erosion. An effective erosion-control strategy, therefore lies in minimizing soil disturbance and keeping the soil surface covered A system of seedbed preparation based on the concept of minimum soil disturbance and maintenance of crop residue mulch is called "conservation tillage". Mulch cover is an important component of conservation tillage. Weed control, a major function of plowing and inversion tillage, is achieved through one of several options: (i) applications of herbicides and growth regulators; (ii) inter-cultivation based on secondary tillage; (iii) manual hoeing/slashing; (iv) smothering by cover crops and planted tallow, and (v) suppression by mulches.

A broad range of conservation tillage systems is used to reduce the risks of soil and water losses (Lal, 1989c, 1990c). Some commonly used variants are shown in Fig. 14 and are briefly described below.

(i) No-Till

This is a tillage system whereby pre-planting seedbed preparation is zero and most crop residue is left on the soil surface (Plate 21). The combination of no soil disturbance and the presence of crop residue mulch minimizes erosion risks. Several experiments conducted throughout the humid tropics have demonstrated the erosion-control effectiveness of no-till farming. The data in Tables 15 and 16 show that no-till with crop residue mulch was effective in runoff and erosion control even on steep slopes of up to 15%. In comparison with a cowpea-maize plow-till system the no-till system reduced runoff by 59% at 1% slope, 87% at 5% slope 63% at 10% slope, and 79% at 15% slope (Table 15). The mean (average of all slopes) reduction was extremely effective in soil erosion control. Soil erosion was negligible even on a steep slope of 15% (Table 16). However, to be versatile and effective a no-till system should be adapted to locale-specific conditions. There are several variants of no-till systems that can be adapted to address soil-specific constraints. These variants include the following:

Fig. 14 Common types of conservation tillage systems

Table 15 Effects of no-till system on runoff from different slopes in western Nigeria in 1973

Total annual rainfall = 1397.4 mm
(Lal, 1976)

Table 16 Effects of no-till system on soil erosion from different slopes in western Nigeria

Lal (1976)

Sod Seeding: Planting is done directly in chemically killed or mechanically suppressed sod, weeds, cover crops or in previous crop residue. Paraquat or any other contact herbicide is often used to suppress weeds or cover crop. Residual herbicides (e.g., atrazine lasso) are also used for pre-emergence weed control. Crops can be planted either by hand planter or by opening a narrow trench (5 to 7 cm wide and 5 cm deep) for the seed (Plate 22).

Live Mulch: This system involves growing grain or food crops through a cover crop specifically grown to produce a protective ground cover. A live mulch system is based on the principles of mixed cropping.

A fast-growing perennial legume is established with the objectives of smothering perennial weeds and growing a seasonal grain crop through it without severely suppressing the growth and yield of the food crop. A small strip is opened with or without herbicides to seed a seasonal food crop through an established live cover crop. The system works if the live mulch is a low-growing non-climber and is not competitive for lights moistures or nutrients. Drastic yield suppression of food crops can occurs however, due to allopathic effects, smothering, and competition for moisture during periods of drought stress. The live mulch is usually a low-growing, shallow rooted legume (Plate 23). However, there can be severe competition between live mulch and the food crop for soil moisture and nutrients. Some live mulches are climbers and can smother the food crop (Plate 24) and drastically reduce the yield.

Ley Farming: This system of no-till farming involves light grazing of the cover crop before sowing a food crop with a no-till system. The system can be useful if the cover crop is managed properly and grazing is strictly controlled.

(ii) Zonal-Till

This system refers to mechanical soil disturbance in the row zone only. The inter-row zone is kept undisturbed and protected with crop residue mulch. Zonal-tillage involves opening a small slot, or rotating a small strip to facilitate seed-soil contact and minimize competition. Common variants of zonal-tillage include the following:

Strip-Tillage: Tillage is eliminated in most of the inter-row areas and a narrow strip is opened to facilitate planting and the placement of fertilizer. The crop residue is left undisturbed in the inter-row zone (Plate 25).

Chisel-Tillage: Primary tillage is replaced by sub-soiling with a chisel plow in the row zone. Sub-soiling is done to a depth of 30 or 50 cm. The primary objective is to loosen the compacted sub-soil.

Para-Plow: A slanted plow is used to loosen the sub-soil without soil inversion. This system requires a large tractor to operate and is expensive (Plate 26).

(iii) Minimum-Till

The term "minimum-till" is commonly defined as "the minimum soil manipulation necessary for crop production or meeting tillage requirements under the existing soil and climatic conditions". It often means any system that has fewer tillage operations than the conventional plow based system. However, "conventional" tillage is soil- and ecoregion specific. The conventional tillage system in the humid tropics is based on a manual source of power or animal traction and consists of a ridge furrow system or hillocks (Plate 27). In some cases, traditional tillage simply means dropping seeds in a hole made with a digging stick and sowing immediately after the first rain. Some variants of minimum-till include the following:

Stale Bed: In this system, soil inversion by moldboard plowing is done at the end of the previous crop cycle or growing season. The next crop is seeded with a minimum of seedbed preparation, such as disk harrow performed at the onset of the next rains.

Ridge Till: The practice of planting or seeding crops on ridges is a widely adopted system in tropical climates. The crop may be planted on the ridge top, along both ridge sides, or in the furrow. Ridge tillage facilitates mixed cropping systems such as a rice-based cropping system, whereby upland crops can be grown on the ridge top and rice in the furrow.

Ridges may be made every season. Alternatively, in a semi-permanent ridge-furrow system only the necessary repair is done at the onset of a new cropping cycle. The ridges may be on the contour with graded furrows draining into a grassed waterway or the ridges may have short cross-ties to create a series of basins to store water. The latter system with cross-ties is called the tied-ridge system (Plate 28).

The wide range of tillage systems described in this section indicates the soil-specificity for appropriate tillage methods. In fact, it is difficult to adopt a tillage system for a wide range of ecoregions, soils. crops and cropping systems. The outline shown in Table 17 is an attempt to describe the suitability of different conservation tillage systems for soils and ecoregions of the humid tropics.

Table 17 Suitability of different conservation tillage systems for soils of the humid tropics

3. Strip cropping

Contour strip cropping divides a steep land into contour strips that cut across the path of overland flow and retard its velocity. Low-growing soil conserving crops (e.g.. cowpea, soybeans, Stylosanthes, Pueraria) are grown in alternate strips with open-row soil-degrading crops (e.g.. maize, rice). A soil-conserving crop is grown in a contour strip down slope of the erosion promoting crop to absorb runoff, retard runoff velocity, and encourage sedimentation of entrained sediment (Plate 29). Various categories of strip cropping include the following:

Contour Strip Cropping: Alternate strips are established on the contour. These contour strips facilitate the performance of all farm operations on the contour.

Buffer Strip Cropping: Buffer strips are installed on a rolling topography with complex slopes where contour strips are difficult to establish. This is done by expanding the filler areas into a continuous buffer strip. Buffer strips are generally planted to cover crops and trees.

Field Strip Cropping: This practice involves establishing rectangular strips parallel to one side of a field. This type of strip cropping can be done only on gentle slopes with soils of low erodibility.

Barrier Strips: These strips involve a single or double row of closely growing grass or cereals established on the contour to provide protection against runoff. Vetiver hedges come under the category of barrier strips (Plate 30).

Border Strips: Property boundaries are often established with hedges of perennial vegetation. These strips also minimize the risks of erosion.

In addition to controlling erosion, cultivating land in alternate strips in alternate years may regenerate soil fertility, improve soil structure, and restore productivity. The biomass produced in these fallow/ buffer strips can be used as mulch, fodder, and for comporting. Buffer strips are usually planted with quick-growing and easy-to-establish forage legumes. Some common forage legumes suitable for the soil and environment of the humid tropics are listed in Table 18.

Strip cropping is generally effective on gentle slopes (< 7%) on rolling terrain. For steep slopes, strip cropping should be reinforced with engineering structures.

4. Contour farming

The simplest approach to erosion control is contour farming (e.g., performing all farm operations on the contour rather than up and down the slope or parallel to field boundaries). Important farm operations to control soil erosion include plowing, ridging, sowing, fertilizer application. and inter-row cultivation.

The effectiveness of contour farming decreases with increase in slope gradient and slope length, and increasing intensity of rains. If the rainfall exceeds the surface detention capacity of the contouring system, concentrated runoff flowing downstream unchecked can lead to accelerated erosion and even severe gulling. Therefore, contour farming alone is not sufficient to control erosion on steep, long slopes, erodible soils, and during erosive rains. The major drawbacks of contour farming are frequent turning involving extra labor and machinery time, and loss of some area that may have to be put out of production.

(Lal. 1984)

5. Cover crops

Growing grass or leguminous cover crops once every two to three years may be necessary for the sustainable management of soil and water resources. Cover crops have many advantages for the sustained use of natural resources (e.g., restore fertility, control weeds, avoid repeated seeding and cultivation traffic, conserve rain, and reduce energy costs). In addition to controlling pests, cover crops improve soil physical properties and soil filth and reduce soil erosion.

Cover crops have long been used in the tropics for soil and water conservation, especially on steep land plantation crops (Plate 31). In addition to augmenting soil fertility, cover crops also improve soil structure and increase macroporosity. However, the principal benefit of a cover crop lies in erosion control. The data in Table 19 show that runoff and soil erosion under a Mucuna cover crop were low and the cover was extremely effective in decreasing erosion losses, even on eroded and compacted soils.

A wide range of cover crops can be used for soil and water conservation in the humid tropics (Table 20). There is considerable scope for selecting appropriate species and cultivars for suitable cover crops. The choice of an appropriate cover crop for different soils and ecological regions depends on many considerations, including:

- the ease and economics of establishment, including the availability of seed;

- quick ground cover and growth rate during the off-season;

- N-fixing rather than N-consuming;

- deep-root system and low consumptive water use;

- feed value for livestock;

- low possibility as alternative hosts for pests and cover for wildlife;

- low canopy height;

- ability to suppress weeds;

- growth duration (i.e., permanent versus annual);

- shade tolerance; and

- ease of management for growing a food crop with conservation tillage.

Table 19 Effects of Mucuna fallowing on runoff and erosion as compared with alley cropping and maize-cowpea rotation in 1982

The data on runoff and soil erosion arc averages of the sub-watersheds listed for each treatment.

All sub-watersheds sown with maize-cowpea were managed with a no-till system of seedbed preparation.

(Lal 1992)

Table 20 Some cover crops used for soil and water conservation in the humid tropics

(Modified from Lal, 1984)

Managing cover crops for food crop production is an important consideration in choosing an appropriate cover crop. A difficult-to-suppress cover crop can be expensive and energy-intensive. Consequently, the concept of live mulch or a green seedbed has been proposed and was discussed in the previous section.

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