Principles and Methods of Soil and Water Conservation

Soil not only provides anchorage to plants but all nutrients and water are absorbed into the plants through the soil. Therefore, all cultural practices in crop production are directed towards improvement and maintenance of the physical condition of the soil so as to create favorable condition for the growth and development of the crop plants.

Water is indispensable for growth and development of plants, too much or too little water, decide between success or failure of  crop production enterprise. In this unit you would treat the different methods of soil and water conservation.

Table of content

1.            Principles of soil water conservation

2.            Methods of water conservation

3.            Agronomic measures to soil erosion

4.            Measures to control wind erosion

1)   Principles of soil water conservation

The principle of soil water conservation is basically that of reducing run-off, percolation and evaporation losses. Essentially, water management entails three basic practices:

1.            Conservation of natural precipitation (in sub humid and arid regions).

2.            Drainage of wet lands.

3.            Supplementation of rainfall with irrigation.

2)   Methods of water conservation

Water generally is a limiting factor for crop production where irrigation is not available. It can be limiting even in humid and sub-humid regions where there is a theoretical need to dispose of excess water. Dry periods with water deficit frequently occur in these regions and positive responses to moisture conservation techniques are commonly obtained.

Over 80% of the agricultural land of the world is not irrigated. In Africa, where a little over 0.3% of the land is under irrigation (FAO 1987), rain fed agriculture prevails. In rain fed system the constraint is not only the erratic rainfall distribution but the amount of rainfall that can be stored in the root zone.

i)       Mulches and green manures

Mulch may be defined as a protective covering over the soil surface that is intended to minimize evaporation losses. Green manures may serve as mulches. Leaves, straw and saw dust are also commonly used. Paper and plastics may also be used.

Importance of mulch

•           Mulch intercepts solar radiation, reflects the light and so keeps the soil temperature low

•           It reduces the effect of wind and air movement.

•           The presence of mulch on the surface of the soil improves infiltration because they reduce the impact of raindrops.       

•            Reduces run-off losses.

ii.    Run-off control

Run-off occurs when rainfall intensity exceeds the infiltration capacity of the soil which is a measure of the ability of the soil to absorb and transmit rain water. Run-off is limited on soils with high a infiltration capacity. This in turn depends on the water transmission characteristics and structural stability of the soil and its ability to maintain continuous pores.

The transmission pores may exist in the soil as a result of coarse texture, good aggregation, or from the burrowing activities of the soil fauna, particularly certain species of earthworms.

The rate and amount of run-off are also influenced by the intensity and amount of rainfall received, the previous soil moisture content, the degree of relief, slope steepness and aspect. These factors manifest themselves in a wide range of run-off management problems and conservation needs.

Ways of minimizing run-off

•      Careful and rational management of crops residues

•       Fallowing

•       Terraces and contouring

•       Strip cropping and ridging

iii.           Fallowing

This is one of the most effective water conservation techniques particularly in areas of limited rainfall. It is the practice of leaving the land unplanted in alternate years or cropping the land for three years and then fallowing for the next few years. In general, the moisture saved in this way is small but it may be critical in dry- land farming.

iv.           Terraces and contouring

These are methods of minimizing the loss of surface water which may occur when water gushes down slopes after intense rainfall or excessive irrigation.

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v.           Contour farming

This is the practice of cultivating and planting on strips of land that are of the same elevation. The system of gently sloping terraces separated from each other by banks helps to hold water on the soil surface and so encourage infiltration. In preventing run-off, contour farming and terraces also reduces soil erosion which will remove valuable nutrients from the topsoil.

vi.           Strip cropping

Spreading vegetation or crops established in a strip which is at right angles to the flow of water or the prevailing wing. This gives protection to adjacent strip or rows of crops or fallow land.

i.      Field strip cropping: crops are grown in strips across the general slope of the land but not following the contour. Spreading crops or grass may be alternated with more upright crops.

ii.     Contour strip cropping: crops are grown in relatively narrow strips which are planted on the contour and at right angles to the natural direction of the slope. Crops are usually planted in strips of grass.

iii.    Wind strip cropping: crop strip are planted which are at right angle to the prevailing winds irrespective of the contour of land. The crops serve as series of miniature windbreaks to minimize wind damage.

vii.   Ridging

Ridges help surface drainage during rains and prevent young plants from being washed away. Ridges are formed by pilling up top soil so the depth of soil for plants roots to grow is increased.

3)    Agronomic measures to control soil erosion

Soil management practices are based on the following broad principles:

•        Those practice which help maintain high soil infiltration rates at sufficiently moderate levels to reduce run-off to a safe amount.              

•         Those practices which help the safe disposal of run-off water from the field.

Cultural practices which help maintain a high soil infiltration rate are essentially based on agronomic measures which maintain a mulch or vegetation cover on the soil, such as non- tillage or minimum tillage, stubble mulching, or the use of cover crops.

i.             Tillage Practices

Beneficial Effect of Tillage

i.              Increase infiltration of rainfall.

ii.             Reduced surface run-off.

iii.            The No- tillage system has the advantage of moisture conservation in the soil profile and decreasing run-off and soil loss to a minimum.

iv.           In the non-tillage system the organic matter content of the surface horizon is also better maintained, as it is the water holding capacity of the soil.

Contour cultivation is to be generally recommended as one of the simplest and cheapest conservation measures.

Surface plants residues very effectively control erosion.

Tied-ridging is another effective practice in controlling erosion. This involves growing crops on ridges made approximately on the contour, adjacent ridges being joined at regular intervals, usually 1.5-3.5 m by barrier or ties slightly slopes with permeable soil of adequate depth in areas not subjected to high-intensity rainstorms, the series of basins so formed can hold the rainfall where it falls, allowing it to infiltrate into the soil and preventing run-off.

ii. Minimum tillage

Repeated ploughing   and harrowing may break down the structure of some soils to such an extent that infiltration is drastically reduced. In most cases, this leads to an increase in soil erosion and decline in crop yields.

iii.   Mulching

Beneficial effects of mulching

i.             Mulching with cut grass or other vegetable refuse prevents surface  sealing by avoiding direct raindrops impact on the soil, and by encouraging enhanced biological activity which leads to the development of  macro pores in the soil.

ii.            It is very effective in reducing run-off and erosion since it protect the ground from  the impact of rains, slows down the movement of water over the surface, and improves the permeability of the soil.

Stubble mulching has similar effects to ordinary mulching. In this system, all or part of the crops residues and weeds is left on the surface of the soil as protective cover.

This system involves shallow ploughing of the land after harvest, either leaving stubble and weeds on the surface or partially burying them.

The greater the quantity of stubble and weeds left on a unit of land, the greater the effectiveness of the practice in reducing run-off and soil erosion.

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iv.   Crop rotation

Growing wide- spaced row-tilled crops such as sorghum, maize, cotton, and sugarcane continuously for a few years makes the soil susceptible to erosion.

On the other hand, growing grasses or legumes protect the soil from erosion because these provide a complete ground cover while they are growing and also improve the soil structure and permeability.

To check or minimize the loss of soil it is therefore advisable to have close-spaced cereal crops such as wheat or finger millet and grasses or legumes, or grass-legumes mixtures in the rotation.

v.     Strip crops

The purpose of strip cropping is the same as that of crop rotation, i.e. to minimize the loss of soil. In strip cropping, strips of erosion permitting crops are separated by strips of close-growing protective crops, in such a way that there are excessive strips of wide-spaced, row- tilled crops such as sorghum or maize; a dense untilled crop, such as grasses or legumes and close-spaced crop receiving little or no cultivation after planting, such as finger millet. In succeeding seasons, these strips are rotated.

vi.  Cover crops

For effective erosion control, proper land use is imperative. The steepest slopes that are unsuitable for cultivation may be left under forest or permanent pastures; less steep land may be use for highly protective tree crops that provide a full canopy of foliage, such as cocoa, gentler slope may be planted with less protective tree crops such as coffee or citrus and grasses and arable crops may be planted on the gentlest slope.

These crops not only provide a protective cover, but also enhance rainfall infiltration by means of improving the organic matter content of the soil.

Vii.  Soil conditioning

In other to prevent the breakdown of soil aggregates due to raindrop impact, their aggregate stability must be improved.

Soil conditioning with bitumen emulsion, polyurethane, latex, asphalt etc. minimizes aggregate destruction and helping to preserve a high infiltration capacity.

Soil conditioners may be applied by incorporation or surface treatment.

viii.        Management

Crops and management practices which help produce an early ground cover are certainly more useful in controlling run-off and erosion than those which take longer for full canopy cover to develop. Practices such as mixed cropping also affect ground cover.

Soil erosion and run-off losses are relatively less from mixed crops than from sole crops. Plant population, time of planting and fertility level are important cultural practices which could be used to control erosion.

4) Measures to Control Wind Erosion

The measures aim principally at

•             Maintaining more moist in the soil

•             Increasing surface roughness

•             Reducing wind velocity.

To achieve these aims above, the following practices should be encouraged:

•             Minimum tillage

•             Ridging at the right angles to the direction of prevailing wind

•             Early planting

•             Cropping in alternate strips

•             Mulching and stubble mulching

•            Planting of windbreaks at intervals across the path of the wind to slow its velocity and to cause the deposition of soil particles already in movement.

Related: The History and Spread of Cultivated Crops