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Practices that reduce soil moisture content

(Crop) residue burning

Burning of maize, rice, other crop residues and natural vegetation in the field is a common practice. Residues are usually burned to help control insects or diseases or to make next seasons’ fieldwork easier. Burning destroys the litter layer and thus diminishes the amount of organic matter returned to the soil. The heat generated by the fire facilitates  the loss of soil moisture.

The organisms that inhabit the surface soil and litter layer are also wiped out. For future  decomposition to take place, energy has to be put first in rebuilding the microbial  community before plant nutrients can be released.

Burning is often practised to improve the quality of grazing land. The philosophy behind  the practice is that destroying the dry and non-palatable parts will induce the sprouting of fresh grass.

Soil tillage and mechanical weeding

It was thought that on soils with low infiltration rates due to surface crusting, shallow soil  tillage and mechanical weeding with disc or tined implements will break up the crust and encourage rainfall infiltration. This practice has to be repeated after every rainstorm on crusting-susceptible soils and has many disadvantages as loss of organic matter or compaction. Therefore increasing the infiltration rate through tillage leads often to soil degradation.

However, the regular use of shallow tillage with disc or tined implements to break-up  surface crusts to increase surface porosity and enhance rainfall infiltration is not recommended. The increase in surface porosity is only temporary and on crusting susceptible soils tillage will need to be repeated after every rainstorm. Tillage leads to the disruption of pore spaces in the soil, and the use of discs, in particular, often causes compaction, which may impede root growth and rainwater percolation. Tillage also accelerates the loss of soil organic matter leading to a progressive deterioration of soil architecture and a reduction in the number and stability of pores that allow growth of roots and movement of rainwater. Regular tillage therefore is not recommended as a solution to restricted infiltration caused  by low porosity of the soil surface.

Excessive tillage leads to pulverisation of soil and small particles can easily be washed away by runoff during rainshowers. These, usually very fine clay or organic particles can easily block micropores at the soil surface, and by doing so form a very thin film-like layer, also referred to as surface sealing. This continuous impermeable layer on the surface prevends rainwater to infiltrate and facilitates run-off.

The less the soil is covered with vegetation, mulches, crop residues, etc. the more the soil  is exposed to the impact of raindrops. When a raindrop hits bare soil, the kinetic energy of the impact of the terminal velocity detaches individual soil particles from soil clods.  These particles can clog surface pores and form many thin, rather impermeable layers of  sediment at the surface, referred to as surface crusts. They can range from a few  millimetres to a centimetre or more and are usually made up of sandy or silty particles.  These surface crusts impede rainwater infiltration. The breaking down of soil aggregates into smaller particles depends on the stability of the aggregates, which largely depends on the organic matter content.

The use of machinery and implements and even the trampling of animals can destroy, or greatly reduce the sizes of soil pores. Compacted soil (Soil compaction module) does not provide adequate space for the storage or movement of soil air and water. Soil animals and root growth are also restricted. Most importantly, large, continuous soil pores are lost or are reduced in size, leading to poor water infiltration, slow drainage and reduced aeration for healthy root growth and nutrient uptake for maximum crop yield.

More important, soil compaction often is not visible at the soil surface and therefore often disregarded as a problem. In case soil compaction is not a serious problem, crops with taproots (sunflower, pigeon pea or radish) might be taken up in the rotation to break
the hard layer.

Drainage

Drainage of water beyond the crop’s rooting zone may reach the groundwater and help to maintain the level of water in wells, streams and rivers. However, the water ‘lost’ by  drainage could have been used for crop production. Deep drainage occurs when rainfall exceeds the amount of water that is needed to bring the rooting zone to field capacity. Sometimes, deep cracks in clay soils at the surface during the dry season can cause deep drainage. In soils that contain montmorillonitic clay, cracks often develop to depths of 30 to 60 cm or even deeper. If precipitation occurs while these cracks are open, some of the water will move quickly to the bottom of the cracks. However, unless the crack is open to the surface, water will not move into the crack regardless to how much precipitation is received. For example, if a soil that is cracked to 30 cm is tilled to a depth of 10 cm, then precipitation that occurs will be largely retained in the upper 10 cm unless the amount of rainfall is very large; and even then it will move downward only by gravitational and capillary forces. On the other hand, if the land is not prepared, some of the water will move quickly to the bottom of the cracks where it will be much less available for evaporation.

Sometimes, deep cracks in clay soils at the surface during the dry season can cause deep drainage. In soils that contain montmorillonitic clay, cracks often develop to depths of 30 to 60 cm or even deeper. If precipitation occurs while these cracks are open, some of the water will move quickly to the bottom of the cracks. However, unless the crack is open to the surface, water will not move into the crack regardless to how much precipitation is received. For example, if a soil that is cracked to 30 cm is tilled to a depth of 10 cm, then precipitation that occurs will be largely retained in the upper 10 cm unless the amount of rainfall is very large; and even then it will move downward only by gravitational and capillary forces. On the other hand, if the land is not prepared, some of the water will  move quickly to the bottom of the cracks where it will be much less available for evaporation.

In addition, drainage beyond the rooting zone can be favoured by biopores – continuous  pores of 0.5 mm and wider formed by earthworms, ants and termites that extend from the soil surface to the subsoil. The amount of water lost through deep drainage is higher
in coarse textured soils, compared to fine textured soils.

Deforestation, overgrazing and the cultivation of one single crop every year (monocropping) are all management practices that lead to a reduction of the moisture content in the soil. These practices result in reduced soil porosity which in turn reduces the infiltration of rainwater and the capacity of the soil to retain moisture.

(Source – http://www.fao.org/ag/ca/training_materials/cd27-english/sm/soil_moisture.pdf)

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