Water retention in soil can be understood as the water retained by the soil after it runs through the soil pores to join water bodies such as groundwater or surface streams. Pores in the soil can be defined as the air-spaces that exist in between soil particles.
Water retention is mainly dependant on the particle size of the soil. The finer the soil particles, the higher the chance that water molecules shall hold on to soil particles, such as in clay, as opposed to sandy soil, that has large and coarse particles that are not cohesive.
The water retention by soil is critical for plants and acts as the chief source of moisture for it in almost all habitats. Other than percolation through the soil, soil moisture can also deplete due to evaporation directly from the soil and by transpiration by plants.
As based on the size of the soil particles, there are four classificatory systems for the identification of soils (J. Mariamma, 2010) –
US Bureau of Soil Classification
Indian Standard Classification
The Indian Standard classificatory system was formulated originally for the classification of soils primarily for engineering purposes. This is because the draft for soil classification was prepared by the Soil Engineering Sectional Committee and was approved by the Civil Engineering Division Council.
The final draft was adopted by the Bureau of Indian Standards on December 19th, 1970. This system divides soils into three broad categories based on the properties of soil particles (Bureau of Indian Standards, 2004) –
i) Clay – the particles are microscopic to sub-microscopic and exhibit plasticity, allowing it to retain the most water.
ii) Silt – the particles are fine grains, but exhibit less plasticity, making this form retain lesser water.
iii) Sand and Gravel – aggregates of comparably larger particles that are coarse and loosely bound thus lacking cohesion. The least water retention is possible in this form of soil.
Different topographic and climactic patterns result in varied behaviour of soils and thus require a variety of approaches to analyze and implement soil management techniques for water retention. Soil can sometimes pose problems for not being as desired, and these problems can broadly be grouped under chemical and physical problems (National Agricultural University, NAU, 2013).
Chemical problems include high salinity or acidity in soils, along with the presence of other toxic chemicals such as phosphorous in soil (NAU, 2013). This problem becomes especially pertinent in agriculture where crop yield or productivity could dwindle due to chemicals used in the agricultural process such as pesticides and herbicides.
Among a large gamut of solutions and applications, the most common one is the use of ecologically beneficial green manure. Agricultural soil should also be frequently and properly drained to achieve effects such as the leaching of saline moisture in soils.
The physical problems can involve soil that is not able to contain much water due to lack of cohesion or due to a rigidity that can occur owing to encrustation, or a very clayey surface. Shallow depth of soil, soil that is too clayey, or the presence of hard opaque surfaces underneath can also present problems to water retention and there can be water-logging when too much water is added to soil.
These require artificial solutions to soil management that frequently involves the mixing of soil with other different forms of soil. Incorporating organic matter and regulating drainage are also frequently applied solutions.
There are various methods to enhancing the water retention capacity of soil. Some methods are more traditional, and also conventional, while some involve the utilization of technology. While most of technological investment regarding water retention in soils involves technologies for enumeration and generation of data, technological solutions can vary from simple, affordable, everyday solutions to solutions utilizing high-end technology.
Some of the simple solutions include application of organic solutions such as drought resistant crop varieties and organisms that increase the fertility of soil, management and design of irrigation according to soil properties, application of biochar – produced from biomass for low-cost carbon sequestration in soil – making soil less porous, use of the roots of plants that grip soil, and application of natural by-products such as poultry litter that provide greater cohesiveness to soil.
The solutions can also range towards using complex technologies such as mapping the global water cycle in relation to water retention in soil, and preparation of dietary fibres that have high water-holding capacity from food sources used in soil. There is however, a leaning in technological progress in engineering water retention in soil to introduce organic elements in the soil instead of inorganic matter.