Soils Store and Filter Water

2015 International Year of Soils

Functional soils play a key role in the supply of clean water and resilience to floods and drought. Water infiltration through soil traps pollutants and prevents them from leaching into the groundwater. Moreover, the soil captures and stores water, making it available for absorption by crops, and thus minimizing surface evaporation and maximizing water use efficiency and productivity. Healthy soils with a high organic matter content have the capacity to store large amounts of water. This is beneficial not only during droughts when soil moisture is crucial to plant growth, but also during heavy rainfall because the soil reduces flooding and run-off by slowing the release of water into streams. Healthy soils are therefore crucial for maintaining food production and clean groundwater supply, while also contribution to resilience and disaster risk reduction.

The amount or percentage of water in the soil (by weight) is generally referred to as soil moisture content. The maximum amount of available water that a soil can retain (the available water capacity) will vary depending on the soil's texture, organic matter content, rooting depth and structure. Soil organic matter is particularly important in that it can retain about 20 times its weight of water. By implementing sustainable agricultural practices, farmers can influence the structure and organic matter content of the soil to improve its water infiltration and retention.

Water is the "lifeblood" of agricultural practice worldwide--improved soil moisture management is critical for sustainable food production and water supply. Reduction of a soil's capacity to accept, retain, release and transmit water reduces its productivity, whether of crops, pasture species, shrubs or trees. The great challenge for the coming decades will be the talk of increasing food production with less water, particularly in countries with limited water and land resources. In order to minimize the impact of drought on food security, soil needs to capture the rainwater that falls on it, store as much of that water as possible for future plant use, and allow plant roots to penetrate and proliferate.

Problems with or constraints on one or several of these conditions cause soil moisture to be a major limiting factor for crop growth. In fact, poor crop yields are more often related to an insufficiency of soil moisture rather than an insufficiency of rainfall. Poor and unsustainable land management techniques also decrease soil moisture content. Overcultivation, overgrazing and deforestation put great strain on soil and water resources by reducing fertile topsoil and vegetation cover, and lead to greater dependence on irrigated cropping. Meeting food scrutiny targets requires the implementation of sustainable agricultural policies that ensure improved soil quality and water retention. As most smallholder farmers in developing countries are reliant on rainfed agriculture, improved soil moisture optimization and management is crucial.

A number of sustainable agricultural and land management practices can help to improve soil moisture retention capacity, including:

• Residue covers, cover crops and mulching protect the soil surface, improve water infiltration rates, and reduce both erosion and evaporation, thus improving soil moisture compared to bare soils, even under low rainfall.
• Conservation tillage is a general term which has been defined as "whatever sequence of tillage operations that reduces the losses of soil and water, when compared to conventional tillage"
• Zero-tillage, which is the practice of leaving residue of the previous season's crops on farmland, can increase water infiltration while reducing evaporation as well as wind and water erosion.
• Conservation agriculture employs the three principles of minimal soil disturbance, permanent soil cover and crop rotations to improve soil conditions, reduce land degradation and boost yields.
• Use of deep-rooting, drought-resistant, or less water-demanding crops can help preserve soil moisture and improve food security
• Capture of runoff from adjacent lands can lengthen the duration of soil moisture availability