101 Questions answered
Why Sandy Soils Sometimes Win: Rain Infiltration, Root Access, and Soil Depth
Without knowing the exact conditions under which this has happened, it is likely due to how water moves into soil, and how that water is stored in the soil.
First – water availability to plants
As clay content increases, so does the total soil water holding capacity. However, clay soils hold a lot of water within the clay structures, which is not available to plants. This means it takes more rainfall to ‘recharge’ clay soils, while most of the rain stored in sandy soils is available to plants.
Second – making use of light rains
As long as a sandy soil has adequate surface roughness (stubble or a growing plant) to allow infiltration, rain will infiltrate more rapidly on sandy soils than clayey soils. A light rain event might only soak in a few centimetres in a clay, but more deeply in a sand.
However, as clays store more water, if subsoil moisture is good, clay soils will supply water for longer than sandy soils.
A key challenge in understanding why different soils perform better under different rainfall conditions, is characterising soils to depth. Many sandy soils have a clay subsoil. The sandy soil has the benefit of rapid infiltration and easy access to soil moisture for plant roots, while the clay subsoil provides greater soil moisture storage for the plant roots to tap into in a dry year.
The most important rule in predicting how different soil types will perform is knowing the rooting depth. It doesn’t matter if the soil type is sandy or clay if the plant roots are only shallow due to other constraining factors.
