Last week’s column focused on water in plants. This week features water in the soil. As always, this is simply a brief overview that covers the highlights. The water contained within plants comes from the soil and is extracted by young, actively growing roots cells. The ability of the soil to hold water and allow roots to extract it is a function of the following:
• Soil texture describes the percentages of sand, silt, and clay that make up the soil. Sand is unable to hold water while silt can hold a minor amount. Clay, because it is negatively charged attracts and hold water. The amount of water a soil can hold against gravity is a function of the amount of clay and the type of clay (there are many). As clay content increases so does the soils ability to hold water. While the soil can hold more water with more clay, less of that water is available so the plant will wilt at a higher soil moisture content. Sandier soils hold less water but a greater percentage of it is available. Clay soils dry out more slowly than lighter, less clay, soils.
• Organic matter, like clay, typically has a net negative charge and can hold many times its weight in water. So if a producer, or gardener for that matter, can increase the humus content of the soil they increase the water holding capacity of the soil. This is practical for producers while changing the soil texture is not. However, it takes careful management and times to significantly increase the humus content of soil on a large scale.
• Pore space matters also. The soil is made up of “stuff” and “not stuff.” The stuff is the mineral and organic components of the soil. The not stuff is pore, or void, space. The ideal soil for plants is approximately fifty percent of each and the half that is space should be about half air and half water. Pores also come in various sizes. Large pores allow water into the soil and for it to move downward. Small pores hold the water in the soil against gravity through capillary action and clay content. Pores also allow gas to be exchanged (plant roots need oxygen too) and plant roots to grow through. A mixture of large and small interconnected pores is best. Too many large or small pores upsets the ideal balanced.
• Finally, what is at/on the soil surface matters. Residue accumulating at the soil surface allows more water to infiltrate, serves as a vapor barrier to decrease soil water loss, and keeps the soil cooler which also decreases evaporation.
Next week will finish this series tying everything together.