This scenario represents water that is wasted, from either precipitation or irrigation. This water runs off the soil surface, promoting erosion, rather than entering the soil to become available for the plant. Crusts prevent water from infiltrating the soil. Soils that contain enough silt and clay are susceptible to crusting. Raindrops striking bare soil can break the bonds holding aggregates together, allowing individual particles to mix and form a tightly knit, dense layer at the soil surface called a surface crust (Figure 2). The spaces between aggregates are important pathways for water and roots to move through the soil profile. Adequate organic matter is usually required for good soil structure in surface horizons. The arrangement of these aggregates in the soil, along with the gaps and fissures between them, are part of what constitute soil structure. Healthy soils tend to have soil particles that are bonded together into clumps or groups. Roots cannot access water held in the soil beyond their physical location. When root growth is impeded by high bulk density or high soil strength, the depth and length of roots are reduced. High values for bulk density and soil strength often result in poor root growth. Generally, an increase in bulk density results in a corresponding increase in soil strength. Soil strength is a measure of the force required by a root to move through a soil and can be measured with a penetrometer. As soil settles after tillage or is compacted by equipment traffic during a field operation, its bulk density increases. Recently tilled soils tend to have a lower bulk density compared to conditions before tillage. Textural influences on plant available water are generally inherent for a given soil and cannot be altered by management.īulk density is a measurement of how tight the soil particles are in the soil. Water stored in this textural class of soils can be extracted by plants.

They have both large pores for infiltration and drainage, and medium-sized pores to store water. Loamy or medium-textured soils, such as a silt loam, tend to have the greatest available water holding capacity. Water held in these small pores is difficult for plant roots to extract. Clay soils drain more slowly but also may have low available water holding capacity because water is held too tightly between the small pores. The large pores between sand particles can allow rapid drainage through the soil, reducing water storage and making the water holding capacity of sands quite low ( Figure 1). Clay particles are the smallest and also have the smaller pores between granules. Sand particles are the largest of these and tend to have larger spaces (pores) between particles. Soil texture refers to the proportions of sand-, silt-, and clay-sized particles in a soil. Soil's Effect on Water Availability Skip to Soil's Effect on Water Availability At the wilting point, the amount of water remaining in the soil is held so tightly by the soil that the roots cannot extract any more-causing wilting of the plant. Conversely, there is a lower limit of soil wetness, referred to as wilting point. Saturation also promotes loss of soil nitrogen via denitrification and limits the conversion of soil’s organic nitrogen to plant available forms. Water entering the soil in excess of field capacity is not plant available because it either drains away too quickly for plant uptake, or if it remains during periods of soil saturation, limits exchange of oxygen and carbon dioxide from roots. This upper limit is referred to as field capacity.

The upper limit to this range is the amount of water in the soil a few days after a heavy, soaking rain, when soil has had adequate time for drainage and aeration. We call this range the available water holding capacity of the soil. A range of soil wetness conditions provides optimal water availability for crop growth. Not all water that enters the soil is available to plants. Available Water Holding Capacity Skip to Available Water Holding Capacity

Does clay absorb water