Water Holding Capacity of Soils

Management of the physical properties of soil often translate to managing a plant’s access to water, either by increasing effective rooting area or increasing water holding capacity. Available water holding capacity (AWC) is the quantity of total plant available water a soil can provide to a growing crop. This is a soil health test offered at Ward Laboratories. Determination of AWC is a lengthy process requiring specialized laboratory equipment.

Soil Characteristics

There are soil characteristics like texture that influence AWC but cannot be controlled. However, there are other soil characteristics like organic matter and compaction that influence AWC and can be controlled. Managing for maximum AWC will buffer a crop from experiencing water stress in periods between precipitation and/or irrigation events. Management practices that increase AWC are those that increase soil organic matter, structure, and porosity. Tillage and traffic are management practices that negatively impact soil structure and porosity, and thus AWC. Compacted soils have reduced water storage capacity.

Soil Health Practices

Farmers who adopt practices like no-tilling, planting cover crops, and controlling traffic can rehabilitate their compacted soil, although increasing AWC may be a long (5-10 year) process. By avoiding soil disturbance and through the influence of microbial activity, soil particles like sand, silt, and clay bind with each other and form into aggregates, which also increase AWC. Many of the practices that alleviate compacted soil will also build organic matter. Building organic matter is a long-term goal. Organic matter increases with reduced soil disturbance, growing perennial crops, growing high residue crops, and growing cover crops.

Laboratory Analysis

As previously mentioned, determining AWC is a time-consuming and labor-intensive laboratory process. Ward Laboratories usually takes 7-10 days to finalize the analysis once we receive the samples. The lengthy turnaround time is due to the time needed for water to reach an equilibrium within each soil sample. The sample is first split, then saturated with water and allowed to come to equilibrium. One part of the sample is placed inside a pressure chamber and pressure is applied at 0.1 bar. This is equivalent to the “permanent wilting point”. The rest of the sample is put in another pressure chamber and pressure is applied at 15 bar. This is equivalent to the “field capacity”. Once each of these comes to equilibrium again, roughly 7-10 days, the sample are weighed, oven dried, and weighed again to determine the amount of water present in each sample. The amount of water present in the “field capacity” sample minus the amount of water present in the “permanent wilting point” sample equals the quantity that would be available to a growing crop.
Originally Printed in April 2021 WardLetter.

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