Water-holding capacity is defined as the amount of water the soil can hold. The type and composition of the soil is the controlling factor in the water-holding capacity. Farmers use the water-holding capacity of soil to determine the ideal crop for the field and if irrigation is feasible or advisable.
Measuring Water Holding Capacity
Field capacity is measured about two days after the field is "thoroughly wetted," according to the Natural Resources Conservation Service. The water in the soil is measured to a depth of 60 inches in irrigated Western lands and 40 inches in the Eastern United States. The water-holding capacity of the soil is reported as a percentage of the soil volume or the number of inches of water the soil holds in the test area.
Soil with fine particles holds more water than coarse soils. Rock fragments hold no water and contribute negatively to the water-holding capacity of the soil. Coarse sands hold between a quarter and three quarters of an inch per foot of soil depth, while clay soils can hold as much as one and a half inches of moisture for each foot of soil. The percentage of organic matter in the soil, decayed plant materials such as compost, also increases the water-holding capacity.
Adding Water-Holding Capacity
Adding 1 per cent of manure or compost to the soil mix increases water-holding capacity by about 1.5 per cent. Compost or manure are commonly added to the top few inches of the soil and largely improve the water-holding capacity of just that segment of the soil.
Knowing the water-holding capacity of the soil is a basic part of the decision to irrigate a field or not. Soils with poor water-holding capacity respond best to irrigation, because the water is supplied on schedule with few of the dry spells associated with natural rain. Plant shallow-rooted crops if only the topsoil has good water-holding capacity. Plant deeper-rooted crops in fields where the water-holding capacity runs deeper into the soil.