How soil holds water
Soil is composed of tiny pieces of rocks. Water adheres to these rocks, and the water’s surface tension pulls other water behind it. This causes water to be drawn into the soil the same way that water is drawn into a dry rag when you wipe up a spill. In effect, the soil sucks or pulls on the water. The less water there is in the soil, the harder it pulls on the water. The more water there is in the soil, the easier it is to remove that water. In this way, soil can redistribute water from wet areas to drier areas. When all of the air space between soil particles is full of water (mud), then gravity can overcome the soil’s pull on the water and water will drain on down through the soil to lower soil layers. Soil water content is often expressed in either a percent of the total volume or in terms of inches of water per foot or its metric equivalent. The maximum amount of soil water that can be held long-term against the pull of gravity is called field capacity (FC). Soil with larger particles (sand) can’t hold as much water as soil with smaller particles (clay and loams).
Enter the plants
Plants have to overcome this pull of the soil on the water in order to move the water out of the soil into the roots, up through the stems, and up to the leaves. As the soil water content decreases, the plants have to work much harder to pull the water out away from the soil particles. After a while, this extra effort starts to affect the plant. The plant growth rate slows down, it may change colors to a slightly darker hue of green, and less energy is available to put into the fruit, grain or other product that we humans like to get from plants. This also slows the uptake of water from the soil. At a certain point the plant can no longer remove any water from the soil no matter how hard it tries, and the plant will die. This soil water content is referred to as the permanent wilting point (PWP).Tables are available listing the FC and the PWP numbers for various classes of soil.
Uses for irrigation management
From the plant’s point of view, the soil is a reservoir that stores water and nutrients. The water-holding capacity of this reservoir depends on the difference between field capacity and the permanent wilting point. Again this is expressed in terms of a percent of the total soil volume, or in inches per foot or millimeters per meter of soil. The total amount of water stored in the soil for the plant’s use is determined by multiplying this water-holding capacity by the effective rooting depth of the plant, which is defined as the depth that would contain 80% of the feeder roots in a deep, uniform, and well-drained soil. For example, let’s find the total available water for potatoes growing in a fine sandy loam soil. The water-holding capacity for a fine sandy loam is about 2 inches of water per foot. The effective rooting depth of potatoes is about 2.5 feet. The total water holding capacity is 2 in/ft times 2.5 ft = 5 inches of water. Of course we would never want to see a five inch depletion because, if you’ll remember, the bottom end of this is permanent wilting point, i.e. dead potato plants. We choose a maximum allowable deficiency. This is the soil water depletion below which the plants see stress and yield reductions result. For potatoes, we may only want a maximum allowable deficiency of 30 percent, because 30 percent of 5 inches is 1.5 inches. Therefore as a management strategy, we would fill up the soil profile to field capacity; then we would only allow a 1.5 inch depletion of water before we came back and refilled the profile with a 1.5 inch irrigation.
Managing irrigation in this manner requires knowledge of how much water a crop uses on a daily basis. This can be calculated from weather data measurements taken by a weather station.