Those who understand the basics of soil water interactions and the major differences between soil water sensors can skip these first sections and go directly to the Placement of Sensors, and the Soil Water Content and Tension-Based Sensor sections.
Soil Water Basics
Soil water fills about 25% of the space in the soil. This water is held in the pore space, or the cracks and empty spaces between soil particles. When all of the pore space is completely filled, the soil is said to be saturated. Excess water will drain out over time to a point where the soil will hold a certain amount of water indefinitely against the downward pull of gravity. This soil water content is called field capacity. As a plant’s roots remove water from the soil, the soil will dry out to a point where the suction or pull of the soil on the water exceeds the plant’s ability to absorb water. At this point, the plant will wilt and die. This soil water content is referred to as permanent wilting point. The difference between field capacity and permanent wilting point is the available water holding capacity (AWC) of the soil (Figure 1).
Different soils have different available water holding capacities (Table 1). Sands can’t hold very much water compared to silts and clays. A plant’s rooting depth is also an important consideration. A plant with deeper roots has access to a much larger volume of soil and, consequently, to a larger reservoir of soil water to draw upon before it runs out, compared to a shallow-rooted plant (such as onions or potatoes). Applying more water than a soil can hold simply results in deep percolation: water that is lost below the root zone of the plant, along with essential plant nutrients and other soluble compounds.
At first, as the soil water is depleted from field capacity (100%