What factors determine the degree to which salt accumulates in irrigated soil?

Evapotranspiration

During a plant's life, its roots grow outward, penetrating the network of pore spaces in the soil. The roots absorb water and dissolved nutrients from the wetted pores. Even the soil water present at field capacity (FC) is gradually extracted by plants — until the remaining water is held so tightly that plants cannot extract any more. At that point, the soil water has reached what is known as the permanent wilting point (PWP). Typically, by the time a field reaches the PWP, the plants have extracted about half of the field capacity moisture. The total water volume extracted is equivalent to about three-fourths of the soil water at saturation.

Figure 3 depicts a gradual reduction of soil water, from field capacity to the permanent wilting point, as roots progressively grow downward and extract water to a depth of about 28 inches.

Figure 4 shows a plant wilting at the PWP because it has expended all of its energy to take up water and cannot extract additional water to meet its ET demands.

The soil water between the FC and the PWP is known as the water-holding capacity available to plants. To avoid water stress to plants, such as wilting, irrigation must commence before 50% of the available water-holding capacity has been depleted.

In summary, the cycle of wetting and drying depicted in the three previous graphs may prove useful as a guide when scheduling the irrigation of landscape plants.

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