Soil Moisture Characteristic Curves | Soil-Water Relationship | Soil Science

In this article we will discuss about the soil moisture characteristic curves.

Soil moisture tension does indicate neither the moisture content of soil nor the amount of moisture that can be withdrawn for plant use at any particular tension. Knowledge of the amount of moisture a given soil holds at various tensions is necessary to know the amount of water available to plants, water that can be absorbed by the plant and the amount of water required for irrigation.

Soil moisture characteristic curves (moisture extraction curves), which are plot of moisture content versus moisture tension show the amount of moisture a given soil holds at various tensions.

Soil moisture characteristic curve is more strongly affected by soil texture. Greater the clay content, the greater the water content at any particular suction and more gradual the slope of the curve. In a sandy soil, most of the pores are relatively large and once these large pores are emptied at a given suction, only a small amount of water remains (Fig. 7.14).

Soil structure also affects the shape of soil moisture characteristic curve, particularly in the low suction range. Soil compaction decrease the total porosity, especially decrease the volume of large inter-aggregate pores. Hence, the saturation water content and initial decrease of water content at low suction is reduced.

On the other hand, the volume of intermediate size pores is likely to be relatively greater in a compact soil, while the inter-aggregate micropores remain unaffected. Hence, the curves for compacted and un-compacted soil may be nearly identical at high suction range (Fig. 7.15). At very high suction range, water is held primarily by adsorption and hence retention is a textural than a structural attribute of soil.

Hysteresis:

The relationship between matric potential and soil moisture can be obtained in two ways:

(1) Desorption, by taking an initially saturated sample and applying increasing suction to gradually dry the soil while taking successive measurements of water content at various suctions and

(2) Sorption, by gradual wetting an initially dry soil while reducing the suction.

Each of these two methods yields a continuous curve, but the two curves will not be identical. The equilibrium soil moisture at a given suction is greater in desorption (drying) than in sorption (wetting). This dependence of the equilibrium content and state of soil water upon the direction of the process leading up to it is called hysteresis (Fig. 7.16). The hysteresis effect may be due to geometric non-uniformity of individual pores, entrapped air or swelling and shrinkage of the soil.