In general plants extract water from the soil in the following pattern-40% of their requirement from the first quarter (top-most layer) of root zone, 30% from the second quarter, 20% from the third quarter and 10% from the fourth quarter (bottom-most layer) of their effective root zone depth, since a greater part of the roots (60-75%) lie in the top half of the depth of root zone, Fig. 2.11. Usually irrigation is applied when the top half of the root zone depth approaches 50% depletion level even though the lower half of the root zone depth may be at 75% of field capacity. Generally the depth of irrigation is taken as 50% of field capacity.
Soil moisture may be determined either from direct weighing of the samples taken from the irrigated fields or by rapid moisture meters commercially developed, embedded in the field.
Soil moisture tension is a measure of the suction required to extract water from the soil. It is low at field capacity (about 1/10 to 1/3 atm) when the soil readily gives up water to plant roots and gradually increases as the moisture is depleted and is about 15 atm at permanent wilting point. Soil moisture tension is often expressed as pF which is the common logarithm (to the base 10) of the numerical value of the negative pressure of the soil moisture expressed in cm of water. Thus, a pF of 2 represents a suction of 100 cm of water or a suction pressure of 100 gm/cm2 (0.1 kg/cm2 =10 kN/m2).
Soil moisture tension is measured by a tensiometer. It consists of a porous cup connected through a glass tube to a vacuum gauge. The cup and the glass tube are filled with water and is then closed. The cup is then embedded in the moist soil sample. The unsaturated soil in contact with the cup tries to draw water through the pores of the cup creating negative pressure which can be read on the dial of the vacuum gauge.
The moisture equivalent in the soil moisture retained after a wet soil sample is subjected to a centrifugal force of 1000 times the acceleration due to gravity (i.e., 1000 g) for 40 minutes in a soil centrifuge and corresponds to field capacity.
Effective root zone is the depth of soil from which the plant draws water during its water sensitive stage of growth. It should not be confused with the true root zone. An effective root zone depth for crops for moisture extraction is given in Table 2.8.
Depth of irrigation-
Where d = depth of irrigation required; wi = % moisture content just before irrigation; wf = % moisture content at field capacity; Gm = apparent specific gravity of the soil = dry unit weight of the soil/unit weight of water = (Ws/V)/γw ; D = depth of soil layer to be irrigated; ∑ = stands for the summation of the soil layers from the surface up to the effective root zone depth and-
ηi = water application efficiency or simply, irrigation efficiency. Irrigation interval or
Frequency = Moisture holding capacity of the soil / Daily consumptive use
The cycle will be short at the period of peak water use. If it rains between consecutive irrigation, the depth and the frequency will have to be corrected according to the moisture contents of the soil. Water in excess of field capacity will be lost to the lower strata and may
build up a water table.
Over-irrigation causes leaching away of the plant nutrients, waste of valuable water, prevents proper aeration of plant roots and also causes water logging. A certain quantity of air in the soil is essential to satisfy the requirements of crop growth. The growth of crop is stimulated by moderate quantities of soil moisture and retarded by excessive or deficient amounts.
Depending on the depth of irrigation and frequency, the average moisture content in the soil may be closer to the lower limit (wilting point) or upper limit (field capacity), Fig. 2.13, the evapotranspiration being more in the latter case than in the case of the former. Most crops react to such variation in soil moisture (and fertilizer dose) and have an optimum soil moisture content, and hence depth of application and frequency, at which maximum yields are obtained.
For example, wheat is quite sensitive to over-irrigation and has a well-defined optimum consumptive use, Fig. 2.14 curve (a), whereas in the case of other crops the increase in yield is not appreciable under increased depths of irrigation, Fig. 2.14 curve (b).
In the former type of crops it is desirable to provide the optimum depth of irrigation whereas in the latter case the increase in yield is not worth the cost of additional water supplied, and water can be more economically utilised for growing other crops. Hence, the depth and frequency of irrigation should be so selected that the average soil moisture is at the corresponding optimum level.