Soil Moisture: Constants, Measurement and Its Loss

After reading this article you will learn about:- 1. Soil Moisture Constants 2. Measurement of Soil Moisture 3. Loss.

Soil Moisture Constants:

(i) Field capacity is the amount of moisture held by the soil after the excess gravitational water has drained away. Negative potential or free energy of soil water is 0.1 to 0.3 bars. Approximate of value is 2.54.

(ii) Moisture equivalent is the amount of water held by the soil against a centrifugal force 1000 times that of gravity. The field capacity of sand is higher than its moisture equivalent, but the field capacity of loams is approximately equal to their moisture equivalent.

(iii) The wilting coefficient represents the soil moisture content which cannot be taken up by roots and so the crop wilts. The negative potential or free energy of soil water is approximately 15 bar.

Approximate p^F value is 4.18.

Field capacity = 0.86 Moisture equivalent + 2.6 Wilting coefficient

Wilting coefficient = Moisture equivalent/1.84

(iv) Hygroscopic coefficient represents that soil moisture content held by the soil in the vapour form. Negative potential or free energy is 31 bars. Approximate p^F value is 4.5.

Available Moisture:

Crops can take up moisture held between a negative potential of 0.1 to 15 bars i.e. between the field capacity and wilting coefficient. Therefore the available moisture of soils is the moisture retained by them between the field capacity and wilting coefficient.

The available water holding capacity of soils depends upon the following factors:

(i) Soil texture:

The pore space volume and the surface area of soils affect their capacity to hold water. Soils of finer texture possess the maximum total water-holding capacity whereas the soils of medium texture i.e. loam to light clay loam, possess the maximum available water holding capacity.

Wilting coefficient of soils increases with the increase in the fineness of their texture but their field capacity increases with the increase in the fineness of their texture up to clay loam only.

Research, as quoted by Foth (1984) has shown that the available water content of many soils is closely correlated with their silt and fine sand contents. As the soil texture becomes finer, a smaller percentage of water held by the soils at their field capacity is available for plant growth.

(ii) Nature of the clay:

Available water holding capacity of black cotton soils is higher than those of red soils. The most important clay minerals in black cotton soil and red soil are Montmorillonite and kaolinite respectively. As the cation exchange capacity of Montmorillonite (80 to 150 milliequivalent per 100gms) is much higher than that of kaolinite (3 to 15 milliequivalent per 100gms), a Montmorillonite soil can retain much more moisture than a kaolinitic soil.

(iii) Soil structure:

As soil moisture at the field capacity is mainly present in the micropores and on the surface of the soil particle, the available water holding capacity of soils increases when their structure is improved because the micropore space and specific surface of soil particles increases.

(iv) Soil humus:

Both the field capacity and wilting coefficient of humus are very high. Hence humus improves the available water holding capacity of soils mainly by improving their structure.

(v) Soluble salt:

Soluble salt decreases the available water holding capacity of soils because they increase the osmotic potential of soil water which in turn increases the wilting coefficient of soils. The available water holding capacity which is the difference between the field capacity and the wilting co-efficient, decreases.

(vi) Depth of soil:

All other factors being favourable, the available water holding capacity of the deeper soil is usually considered higher than that of shallow soil because the volume of soils from which roots extract moisture is higher in the deeper soils than shallower soils. The presence of hard pan restricts root development and therefore reduces the soil volume from which the roots extract moisture. Hence in such cases, the total available water in that soil is decreased.

(vii) Rooting habits and drought:

Other factors like the rooting habit of crops and their capacity to tolerate dry period, their stage and rate of growth, usually influences the rate of absorption of moisture by crops.

Mechanism of Absorption of Moisture by the Root:

When root hairs begins to absorb water from any point in the soil the soil moisture content at that point or zone decreases where the negative potential of the remaining water increases. This means that water molecules are held by the soil particles with a greater force than before, at that point in the root zone.

Therefore, water from the surrounding, relatively wetter zone where the negative potential of the soil water is much less than in the root zone, move to the root zone. The magnitude of the movement of the soil water depends upon the difference of the negative potential of the soil water at the root zone and that of the soil water at the surrounding zones, and the ease with which the soil pores allow the water to move through them.

This kind of movement of the soil water is relatively more rapid in sandy soils than in the clayey soils. The soil water usually does not move by more than a few millimetres by this mechanism.

Roots also rapidly elongate towards the surrounding moist zones and absorb moisture. Well aggregated soils are usually well permeated with roots and root hairs where the soil-water movement of a few millimetres is quite sufficient meet the water demands of the crop.

Measurement of Negative Potential or Tension of Soil Water:

Negative potential or tension of soil water is measured with a tensiometer, a simple form of which is shown in Fig. 5.3. One porous clay bottlenose Deen attached with a long glass tube provided with a narrow side tube of one square centimetre cross-sectional area that has been bent twice at right angles, near its upper end as shown in the Fig. 5.3.

The tensiometer is filled with water and is placed in the soil. The lower end of the bent side tube is placed in a dish containing mercury.

Water moves out of the clay bottle and therefore the mercury rises in the manometer till the force of attraction between clay and humus and water is satisfied.

The mercury column becomes stationery at this stage. The weight of this mercury column per square centimetre represents the force with which water is held by the soil.

The product of the height in centimetres of this mercury column, and the density of mercury gives the height of an equivalent column of water in centimetres, the logarithm to the base ten of which gives the p^F value of the soil moisture. For example, suppose the height of the mercury column of unit cross-sectional area is 25 cm.

Then equivalent height of the water column of unit cross-sectional area is 25 x 13.6 = 340 cms. Here density of mercury is 13.6gm/cc.

.^.. p^F of the soil water = log ₁₀ 340 = 2.54

The tensiometer works only up to a tension of 0.85 atmospheres, beyond which air enters into it, after which it no longer operate. So tensiometers are more suitable for measuring the tension of soil moisture in sandy soils.

They may also be used for crops that require a lot of water, e.g. potato. Tensiometers actually used in the field are provided with a vacuum gauge, which measures the tension or negative potential of soil water directly in bars or atmospheres.

Measurement of Soil Moisture:

1. Gravimetric Method:

The moist soil is weighed. Then it is oven dried and re-weighted.

Soil moisture percentage = Wt. of moist soil – Wt. of oven dried soil/Wt. of oven dried soil x 100

2. Electrical Resistance (Gypsum Block Method):

Two electrodes are embedded in a gypsum block, which is buried in the soil permanently. When the soil moisture is to be measure, these two electrodes are connected with the two terminals of a battery operated moisture meter.

The gypsum block absorbs moisture till its moisture content is equal to the soil moisture content. This increases the flow of electricity through the gypsum block, which is recorded by the moisture meter that has already been calibrated to read zero at wilting coefficient and 100 at field capacity. This reading of the moisture meter thus expresses the amount of available water as the percentage of the available water holding capacity of the soil.

3. Neutron Scattering Method:

A neutron emitting material, for example radium, and a detector of radio activity is lowered in a hole dug in the soil. The high velocity neutrons emitted by the radioactive material collide with the hydrogen nuclei of the water molecule, and are slowed down and deflected.

A certain percentage of these slowed neutrons ultimately reach the detector. This is proportional to the number hydrogen nuclei of water molecules. The reading of the detector gives the amount of water present in the soil as it has been calibrated against the soil moisture percentage.

Loss of Soil Moisture:

Moisture is lost from the soil in the vapour form and the liquid form.

1. A Loss of Moisture in the Vapour Form:

About 5 to 20 per cent of the annual rainfall is intercepted by the foliage of green plants and evaporated back to the atmosphere even before it reaches the surface of the field. More moisture is lost in this way if the rain is received as light showers.

Water vapour is lost from the soil by evaporation and transpiration. Evaporation is the process by which the liquid water is gradually changed at the surface of the soil or water to the vapour.

Transpiration is the process by which water vapour leaves the living foliage of green plants and enters the atmosphere.

The combined loss of moisture by evaporation and transpiration is called evapotranspiration. Water vapour exerts a certain amount of pressure which is called the vapour pressure.

The difference of vapour pressure at the soil surface or plant surface and the atmosphere is called the vapour pressure gradient.

Evapotranspiration depends mainly on the vapour pressure gradient which depends on the following factors:

(i) Radiant energy:

More radiant energy is received from the sun in the arid regions than in the humid regions and also during cloudless days than during cloudy days in any region. This radiant energy evaporates water from the surface of the soil and the plant.

(ii) Atmospheric vapour pressure:

If the vapour pressure of the atmosphere is much lower than the vapour pressure at the surface of the green plant of soil, more water is lost by evapotranspiration. If it is high, much less water is lost by evapotranspiration.

(iii) Temperature:

When temperature increases, then vapour pressure at the surface of the green plant of soil increases, but the vapour pressure of the atmosphere is not increased. So evapotranspiration increase.

(iv) Wind Velocity:

Wind blowing at a high velocity continuously removes moist air near the surface of the plant or soil. Hence more water evaporates from the surface of the plant or atmosphere.

(v) Soil Moisture:

The other factors affect the evaporation or evapotrans­piration as long as the soil contains enough moisture. If the soil contains less moisture, evaporation and evapotranspiration will be obviously be less. Consumptive use of water is a measure of the total quantity of water lost by evapotranspiration. All the factors which affect the evapotranspiration also affect the consumptive use of water which also depends upon the nature of the crop.

Efficiency of Water Use:

A certain number of crops can be grown if a given amount of water is supplied to the soil. The efficiency of water use maybe expressed in terms of consumptive use, which is the total quantity of water required for evapotranspiration (evaporation + transpiration) and metabolic activities of the crop in any specified time.

It is the kilogram of water consumed to produce each pound of plant tissue or may also be expressed in terms of the transpiration ratio which is the amount of water transpired by the crop for producing unit quantity of dry matter. Like consumptive use, Transpiration ratio also depends on the nature of the crop. e.g., this ratio of pea, wheat and maize have been found to be 563, 544 and 337 respectively at Pusa in Bihar (India).

Reducing Loss of Moisture by Evaporation:

The surface of the land should be kept covered with waste organic material in order to reduce evaporation. This material left on the surface of the land in order to reduce evaporation is called the mulch.

In USA plastic sheets are spread between the rows of crops, to reduce evaporation and weed growth. Evaporation may be conveniently reduced if the refuse of the previous crop, the stubble mulch, is left on surface. The seed bed is prepared and seeds are sown.

Sometimes no crop is sown on certain portion of the field. This practice is called fallowing. This is done when the soil is not moist enough to produce crops in that season and the water can, be stored in the soil for the crop which will be grown in the next season.

2. Loss of Moisture in the Liquid Form:

Water moves down the soil profile. This phenomenon is called percolation, and depends upon rainfall characteristics, soil characteristics and crop characteristics.

When the water which then is more than the field capacity of the soil, enters the field and the excess water per-folates down the profile. Percolation loss increases when rainfall increases. The presence of crops of the field reduces percolation loss, because they absorb considerable amount of water.

Considerable amount of plant nutrients are lost from the soil along with the percolation water. More important among them are usually nitrate, potassium and calcium. This loss of nutrients is usually greater in sandy soils than in clayey soils because the sandy soils cannot retain them in sufficient quantities. Therefore, fertilizers should be applied to sandy soils in split doses at the root zone at the time when the crop requires them.