In this article we will discuss about how to conserve soil moisture.
The methods that are adopted to prevent the loss of water helps in the conservation of soil moisture.
There are some methods useful for conservation of soil moisture as follows:
(i) Terracing and contour farming are the best method to control the loss of water due to run-off in the sloppy land. Bund around the land have significant effect of preserving moisture in the land.
(ii) Organic manure should be applied in the land to minimize the downward movement of water.
(iii) Mulching is the best way to check the loss of water due to evaporation. Mulching may be natural and artificial. In case of natural mulching, ploughing is given followed by planking. This process disconnects the capillary movement of water and helps to preserve the moisture to some extent. In case of artificial mulching, crop residues and plastic papers are applied on the surface of the soil which reduces the evaporation losses of water.
(iv) The water losses due to transpiration can be minimized by the use of wind break, selection of proper crops and their varieties though man has practically very little control to transpiration, the natural and physiological process of plants.
(v) The weeds should be controlled at proper time to check the loss of water by them from the crop field.
The soil moisture constants represents definite soil moisture relationship and retention of soil moisture in the field.
They are as follows:
1. Maximum Retentive Capacity or Maximum Water Holding Capacity:
It may be defined as the percentage of moisture in the soil when all the porespaces are completely filled with water. The cropped soil remains at this stage for a long period suffers from good aeration and this condition is consequently harmful to most crop plants.
2. Field Capacity:
The maximum amount of capillary water remaining in a soil after the removal of gravitational water is called its ‘field capacity’. Veihmeyer and Hendrickson (1931) have defined the field capacity as “the amount of water held in the soil after the excess gravitational water has drained away and after the rate of downward movement of water has materially decreased.”
This value has been referred to as the field earring capacity, normal field capacity, normal moisture capacity and capillary capacity. It is the capacity of the soil to retain water against the downward pull of the force of gravity. At this stage, only the micropores or capillary pores are filled with water and the water is held with a force of 0.33 atmosphere. Water at field capacity is readily available to plants and microorganism.
3. Hygroscopic Co-Efficient:
It may be defined as the percentage of moisture in the soil when the soil is kept in an atmosphere having 100 per cent relative humidity. Dry soil kept in an open place absorbs water vapour from the atmosphere. The water absorbed in this way is called ‘hygroscopic water’. It may also be called ‘water of hydration, water of adhesion’. The hygroscopic coefficient for mineral soils is about 31 atmosphere. The forces operating at hygroscopic coefficient are mostly due to attraction of water molecules by soil colloids. P.F. of this water is 4.5.
4. Wilting Co-Efficient:
The growing plant absorbs water from the soil and the quantity of moisture remaining in the soil reduces. As the moisture contents falls, a point is reached when the soil is unable to supply water at a rate sufficient to maintain turgidity of plants and the plants wilt permanently. At this stage even if the plant is kept in saturated atmosphere, it does not regain its turgidity and wilts unless water is applied to the soil. The stage at which this occurs is termed the ‘wilting point or wilting percentage’ and the percentage of moisture held by the soil at this stage is known as ‘wilting co-efficient’.
Briggs and Shantz introduced the concept of wilting point as an important soil moisture constant. Wilting coefficient may be defined as the percentage of moisture in the soil at which the soil cannot supply water to the growing plant at a sufficient rate to maintain their turgidity and the plants wilts permanently. Wilting coefficient could be calculated by dividing the moisture equivalent by factor 1.84.
Wilting co-efficient differs from soil to soil. It is about 3-5 per cent with sandy soil, 7-12 per cent with loam Soil and 15-20 per cent with clay soil. The tension of soil water when permanent wilting occurs is about 15 atmosphere. Water is held probably as a thin film around the particles at this tension.
Wilting co-efficient may be calculated as under:
Wilting Co-efficient = Hygroscopic coefficient/0.68 or Moisture equivalent/1.84
The difference between the field capacity and wilting co-efficient is the maximum available water.
5. Transpiration Ratio:
The ratio of the quantity of water used to the quantity of dry matter produced is called ‘transpiration ratio’. This ratio varies from 200 to 500 in humid region to twice (i.e. 400 -1000) in arid region. Abundance of soil moisture, dry atmosphere, high temperature and strong winds increase transpiration ratio.
Crop differs in transpiration ratio as mentioned below:
High transpiration ratio – Paddy, Sugarcane, Potato and Banana.
Medium transpiration ratio – Wheat, Barley, Linseed and Cotton.
Low transpiration ratio – Sorghum and Millets.
Crops having low transpiration ratio are suitable for cultivation in dry soil and low rainfall tracts.
(i) Plant fails to absorb nutrient if soil moisture becomes deficit.
(ii) Decrease in soil moisture, decreases the absorption of water and the opening of stomata decreases and the stomata begins to close much earlier than the permanent wilting point of the soil.
(iii) The entry of carbon dioxide is increased if the stomata closes due to lack of absorption of water and the photosynthesis is reduced due to decrease in the entry of carbon dioxide.
(iv) The rate of photosynthesis will also decrease due to dehydration of protoplasm as a result of decrease in soil moisture. Dehydration of protoplasm reduces the photo synthetic capacity of a plant.
(v) The respiration will increase with the decrease of soil moisture but lastly the respiration will decrease because of the limitation of respiratory substrate.
(vi) Net growth of the plant will also decrease due to the deficit of soil moisture.
(vii) Under deficit soil moisture condition, the maturity of plant is hastened and the succulency is decreased. But the protein content in grain and other plant parts are increased.
(viii) The sugar content of a sugarcane crop is increased, when the irrigation is withheld prior to harvesting of the crop.
(i) Adhesive forces – It is the attraction of forces between soil particles and water molecules.
(ii) Cohesive force – It is the attraction between the molecules of water.
(iii) Imbibitional forces – The soil colloids absorb water and they swell due to the absorption of water. This is the process of imbibition.
Soil Moisture Tension:
It may be defined as the total force with which water is held in the soil. It is expressed in atmosphere.
1 atmosphere = 14.7 lbs/sq. inch
= 1036 cm height of water column.
pF is the logarithmic value of soil moisture tension expressed in centimeters height of water column required to produce a suction with which water is held by the soil. Soil moisture is commonly expressed in terms of pF values, whose numerical values are the same as pH values denoting hydrogen ion concentration. The pF value varies according to soil condition as follows (Table 6.1).
1 atmosphere = 1036 cm = 1000 cm (approx.) = 103
1 atmosphere = 3 log 10 = 3 pF
(i) Maximum retentive capacity – 0.
(ii) Field capacity – 0.1-0.3 atmosphere.
(iii) Permanent wilting point – 15 atmosphere.
(iv) Hygroscopic co-efficient – 31 atmosphere.
There are some factors that are responsible for available moisture content of soil as follows:
1. Climatic Factors:
Out of the different climatic factors, temperature and humidity have influence on the availability of soil moisture. The evapotranspiration increases with the increase of temperature and the availability of soil moisture to plants decreases.
2. Plant Factors:
Plant factors also have influence on the availability of soil moisture. Rooting habits, basic drought tolerance and stage and rate of growth are the important plant factors.
3. Soil Factors:
The soil factors which are mainly responsible to determine the availability of moisture to plants are as follows:
(i) Soil Moisture Tension Relationship:
The moisture which is held between field capacity and wilting coefficient is said to be available moisture. The factors that will affect these two values, will affect the available moisture content of the soil. The texture, structure, and organic matter content of the soil are the most important factors which will influence the quantity of water a given soil can supply to the growing plants. As the fineness of the soil texture gradually increases, both the field capacity and wilting coefficient increases simultaneously.
The well granulated soil will hold more available moisture than the soil with coarse structure. Because more pore space is available in good structure soil for holding water and this will increase the field capacity of soil. The organic matter content of a mineral soil have influence on the available moisture to plants, because organic matter increases the available moisture holding capacity of the soil due to soil aggregating power of organic matter.
The addition of organic matter to the soil increases both field capacity and wilting coefficient side by side, that is why addition of organic matter in sandy soil is helpful for increasing the water holding capacity of the soil.
(ii) Soil Depth:
The availability of soil moisture depends on the depth of soil. If the depth of soil is more, then the amount of water which may be available to the plants will be more, because when the soil of surface region is dried up, the plants are in a position to absorb water even from greater depth. For deep rooted plants, this is of practical significance, especially in those sub-humid and semiarid region, where supplemental irrigation is not possible.
(iii) Soil Stratification or Layering:
Soil stratification will influence markedly the available water and its movement in the soil. If impervious layers or hard pans are present in the soil profile, the movement of soil water from the lower horizons and also from upper layer is affected. So the presence of such layer will affect the moisture content of soil and this layer will restrict the penetration of roots to lower layer. This will limit the absorption of water from the lower depth.