Reduction of evaporation loss
Evaporation happens to maintain soil thermal regime and is governed by soil moisture content, vegetative cover on surface, soil type, temperature gradient between soil and atmosphere and atmospheric water demand.
Higher soil moisture content, especially a wet surface soil increases evaporation rate. As the surface soil dries up, continuity of capillary pores is disrupted and moisture movement upwards from deeper layers is reduced. Soil surface that is exposed to radiation without any vegetative cover offers more scope for evaporation due to over heating. Evaporation loss in a cropped field is more in the early growth stage when canopy cover is less, especially in widely spaced crops and slow growing species.
Vegetative cover prevents direct exposure of soil surface to radiation, reduces heating of soil layers and thus checks the necessity for evaporation. Black soils tend to absorb more heat and may evaporate more water. When cracks are formed during drying, evaporation takes place from the sides of the cracks also. With high temperature, low humidity and dry winds, atmospheric water demand increases the rate of evaporation.
(a) Measures to reduce evaporation loss
(i) Shallow surface tillage: When surface soil is stirred by tillage, the continuity of capillary pores is broken and the rise of water through capillary movement is obstructed. Shallow tillage after summer showers is beneficial in this regard. This process is called dust mulching. Inter tillage between crop rows during early dry spells has a similar effect.
(ii) Mulching: Mulching means covering the soil surface with any material such as organic wastes, plastic, polythene sheets etc. The organic wastes used for mulching include crop stubbles, straw, coir pith, groundnut shell, husk etc. These wastes at 5–10 t ha−1 are spread on the soil surface to a thickness of 5-10 cm.
Mulching provides the following benefits:
• reduces direct impact of rain drops on soil particles and controls splash erosion. • increases infiltration.
• reduces velocity of runoff water.
• controls erosion.
• improves soil moisture storage from rainfall.
• controls evaporation loss.
• suppresses weed growth.
• influences thermal regime of soil by reducing soil temperature.
• improves microbial activity.
• controls salinity development.
• can be incorporated as manures later.
Vertical mulching is a technique where in trenches of 40 cm wide, 15 cm deep are dug at 2–4 m interval across slope and filled with stubbles or organic wastes to a height of 10 cm above soil surface. Runoff is checked, collected in the shallow trenches and redistributed to adjoining soil layers. This method can be considered as precursor method to broad bed furrow method.
Live mulching is the term used to describe the covering soil surface through the plant canopy in intercropping system.
e.g., sorghum + forage cowpea, sorghum + sword bean.
Dust mulching refers to the soil condition associated with tillage. When land is ploughed or stirred, the surface soil is disturbed and this breaks the continuity of capillary pores from subsoil to surface. As a result, evaporation is checked and soil moisture is conserved. Guntaka (Blade harrow)/Danti/hand hoe are the implements used for dust mulching.
Stover mulch or straw mulch refers to covering the soil surface with cumbu/sorghum straw, sugarcane trash reduces the evaporation and increases soil moisture efficiency. Similarly mulching with organic waste, crop residues, plastic material can be done. Stubble mulch is referred to the stirring of the soil with implements that leave considerable part of the vegetative material or crop residues or vegetative litter on the surface as a protection against erosion and for conserving moisture by favouring infiltration and reducing evaporation.
Stubble mulch is very effectively done in western countries, where crop residue or by products like straw, stover or haulms are not given to animals as fodder. Special farm implements are available to create minimum disturbance and leave large surface area undisturbed. It also acts as minimum tillage and conservation tillage.
Pebble mulch where small pebbles like stone are placed on the soil surface. This mulching will be successful in dry land horticulture (fruit tree culture). The pebbles placed on the basins of trees not only reduce evaporation but also facilitate infiltration of rainwater into the basin.
Use of anti-evaporating chemicals: Chemicals like hexadecanol are used as anti-evaporants. When sprayed and mixed with soil surface, hexadecanol is reported to reduce evaporation by 43%. The treated surface layer dries up fast and creates a diffusional barrier for upward movement of water vapour.
It is resistant to microbial activity and degradation. It remains in soil for more than a year. It also increased the soil aggregate stability. Evaporation from free water surface, farm ponds, lakes etc., can be reduced to 80% by wax emulsions, rubber/plastic boats or saw dust.
Shelter belt: In arid and semiarid regions, the hot winds dry the surface soil and create vapour pressure gradient and continuous vapourisation takes place. This continuous vapourisation can be arrested by raising shelterbelt. It is a practice of growing one or multi rows of trees/shrubs or crop plants across the wind direction either in the field or field boundaries to reduce the wind effect and to reduce the wind velocity. Shelterbelt reduces the evaporation and increases soil moisture content by 3–5% and this will be useful to alleviate the terminal moisture stress in crops grown in adjoining area. The increase in soil moisture percentage is due to favourable microclimate created by shelterbelts. It can be used as resting place of livestock in dry lands. Due to reduction in wind velocity, the pollen drift in orchard crops is minimized, there by pollination percentage is increased and fruit setting is improved. Many trees in shelterbelt are economically important. After long period of maintenance, the trees can be disposed off as timber and raw material for industrial use. Fruit trees grown in shelterbelt give fruits, which fetch higher economic returns. Windbreak is also a form of shelter belt, but only one row of tall trees having good leaf canopy are grown in North-South direction in order to reduce wind velocity and there by reduce soil erosion. Tall trees like eucalyptus, casuarinas, and wood apple are grown as wind breaks. After years of maintenance, these trees can be disposed of economically.
(iii) Measures to reduce transpiration loss: Though transpiration is necessary and unavoidable evil, excessive transpiration has to be controlled especially when soil moisture stress develops during critical stages of crop growth. The rate of transpiration is governed by soil moisture potential, atmospheric water demand and plant canopy characters such as leaf area, leaf orientation, stomatal resistance, etc. Transpiration loss can be reduced by the use of antitranspirants and by some cultural methods also.
A. Antitranspirants
Antitranspirants are substances or chemicals applied on plant-foliage to control rate of transpiration.
The important points to be considered in using antitranspirants are:
(a) They should restrict water loss from leaf surface without restricting entry of carbon dioxide for photosynthesis, and
(b) Transpiration necessary for cooling of leaf surface should not be completely stopped by the application of antitranspirants leading to rise in leaf temperature.
Based on their mechanism of action, antitranspirants are classified into various types.
Stomatal closing type: They cause partial or complete closure of stomata by inducing the guard cells to close. But complete closure of stomata adversely affects gas exchange and photosynthesis. These chemicals may also cause phyto-toxicity and are very expensive too. E.g., Phenyl mercuric acetate (PMA) and alkanyl succinic acid (ASA).
Film forming type: They cover the stomata by forming a thin film over leaf surface. These substances are nontoxic, non-degradable and very easy to apply but they adversely affect photosynthesis. E.g., Paraffin and wax emulsions, folic 2%, and power oil 1%.
Reflectant type: When sprayed on leaf surface, the reflectant type antitranspirants increase the leaf albedo or leaf reflectance of sunlight. As a result, heating is reduced, leaf temperature inside is low and need for transpiration is reduced. E.g., Kaolin and lime solution. Spraying kaolin at 3–6% concentration reduced leaf temperature by 3–4 °C and transpiration by 22–28%. These are less expensive, non phytotoxic and do not interfere with photosynthesis, since stomatal closure does not take place.
Growth retardant type: Chemicals like cycocel (ccc-chloro choline chloride, chlor mequat) when sprayed on foliage, reduce leaf area and thereby reduce the transpiring area and transpiration.
B. Cultural methods
(a) Weed control: Most weeds have a high transpiration coefficient i.e., amount of water transpired to produce unit quantity of dry matter. Early weed control prevents unwanted transpiration loss through weeds.
(b) Shelterbelts: Rows of trees grown across the direction of wind reduce air movement, reduce temperature of air and plant canopy, increase humidity in the protected strips and thereby reduce the atmospheric water demand and control transpiration in the inter space between shelterbelts.
(c) Alley cropping: This practice refers to raising perennial shrubs or tall crops as hedge rows up to 1-2 m height at 48 m intervals and raising short stature annual crops in the alleys (inter space between hedge rows). A similar effect on reduction in atmospheric water demand and transpiration as described under shelterbelts is caused in alley cropping. This method is also called as hedgerow intercropping.
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