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Soil moisture constants
- 1. FUNDAMENTALS OF SOIL SCIENCE(2+1) SOIL MOISTURE CONSTANTS BY TO BASKAR S Dr.RAJAKUMAR R BATCH-’A’ Asst.Prof. CB.AG.U4AGR19018 19SAC111
- 2. SOIL MOISTURE Soil moistureis the water or moisture content stored in the soil and it is affected by precipitation, temperature and soil characteristics (texture & structure). Soil moisture held in the spaces (pores) between the soil particles.
- 3. SOIL – WATERRELATIONSHIP
- 4. SOIL MOISTURE CONSTANTS Theamount of water that a soil contains at each of the equilibrium points (hygroscopic coefficient, capillary point) is known as soil moisture constant. It represents definite soil moisture relationship and retention of soil moisture in field. Soil moisture constants are the reference point for irrigation water management. Based on soil characteristics, it will differ.
- 5. FIVE SOIL MOISTURE CONSTANTS Fieldcapacity Permanent wilting point Hygroscopic coefficient Available water capacity Maximum water holding capacity
- 6. FIELD CAPACITY Field capacityis the capacity of the soil to retain water against the downward pull of the gravitational force. It takes place within 1-3 days after rain or irrigation. At this stage, micro and capillary pores are filled with water and water is available to plants. Field capacity is higher in clay (40%) than coarse sand (10%).
- 7. DETERMINATION OF FIELD CAPACITY Itis determined by ponding water over the area of 2- 5m2 for 2-3 days ( spreading polythene sheet on straw mulch over surface prevents evaporation). As a thumb rule, 1 day of drainage will be adequate for sandy soil, 2 days for silt loam soils & 3 days for clay loam soils. It is also known as maximum capillary capacity. At this stage, water is held with a force of 0.33 atm. It is the upper limit of hygroscopic coefficient.
- 8. PERMANENT WILTING POINT Permanentwilting point is considered as lower limit of available soil moisture. The stage at which plants start wilting for want of water is termed as wilting point. The percentage amount of water held by the soil at this stage is known as wilting coefficient. It is the point at which soil is unable supply water to plants. At this point, water is held with a force of 15 atm.
- 9. HYGROSCOPIC COEFFECIENT The hygroscopiccoefficient is the percentage of maximum amount of hygroscopic water absorbed by dry soil under the standard conditions of relative humidity (50%) & temperature (150C). It is the lower limit of capillary capacity. At this stage, water is not available to plants & may be to microbes. At this, water held with the force of 31 atm.
- 10. AVAILABLE WATER CAPACITY Availablesoil moisture is the amount of water required to apply to soil at the wilting point to reach the field capacity. The available water is the difference in amount of water in field capacity (0.33 bars) and permanent wilting point (15 bars). The available soil moisture is expressed as depth per unit of soil.
- 11. FORMULA ASM = (0FC- 0PWP)*drz ASM = Available soil moisture (cm) 0FC = Volumetric moisture content at field capacity ( fraction) 0PWP= Volumetric moisture content at permanent wilting point drz = Depth of root zone (cm)
- 12. MAXIMUM WATER RETENION CAPACITY Itrefers to the condition when all the macro and micro pores are completely filled with water. At this stage, soil is at maximum water retention capacity. It is also known as saturation capacity or maximum retentive capacity. Soil moisture tension is very low between 1/100 to 1/1000 (almost zero) and equal to free water surface. It is the measure of total pore space of soil.
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