Classification of soil water & soil moisture characteristics curve

Classification of soil water & soil moisture characteristics curve

1. Classification of Water Prof. S.R. Suryavanshi , Asst. Professor of Agronomy, Dr. D.Y. Patil College of Agriculture, Talsande

2. Water is retained by the soil-particles(on their surfaces) especially colloidal particles and the pore spaces by the force of adhesion and cohesion . Such water present in the soil and is called as soil-water. Classification of Soil Water Gravitational water Capillary water Hygroscopic water Physical classification Biological classification Available water Unavailable water Super available water (Drainage water)

3. When is water added to a dry soil either by rain or irrigation, it is distributed around the soil particles where it is held by adhesive and cohesive forces; it displaces air in the pore spaces and eventually fills the pores. When all the pores, large and small, are filled, the soil is said to be saturated and is at its maximum retentive capacity .

4. Adhesion: It is the attraction of solid surfaces for water molecules. Adhesion is operative only at the solid-liquid interface and hence the film of water established by it is very thin. Cohesion: It is the attraction of water molecules for each other. This force makes possible a marked thickness of the films of water established by hydration until they attain microscopic size.

5. Classification of Water The following re the three main classes of soil water: A) Capillary water: Capillary water is that part, in excess of hygroscopic water, which exists in the pore space of the soil by molecular attraction. Water held by forces of surface tension and continuous films around soil particles and in the capillary spaces. Water left out in capillary pores after excess water has drained– Held by surface tension – cohesive force 1/3-15 atmp .– Available to plants

6. B) Gravitational water: Gravitational water is that part in excess of hygroscopic and capillary water which will move out of the soil if favorable drainage is provided. Excess water in soil pores– drains out due to gravitational force– Not available for plant growth. Water that moves freely in response to gravity and drains out of the soil.

7. C) Hygroscopic water: When an oven dried sample is kept open in the atmosphere, it absorbs some amount of water from the atmosphere. This is not capable of movement by the action of gravity or capillary forces. Water held tightly to the surface of soil particles by adsorption forces. Water absorbed by a oven dry soil when exposed to a moist air. Held at high tension - tightly held by adhesion force – water of adhesion 10000-31 atm p., water not available – permanent wilting point

8. Water contents present in soil under certain standard conditions It represents definite soil moisture relationship and retention of soil moisture in the field. The soil moisture tension is measured with “ TENSIOMETER ” Soil Water Content Soil Moisture Content

9. Maximum water holding capacity Field capacity Maximum Capillary capacity Moisture equivalents Permanent wilting point Hygroscopic coefficient Available soil-moisture Air capacity Total pore volume While studying soil water and discussing its availability or other wise to plant, some specific terms called as soil moisture constants are used and they are as follows :

10. I. Field Capacity ( FC ) : It is the non-saturated but still very wet soil condition . Where gravity drainage becomes negligible and only micropores retain water . Factors affecting FC : a .) S oi l -t e x t ure b .) s o il s t r uc t u r e c . )t y pe o f c l ay d .)o r g a nic m a t t e r c o nt e n t e.)soil-compaction f.) Impedition layer

11. I. Permanent Wilting point (WP or wp ) : Also known as “ wilting coefficient ”. It is the soil moisture content at which the plants can no longer be able to meet their transpiration requirement, become water-stressed .There is still some water in the soil but not enough to be used by the plants. WP is of two types : Temporary wilting point . Ultimate wilting point FACTORS AFFECTING PWP Soil properties Plant properties Soil texture & structure Types of clay Organic matter content

12. SATURATION FIELD CAPACITY WILTING POINT 100% Moisture in soil pores (both macro and micro) When water is no longer drained by gravity When plants have extracted as much water as they can ombine to Capillarity and surface attraction c pull more strongly than gravity on: water in “micropores” and water close to the “soil skin” Fig. : Diagrammatic representation of saturation, field capacity and wilting point

13. This characteristic curve describes the relationship between water tension and water content for a specific soil.

14. Appearance of soil Type of Soil Soil Moisture Constant Moisture Tension in Atmosphere(in bar) Wet soil Gravitational water Maximum water 0.00 (~ 0.001 ) Moist soil Available water Field capacity 0.33 (1/3) Water held in micro pores Wilting point 15 Dry soil Unavailable water tightly held to the soil particles H y g r os c oic coefficient 31 Air dry 1000 Oven dry 10,000 Moisture tension of soil moisture constants

15. Saturated flow Unsaturated flow Water-vapour movement The major principle of movement of soil-water is that it is “ along the gradient ”. Wet soil to Dry Soil low soil moisture tension to high SMT high soil water potential to low soil water potential

16. water move in the macropores since all of the pores are filled. Saturated flow is water flow caused by gravity’s pull. This water moves at water potentials larger than – 33 kPa. Factors affecting saturated flow : Texture Structure Amount of organic matter Temperature Depth of soil to hard pan Pressure Amount of water in the soil Saturated flow due to gravity

17. Macropores full of air Micropores = water + air Moisture tension gradient creates unsaturated flow It is flow of water held with water potentials < -1/3 bar. Factors Affecting the Unsaturated Flow Distribution of pores i. Nature of soil Size of pores i. Soil moisture content : The higher the percentage of water in the moist soil, the greater is the suction gradient and the more rapid is the delivery. Unsaturated flow Micropores Macropores

18. Movement from The movement of water vapour from soils takes place in two ways: Internal movement External movement There are mainly two soil conditions that affect the water vapour movement. Moisture regimes Thermal regime

19. Method of downward entry or movement of water into the soil surface Determines the part of the precipitation that would become the surface runoff. Infiltration is governed by two forces: a.) Gravity b.) capillary action

20. Rate at which water enters the soil at the surface. The rate of infiltration can be measured with Infiltrometer . It is measured in inches per hour or millimeters per hour . occurs when supply of water at surface is not limited . accumulated depth of water infiltrated during a given period of time.

21. Type of soil and its properties - Porosity and hydraulic conductivity, soil-texture and structure, soil-temperature Moisture content of soil Condition of soil surface and its vegetative cover rainfall intensity

22. Soil moisture characteristic curves: Soil moisture characteristic curves (moisture extraction curves), which are plot of moisture content versus moisture tension show the amount of moisture a given soil holds at various tensions.

23. Soil moisture characteristic curves: Soil moisture characteristic curve is more strongly affected by soil texture. Greater the clay content, the greater the water content at any particular suction and more gradual the slope of the curve. In a sandy soil, most of the pores are relatively large and once these large pores are emptied at a given suction, only a small amount of water remains

24. Hysteresis: The relationship between matric potential and soil moisture can be obtained in two ways : Desorption , by taking an initially saturated sample and applying increasing suction to gradually dry the soil while taking successive measurements of water content at various suctions and (2) Sorption, by gradual wetting an initially dry soil while reducing the suction.