Soil water is one of the vital bases of plants ans has a huge impact on soil formation processes The amount of water needed by plants to create a unit of organic matter during the growing season is called the transpiration coefficient.The water content in the soil determines the processes of weathering and soil formation, as well as the formation of the soil profile. The intensity of biological, chemical and physico-chemical processes in the soil, the movement of substances, soil regimes depend on the quantity and quality of water in the soil.
The forms of water in the soil.
The nature of the connection with the solid phase of the soil and the degree of mobility of water distinguish the following forms in the soil: chemically bound, physically bound, solid, vaporous, free, gravitational and soil.
- Gravitational water or free water: It is a form that is not retained by solid soil particles and, as a consequence, can move freely through the pores. The driving force acting, in this case, is the action of terrestrial gravity and, as a result of downward vertical movements, is eliminated by internal drainage of the soil towards deeper horizons. A part of this water, which fills the larger pores, drains quickly while the water that saturates the finest pores does it more slowly, taking, sometimes, enough time to drain completely. The soils with insufficient drainage (“heavy” or fine granulometry) present problems for the elimination of this water and in them the crop can suffer significantly.
- Capillary water: It is the water retained by solid soil particles by surface tension forces. Part of this water, specifically that retained by the capillaries of the soil of greater diameter, can be used by the plants, since the retention force is lower than that of suction exerted, by osmotic pressure, by the root system of the plants. On the other hand, the water retained by the smaller diameter capillaries is so energetically that root suction is incapable of overcoming these retention forces and, consequently, cannot be absorbed by the plant. It should be noted, therefore, that all water retained by the soil and usable by the plant is capillary but, conversely, not all capillary water is usable by the plant.
- Hygroscopic water: Equivalent to the maximum amount of water that soil particles can absorb when they come in contact with a saturated atmosphere of water vapor. This water is fixed to the solid phase by electrical forces since, in this case, the water behaves like a dipole.
This form of water represents, in all cases, a very small fraction of the total and is so strongly retained – with tensions from 30.6 to 10,000 technical atmospheres – that in no case can it be absorbed by the plants.
- Water of constitution: Different components of the soil (organic matter, clays, salts, etc.) are formed by molecules in whose composition the water intervenes. This form of water in the soil, linked to the constitution of matter itself, turns out to be a quantitatively very insignificant fraction and, being retained in a very energetic way, is also absolutely unusable by plants.
Water-lifting capacity of soil (VA Kovda)
|Grading||Height of rising water, m|
|Loess||4.0 – 5.0|
The evaporating capacity of the soil depends on the granulometric composition, physical properties, steepness and slope exposure, the nature of the vegetation cover, air humidity, etc. Clay and loamy structureless soils, in which capillary pores predominate, lose a lot of water for evaporation. Structural soils lose much less moisture, which is associated with disunity of capillaries by large inter-aggregate pores, which weaken the water-lifting capacity. Evaporation of moisture increases with increasing wind speed, dry air and its temperature. The southern slopes lose more water than the northern ones.
Physical And Mechanical characteristics of soil Water
The physical – mechanical characteristics of the soils are important for their use as building materials and for the purposes of irrigation and drainage, which is why they must be carefully determined in the laboratory among other things.
The main physical – mechanical characteristics of soils are:
- Land cohesion |
- Soil texture
- Actual soil density and apparent soil density
- Soil structure
- Porosity of the soil | Porosity
- Hair potential
- Water potential.