Absorption and Movement of Water in Soil
The movement of water from the soil surface into and through the soil is called water intake. It is the expression of several factors including infiltration and percolation.
Infiltration is the term applied to the process of water entry into the soil generally (but not necessarily) through the soil surface and vertically downward. This process is of great practical importance since its rate determines the amount of run-off over the soil surface.
In other words, infiltration refers to the entry and downward movement of water in to the soil surface. Infiltration is a surface characteristic of a soil.
It is the rate at which the water enters from the surface to the soil. Initially the infiltration rate is more but afterwards it decreases because the soil gets wet. According to the rate of entry of water from surface to the soil, infiltration rate is grouped in to four categories.
Very Slow: soils with less than 0.25cm per hour e.g. – very clay soils.
Slow: infiltration rate of 0.25cm to 1.25cm per hour e.g. Soils with high clay.
Moderate: infiltration rate of 1.25 to 2.5cm per hour. e.g. – sandy loam/ silt loam soils.
Rapid: infiltration rate is more than 2.5cm per hour e.g. deep/sandy silt loam soils.
Factors affecting the rate of infiltration:
Compactness of soil surface: A compact soil surface permits less infiltration whereas more infiltration occurs from loose soil surface.
Impact of rain drop: the force (speed) with which the rain drop falls on the ground is said to be impact of rain drop. Ordinary size varies from 0.5 to 4mm in diameter. The speed of raindrop is 30ft per second and force is 14 times its own weight. When impact of raindrop is more then it causes sealing and closing of pores (capillaries) especially in easily dispensable soils resulting in infiltration rate
Soil cover: Soil surface with vegetative cover has more infiltration rate than bare soil because sealing of capillary is not observed.
Soil Wetness: If soil is wet, infiltration is less. In dry soil, infiltration is more.
Soil temperature: Warm soil absorbs more water than cold soils.
Soil texture: In coarse textured soils, infiltration rate is more as compared to heavy soils. In coarse textured soil, the numbers of macro-pores are more. In clayey soils, the cracking caused by drying also increases infiltration in the initial stages until the soil again swells and decreases infiltration.
Depth of soil: Shallow soils permit less water to enter into soil than too deep soils.
A coarse surface textured, high water stable aggregates, more organic matter in the surface soil and greater number of micro pores, all help to increase infiltration. As it is a dynamic and quite variable character of soil, it can be controlled by management practices. Cultivation practices that loosen the surface soil make it more receptive for infiltration e.g. course organic matter mulches increases infiltration.
It is defined as the characteristic that determines how fast air and water move through the soil describes what is known as permeability.
Once the water has entered into the top layer, its subsequent slow or rapid movement within the soil indicates its rapid or slow permeability. The permeability basically depends upon pore size distribution in the soil. Larger the number of macro pores (non-capillary pores), the greater is the permeability. The movement of water becomes slow in subsoil layers due to their compactness and low organic matter content but with deep-rooted plants, there is an increased permeability even in such sub soil layers. Permeability increases with the increasing fine texture.
Permeability depends up on:
Number of micro pores: More the number of macro pores higher is the permeability.
Soil aggregates: Larger the size of capillary pores, greater is the permeability.
Depth of soil: Permeability decreases with the depth, as the sub soil layers are more compact and have less organic matter.
Coarseness of soil texture: In coarse textured soil, permeability is more, however fine textured soil is less.
Salt concentration: Salt concentration affects permeability adversely. If the sodium is high in water; it would cause ready dispersion of soil and thus reduces permeability.
Soil moisture status: Permeability decreases as the soil becomes drier and increases when soil becomes wet.
Organic matter content: more organic matter in the soil results in more permeability.
The permeability is considered slow, if it is less than 2.5 cm per hour, moderate if it is about 5.0 cm per hour. Like infiltration, permeability can be also controlled to a extent by suitable management practices. Continuous tillage reduces permeability, while the growth of deep-rooted crops like pulses or legumes, grasses and tress increases permeability. The permeability of soil varies with its moisture status and usually decreases as the soil becomes drier because air enters in to soil and reduces the permeability.
The down ward movement of water through saturated or nearly saturated soil due to the forces of gravity is known as percolation. Percolation occurs when water is under pressure or when the tension is smaller than about 1/3 atmosphere.
Percolating water goes deep into the soil until it meets the free water table. Percolation studies are important for two reasons-
1)Percolating water is only source of recharge of ground water, which can be again be profitably used through springs and wells for irrigation.
2)Percolating water carries plant nutrients like Calcium, Magnesium deep into lower layers and depositing them beyond the reach of roots of common field crops. In sandy or open textured soils, there is a rapid loss of water through percolation.
Percolation depends up on:
(i) Climate: If the rainfall is more than evaporation, then there will be appreciable amount of percolation. In dry region, percolation is almost negligible.
(ii) Nature of soil: sandy soils permit more percolation as these occupy large number of macro-pores. The macro-pores serve as the main channels of the gravitational flow. However, clayey soil permits less water to percolate.
Once the flow due to gravitational forces has been ceased (stopped), the water moves in the form of thin or capillary film from a wet region to dry region. This type movement goes through the finer or micro-pores and it continues until the thickness of moisture film surrounding the soil particles is equal to both the regions (wet and dry regions). Capillary may be in all directions i.e. it may be downward, lateral or upwards from a low tension to high-tension area, since thicker film have lower tension; water from thicker film around the soil particles flows to thinner film. The greater the difference between the thicknesses of the film, the quicker is the capillary movement up to certain point and as difference narrows, the movement of water film also becomes slow and may cease (stop).