Factors Affecting Soil Water

1. Texture: The finer the texture, the more pore space, and surface area, and the greater the retention of water.

2. Structure: Well-aggregated porous structure favors better porosity, enhancing water retention.

3. Organic matter: The higher the organic matter more is the water retention in the soil.

4. Density of soil: The higher the density of the soil, the lower the moisture content.

5. Temperature: The cooler the temperature, the higher the moisture retention.

6. Salt content: The more the salt content in the soil, the more water available to the plant.

7. Depth of soil: The more the depth of soil more is the water available to the plant.

8. Type of clay: The 2:1 type of day increases the water retention in the soil.

 Soil Water Management 

The losses of soil water generally occur through percolation and the Evapotranspiration process.

Percolation loss 

The soil water losses through percolation are maximum under humid climates with high precipitation. Different essential plant nutrients are lost from soils through such percolating water.

Percolation losses of water can be reduced by applying organic matter in the soil, especially in soils containing moderate to high amounts of sand.  

Evapotranspiration : 

Water vapor losses from soils occur in two ways: a)by the evaporation of water at the soil surface and b) by transpiration from the leaf surfaces of water absorbed by plants and translocated to the leaves. The combined loss resulting from these two processes is called Evapotranspiration (ET) and it is responsible for most of the water removal from soils under normal field conditions.

The rate of evapotranspiration increases when the air is dry (low relative humidity ), warm, or moving (winds )and if the soil water is near the field capacity to the absorbed by roots or evaporated from the soil surfaces.

Evapotranspiration losses can be reduced by selecting more growth-efficient plants, reducing total growth by shading (mulching ) / cooling the area, or using a moisture barrier.

water stress 

Water stress occurs when the demand for water exceeds the available amount during 

In agriculture, drought creates a deficiency in soil moisture and water logging creates flooding conditions (excess soil moisture ) both of which impair crop production. thus there should be optimum soil moisture for plant growth.

stress :

stress is any external factor that results in less than optimum growth rates. i.e. any factors (moisture ) that interrupt, restrict, or accelerate normal metabolic processes of a plant or its parts.

Moisture stress

Too much water (waterlogging conditions) or too little water (drought conditions) can cause problems for crop production.

Drought:

It is one of the major factors limiting crop production in Nepal. Drought occurs in arid climates with insufficient rainfall in areas with unpredicted precipitation and reduced rainfall. In addition, factors such as high temperatures, winds, and low humidity can speed up drought through the increase of evapotranspiration. Plant response to drought accompanies changes in plants' physical and physiological components.

Physical effects:

Water activity in plants is lowered. There are changes in moisture tensions of leaves, roots, and soils. Root growth is checked. Turgor pressure is decreased. There is shrinkage of reduction in shoot and leaf growth.

Transpirational crop demand for water will not be met. Automatical changes such as cavitation of the xylem and blockage by vapor space occur. In severe cases leaves wilt and shed. Finally, plants will die.

 Physiological effects: 

The following changes in the internal constituents of plants are noticed. Cell expansion and cell division are inhibited. Cell wall and protein synthesis decrease. In general enzyme level is decreased. Hormones such as abscisic acid and ethylene are increased whereas cytokinin is decreased. In mild stress, the rate of photosynthesis is not affected. However, severe drought inhibits photosynthesis and stomatal openings. The translocation of photosynthesis is also checked. It reduces the rate of respiration. Drought-tolerant cultivars have been shown to accumulate higher proline content than susceptible cultivars.

 Drought overcome 

Drought can be avoided by using irrigation, planting drought-resistant crops, and mulching, and also by increasing the field capacity by adding organic matter.

Water logging: 

Excessive rainfall, excessive water retention by the soil, too much field irrigation, poor drainage, high water table, etc. cause excessive soil moisture. Increasing soil moisture means decreasing the levels of oxygen in the soil. This, in turn, decreases respiration in the plant roots, causing poorer plant development and eventual death. Roots are the first to die due to the water logging, turning healthy white to brown in color and rotting from microbial attacks. At this stage, the water uptake is significantly reduced and the plants wilt. In addition, waterlogging increases the leaching of nutrients and may cause nutrient deficiencies. Excess moisture during ripening causes abnormal splitting and cracking of fruits. Some disease problems are also noticed under waterlogged conditions. To overcome water logging proper drainage should be established. 

Water logging:

·         It is caused by a rise in groundwater close to the soil surface or inadequate drainage of surface water.

·         Often, resulting from poor irrigation management.

Problem /constraints:

·         As a result of water logging, water saturates the root zone leading to oxygen deficiency.

Main causes:

·         Surface flooding/rise of the water table

·         Excess of waterlogging

        Improper  drainage design

·         No provision of drainage below the dams and other water harvesting structures,

·         excessive rain etc

Management of water-logged soils

·         Leveling of land

·         Drainage

·         Controlled irrigation

·         Check the seepage in the canals and irrigation channels

·         Flood control measures

·         Plantation of trees having a high transpiration rate

·         Selection of crops and their proper varieties 

Time of irrigation

Plants obtain water from the moisture present in the soil. When the moisture content is low in the soil, then the plant requirements are not met. Likewise, when there is excess moisture in the soil, the air supply depletes, which results in limited growth. Thus, there is a range of moisture content called the optimum soil moisture range for plant growth in between.

Field capacity (-0.01 to -0.03 mpa) is the upper limit of the optimum soil moisture range, and the wilting point (-1.5 mpa) is the lower limit. Thus, the main objective of irrigation is to store soil moisture in between these limits. As soon as irrigation is supplied, all the micro and macropores are filled with water. All the water in macropores drains to subsoil within 48 hours, and the moisture present in the micropores will be available to plants.

As the soil dries because of transpiration and evaporation loss, plants wilt during the daytime and regain normality during nighttime to conserve moisture. When the same condition continues, the plant wilts without dying. This condition is called the wilting coefficient. Thus, irrigation is supplied whenever plants require water. This is determined by the crop and soil appearance.

Crop water requirement

Water requirement is the quantity of water needed for obtaining normal crop growth and yield, which may be supplied through precipitation/irrigation or both. In other words, water requirement is the amount of water required by a crop or a diversified pattern of the crop in a given period for its normal growth under field conditions.

Crop water requirement is the amount of water needed by plants to survive, grow, and develop. The required quantity of water is either supplied naturally through precipitation or artificially through irrigation. Mathematically, the total water requirement can be illustrated as
follows:

Total water requirement = Transpiration + Transpiration + Evaporation from Soil + Water Used in Metabolic Process + Water Loss via Runoff, Percolation + Water Required for Land Preparation and other purpose