Soil water

Water contained in soil is called soil moisture. The water is held within the soil pores. Soil water is an important component of the soil which influences soil organisms and plant growth. Unlike open source water like ponds and lakes, soil water exists in different forms. Some forms are available to plants while others are not. Soil water with dissolved salts is called soil solution. Soil solution supplies nutrients to growing plants.

Importance of Soil Water 

• Water is essential for life including soil organisms and plants.

• Soil water serves as a solvent and carrier of food nutrients for plant growth.

• Yield of crop is first determined by the amount of water available in soil followed by the availability of other food nutrients.

• Soil water itself is a nutrient.

• Soil water regulates soil temperature 

• Soil water is essential for soil forming processes and weathering

• Microorganisms require water for their metabolic activities

• Soil water helps in chemical and biological activities of soil

• It is a principal constituent of the growing plant

• Water is essential for photosynthesis

Classification of soil water

Soil water has been classified from a physical and biological point of view as Physical classification of soil water, and biological classification of soil water.

Physical classification of soil water

i) Gravitational water ii) Capillary water and iii) Hygroscopic water 

1. Gravitational water:

• Gravitational water occupies the larger soil pores (macro pores) and moves down readily under the force of gravity.

• Water in excess of the field capacity is termed gravitational water. Gravitational water is of no use to plants because it occupies the larger pores.

• It reduces aeration in the soil. Thus, its removal from soil is a requisite for optimum plant growth. Soil moisture tension at gravitational state is zero or less than 1/3 atmosphere.

2. Capillary water:

• Capillary water is held in the capillary pores (micro pores). Capillary water is retained on the soil particles by surface forces.

• It is held so strongly that gravity cannot remove it from the soil particles. The molecules of capillary water are free and mobile and are present in a liquid state.

• Due to this reason, it evaporates easily at ordinary temperature though it is held firmly by the soil particle; plant roots are able to absorb it. Capillary water is, therefore, known as available water. The capillary water is held between 1/3 and 31 atmosphere pressure.

3. Hygroscopic water:

• The water that held tightly on the surface of soil colloidal particle is known as hygroscopic water.

• It is essentially non-liquid and moves primarily in the vapour form.

• Hygroscopic water held so tightly (31 to 10000 atmosphere) by soil particles that plants can not absorb it. Some microorganism may utilize hygroscopic water.

B. Biological Classification of Soil Water

There is a definite relationship between moisture retention and its utilization by plants. This classification based on the availability of water to the plant. Soil moisture can be divided into three parts.

i. Available water: The water which lies between wilting coefficient and field capacity. It is obtained by subtracting wilting coefficient from moisture equivalent.

ii. Unavailable water: This includes the whole of the hygroscopic water plus a part of the capillary water below the wilting point.

iii. Super available or superfluous water:

The water beyond the field capacity stage is said to be super available. It includes gravitational water plus a part of the capillary water removed from larger interstices. This water is unavailable for the use of plants. The presence of super-available water in a soil for any extended period is harmful to plant growth because of the lack of air.

Measuring Soil Moisture

     i) Direct measurement of moisture content

    ii) Measurement of soil moisture potential (tension or suction)

Measuring soil moisture content in laboratory 

1. Gravimetric method:

• This consists of obtaining a moist sample, drying it in an oven at 105°C until it losses no more weight and then determining the percentage of moisture.

• The gravimetric method is time consuming and involves laborious processes of sampling, weighing and drying in laboratory.

2. Electrical conductivity method:

• This method is based upon the changes in electrical conductivity with changes in soil moisture.

• Gypsum blocks inside of with two electrodes at a definite distance are apart used in this method. These blocks require previous calibration for uniformity.

• The blocks are buried in the soil at desired depths and the conductivity across the electrodes measured with a modified Wheatstone bridge.

• These electrical measurements are affected by salt concentration in the soil solution and are not very helpful in soils with high salt contents.

3. Pressure plate and pressure membrane apparatus:

• The pressure plate apparatus is used for moisture measurements up to 1 atm. and pressure membrane apparatus is used for moisture measurements up to 20 atm.

4. Neutron scattering method:

• Fast neutrons are emitted by neutron emitters like radium-beryllium or americium-beryllium.

• The fast moving neutrons interact with hydrogen atoms and get slowed down. The number of slow moving neutrons returned back after colliding with hydrogen is proportional to the quantity of water present in soil as water is considered to be the sole source of hydrogen atoms.

• The slow moving neutrons are detected by proportional or scintillation counters and interpreted in terms of moisture percentage at different depths.

5. Profile probe and theta probe:

This new technology uses electro-magnetic radiation. The emitted radiation is intercepted by soil water and the radiation reflected is detected and interpreted in terms of per cent moisture content in different layers.

6. Measuring soil moisture potential in-situ (field)

Suction method or equilibrium tension method:

• • Field tensiometers measure the tension with which water is held in the soils. They are used in determining the need for irrigation.

  • The tensiometer is a porous cup attached to a glass tube, which is connected to a mercury monometer.

  • The tube and cup are filled with water and cup inserted in the soil. The water flows through the porous cup into the soil until equilibrium is established.

  • These tension readings in monometer, expressed in terms of cm or atmosphere, measures the tension or suction of the soil.

  • If the soil is dry, water moves through the porous cup, setting up a negative tension (or greater is the suction).

  • The tensiometers are more useful in sandy soils than in fine textured soils and can be used only up to a tension of 1 bar.

  • At tensions above one bar the water column is broken and air enters the ceramic cup. Once the air gets entrapped in the tensiometer, the reliability of readings is questionable.