Introductory Agrometeorology/Soil moisture

Objectives of the chapter

• To understand:
  • soil moisture balance
  • significance in crop production

Soil Moisture

• water present in soil
• water holding capacity depends upon the porosity
• Moisture content can be determined by following methods:

a) Gravimetric method (oven dry method) b) Electric resistant blocks c) Tensiometer

Soil water classes

1) Gravitational water

a) Water contained at macro pores are affected by gravity. b) It is the water in excess of hygroscopic and capillary water. c) not available to the plants. d) it is the amount of water between saturation capacity and field capacity. So, FC is lower limit of gravitational water.

2) Capillary water

a) Water retaining in micro pores after drain off of gravitational water. b) It is held in capillary pores by surface tension. c) It is the principle source of water for plant growth. d) It is the amount of water present between FC and PWP. So the upper limit is FC and lower limit is PWP.

3) Hygroscopic water

Water absorbed by dry soil particles from atmosphere and is held as a very thin film on the surface of soil particles due to adhesive force. Its upper limit is PWP. It is not available to plants.

Soil moisture content (SMC)

It is the measurement of the water occupying the pore spaces between soil particles, determined by a series of wet and dry weights

Soil moisture characteristics (SMCs)

It is the curves descriping the water storage capacity of a soil as it is subjected to various soil suctions

Soil moisture constant (SMCon)

It is the imaginary concept of soil moisture content that is named according to available to the plant.

a) Saturation capacity (SC)

Maximum water holding capacity of soil, all pores filled with water. Soil moisture tension = 0 atm.

b) Field capacity (FC)

It is the maximum water holding capacity against gravity. It is the upper limit of water available to the plants. Also called as field carrying capacity, normal moisture capacity, and capillary capacity.

Tension of FC = 1/10 to 1/3 atm from coarse textured soil to fine textured soil.

After 24 hrs. to (3- 4) days of water drained out from saturation capacity, depending upon soil type, field comes to field capacity.

c) Permanent wilting point (PWP)

It is the lower limit of water available to the plant which is also called as coefficient or permanent wilting percentage Tension 7 to 32 atm (coarse to clayey soil).

d) Temporary wilting

Plant wilted due to high sunlight intensity and wind. Plant can’t meet the transpiration. When there is cool cells becomes turgid and regains its shape, it doesn’t have relation to moisture. So, PWP is the amount of water that plant is permanently wilted I.e. Cannot regain its turgidity when exposed to saturated atmosphere (100 % ). Water availability of coarse textured soil is 1/10 to 7 atm and for clay soil is 1/3 to 32 atm.

e) Ultimate wilting (UW)

Plant is completely wilted and dies at the tension˃ 60 atm.

f) Wilting range (WR)

If little water add to PWP, plant can continue its growth and development. Further depletion of soil moisture results plant dead, no effect of any supplied moisture. So at WR, plant hardly derives water and more tension will require.

PWP is nearly equal to FC / (2 to 2.4)
When silt is low, PWP= FC / 2
when silt is high, PWP = FC/2.4
Ultimate wilting (UW) = 67% of PWP

i.e. Hygroscopic coefficient = 67% of PWP.

Terms

Moisture Equivalent

When initially saturated soil sample being subjected to centrifugal force of 1000 times against gravity for 30 minutes, the remaining moisture in soil sample is expressed as moisture equivalent, which is determined by gravimetric method.

Soil moisture characteristic curves (SMCC)

The functional relationship of the soil moisture tension with respect to its available moisture content of the soil is called as SMCC.

Infiltration

It is the process of entry of water from the surface to soil, downward movement of water. Infiltration rate (mm/hr, mm/min, cm/hr)

Percolation

At saturation or nearly saturated soil. Downward movement of water occurs called percolation.

Seepage

It is the infiltration of vertically or downward movement as well as laterally movement into soil or from source of water supply(irrigation canal).

Signifiacance

Water Availability Theory

A. Equal amount of water is available between PWP and FC.

B. When water used by plant from FC to PWP i.e. Moisture decreases, plant growth rate also decreases.

C. There is critical moisture level in available moisture range and beyond this, there is decreasing availability of moisture to plant as PWP is reached. Significant decrease in growth of plant is noticed below this critical moisture tension(CMT).yield of several crops are not reduced if irrigation is scheduled at 50% available moisture depletion.