Soil Moisture Constants

Definition

Soil moisture constants, also known as soil water constants or hydro-physical constants, refer to specific levels of water content held in the soil at various potentials (suction pressures). These constants represent critical thresholds that dictate how much water is available for plant growth, how much is held tightly by soil particles, and how much drains away due to gravity.

Main Content

1. Saturation (Maximum Water Holding Capacity)

This is the state where all soil pores (both macro and micro) are completely filled with water, and there is no air present in the soil profile.
At this stage, the soil water potential is zero (or atmospheric pressure), and water will freely drain away if a drainage path exists.

2. Field Capacity

This represents the amount of water remaining in the soil after the excess gravitational water has drained away, typically 24 to 48 hours after heavy rain or irrigation.
It is considered the upper limit of water availability for plants and is held at a suction pressure of approximately -0.1 to -0.3 bar.

3. Permanent Wilting Point

This is the minimum amount of water in the soil at which a plant can no longer extract sufficient moisture to meet its transpiration needs, causing the plant to wilt and not recover even in a humid atmosphere.
It is held at a high suction pressure, typically around -15 bars, where water is tightly bound to soil particles.

Soil Water Spectrum:
|----------------------------------------------------------|
0% (Dry)     PWP         Field Capacity      100% (Saturation)
               |               |                      |
      [Unavailable] [Available Water]      [Gravitational Water]

Working / Process

1. Determination of Saturation

A soil sample is placed in a saturation container where it is allowed to absorb water from the bottom via capillary action until the surface glistens.
The weight is recorded, and the sample is then oven-dried at 105°C to calculate the moisture percentage by weight.

2. Determination of Field Capacity

A representative soil plot is saturated with water and covered with plastic to prevent evaporation.
After 48 hours (when drainage has effectively ceased), soil samples are taken from the root zone depth, weighed, dried, and weighed again to determine the constant.

3. Determination of Permanent Wilting Point

Using a pressure plate apparatus, a saturated soil sample is subjected to a pressure of 15 bars.
The pressure forces the loosely held water out of the soil until only the water bound at the wilting point remains, which is then measured using the oven-dry method.

Advantages / Applications

Irrigation Scheduling: Farmers use these constants to calculate exactly when to irrigate to prevent plant stress and optimize water usage.
Crop Selection: Helps in choosing crops that can thrive in specific soil types by comparing the soil's "Available Water Holding Capacity" with the plant's water needs.
Drainage Design: Engineers use saturation and field capacity data to design efficient subsurface drainage systems that prevent waterlogging.

Summary

Soil moisture constants are the specific points on the soil moisture retention curve that define the relationship between soil water tension and water content. By identifying levels like Field Capacity and Permanent Wilting Point, we can determine the "Available Water" that plants rely on for survival and productivity.

Important terms to remember: * Gravitational Water: Excess water that drains away. * Available Water: The range between Field Capacity and Permanent Wilting Point. * Hygroscopic Water: Water held so tightly by soil particles that it is entirely unavailable to plants.