02-08-2014, 11:27 AM
SOIL MOISTURE CONSTANT'S AND IT'S IMPORTANCE
IN CROP PRODUCTION
INTRODUCTION
For a particular soil, certain soil water proportions are defined which dictate whether the water is available or not for plant growth. These are called the soil water constants.
Water contents under certain standard conditions are referred as soil moisture constants.
Under field conditions, water content of soil is always changing constantly with time and depth of soil and is not static or constant. However, the concept of soil moisture constants greatly facilitates in taking decision in irrigation.
While studying soil water and discussing its availability or other wise to plant, some specific terms called as soil moisture constants are used. A brief explanation of some important and commonly used terms is given below and the methods of expressing them are indicated in the table below.
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.
Soil Moisture Constants
For a particular soil, certain soil water proportions are defined which dictate whether the water is available or not for plant growth. These are called the soil water constants.
Saturation
Soil is a porous medium and when all the pores of the soil are
filled with water, it is referred to as saturated soil.
Saturation percentage or the water held by soil can be
determined by: Bringing the soil core/soil sample in contact with
water column at a height corresponding to the midpoint of the
sample and determining the moisture held by soil, or by using
the formula for porosity:
P = {1 - BD/PD} x 100
Where,
P is the porosity in percent, BD is bulk density (g cm-3) and PD is particle density (g cm-3) of soil. PD can be generally assumed to be 2.65 (g cm-3) for mineral soils.
It can also be estimated by determining the moisture content of the core of which saturated hydraulic conductivity has been determined without allowing any water to drain out of the sample.
2.Field capacity
It is the term used to describe the maximum amount of water that a soil will retain after allowing free drainage.
It does not generally correspond to a fixed soil water suction (or potential) varying from 1/10 bar for coarse textured soils to 1/3 bar for fine textured soils.
It is, therefore, best estimated in the field by saturating the root zone and determining soil moisture after free internal drainage ceases. It takes about 24 to 36 hrs in coarse and 2-3 days in medium textured soils.
Conclusion:
1. It is important factor in management of irrigation scheduling and determination various soil moisture levels present.
2. As soil water content decreases plant suffer water stress and reduced yield and/or quality well above permanent wilting point because of the increasing effort required to extract water from the soil.
3. Soil texture, structure, layering, and chemistry along with crop type, rooting characteristics, stage of development, as well as environment are just some of the many factors that can impact on available water capacity.
4. The procedures for more accurate determination of PAW are often complicated, requiring specialized equipment and an extensive number of measurements, because it is a function of the interactions between the plant, the soil, and the environment.