2. Water makes up approximately 90% of a plant's mass and performs many functions,
it is required for seed germination, it carries minerals into and through the plant,
transports photosynthates and other biochemicals, cools the plant by transpiration and
also involved in photosynthesis.
Water is present in soil in three forms:
1. Gravitational water is present in macro pores. It moves downward by the action
of gravity and this water is not available to plants for their growth and
development because it rapidly leaches down from the soil.
2. Hygroscopic water is the moisture absorbed by the soil colloids from the
atmosphere. This moisture is held very tightly by the soil colloids hence it
becomes unavailable for plants.
3. Capillary water is present in the soil at field capacity. After rain soil becomes
saturated, both macro and micro pores are filled with water. Water remaining in
the soil after gravitational flow is referred as capillary water and it is very easily
available to plants for their growth and development. At this situation micro
pores are filled with water while macro pores are filled with air.
Water is present in the soil in liquid and air phases and moves through water filled
spaces and air filled spaces respectively. When rain occurs some water drains out and
some percolates into the soil. It reaches the ground water passing through water table.
Three forces I.e. gravity, cohesion and adhesion act on the water molecule and are
responsible for water movement in the soil. Cohesion forces are between water
molecules. Water molecules are attached to each other by these forces while the
adhesion forces are attractions between water and soil particles and effect the water
movement in soil. Gravity forces move water downward. Cohesion and adhesion
together are called capillary rise and move water in all directions. These are the main
responsible forces that cause water movement in soil.
During normal conditions micro pores are filled with water while macro pores are filled
with air and water is easily available to plants. In saturated soil conditions all the pores
are filled with water and water become unavailable to plants due to lack of oxygen. In
too dry conditions all the pores are filled with air and there is no water in the soil for
plant uptake.
3. Water is available to plants very easily at field capacity due to sufficient amount of
water and air in the soil. Water between field capacity and saturation capacity is not
available to plants due to lack of air and water below field capacity is very tightly bound
with soil particles thus unavailable to plants.
Cohesion and adhesion forces are indirectly related to water availability. When
cohesion and adhesion forces are strong there will be no water availability for plants.
If salts are there in soil, these bound the water molecules and restrict the water
movement from soil to the plant. Plant with deeper and strong root system are able to
extract water from depth of soil. Development of hard pan below soil also stop water
movement and root penetration. Friction force between soil and water called hindrance
also plays a vital role in restriction of water movement in soil. Different soils have
capacity to hold water, clay soils hold more water than sandy soils therefore more
water isavailable to plants in clay soils. Water potential plays a key role inwater uptake
form soil by plant roots. Water moves from higher to lower potential. Water potential
of root is lower than that of soil.
Soils should be carefully studies before irrigation for effective use of water.
