soil water plant relation ship

Instructional objectives
On completion of this less
on, the student
shall learn:
1. The soil and water system t
hat is needed for plant growth
2. Classification of soils
with regards to agriculture
3. Classification of wa
ter held within soil pores
4. Soil – water constants and their significance
5. Watering interval for crops
6. Importance of wate
r for plant growth
3.2.0 Introduction
Both soil and water are essential for plant gr
owth. The soil provides a structural base to
the plants and allows the root
system (the foundation of th
e plant) to spread and get a
strong hold. The pores
of the soil within the
root zone
hold moisture which clings to the
soil particles by surface tension in the driest
state or may fill up t
he pores partially or
fully saturating with it useful
nutrients dissolved in water, e
ssential for the growth of the
plants. The roots of most plants also r
equire oxygen for respir
ation. Hence, full
saturation of the soil pores leads to restricted root growth for these plants. (There are
exceptions, though, like the rice plant, in
which the supply of ox
ygen to the roots is
made from the leaves through aerenchyma ce
lls which are continuous
from the leaves
to the roots).
Since irrigation practice is essentially,
an adequate and timely supply of water to the
plant root zone for optimum cr
op yield, the study of the inter relation ship between soil
pores, its water-holding capacity and plant wa
ter absorption rate is fundamentally
important. Though a study in de
tail would mostly be of im
portance to an agricultural
scientist, in this lesson we discuss the essentials which are important to a water
resources engineer contem
plating the development
of a command area through
scientifically designed irrigation system.
3.2. 1 Soil-water system
Soil is a heterogeneous mass consisting of
a three phase system of solid, liquid and
gas. Mineral matter, consisting of sand, silt
and clay and organic ma
tter form the largest
fraction of soil and serves as a framework
(matrix) with numerous pores of various
proportions. The void space within the solid pa
rticles is called the
soil pore space.
Decayed organic matter derived from the plant and animal remains are dispersed within
the pore space. The soil air is
totally expelled from soil when
water is present in excess
amount than can be stored.