Soil water is mainly found within the pore
spaces of the soil. It competes with air for pore spaces and therefore occupies
the pore spaces not occupied by soil air. It is also found in the spaces
between the mineral grains of the soil.
The volume of water
in soil is highly dynamic. It depends on many factors, such as; the volume and
frequency of water entering the soil, rate of soil drainage, which depends on
the abundance of the large-sized pores in the soil, plant and microorganism
utilization, soil temperature and rate of evaporation etc. However, the average
amount of soil water in the ideal mineral soil is approximated 25%, fluctuating
with soil air.
Rainfall and irrigation are the major means
through which water enters the soil from the surface. However, water may enter
the soil from the underground table water through capillarity and negative atmospheric
water demand due to low relative humidity of the atmosphere.
In a well-drained soil, soil water will be
found in the micro-pores (the small-sized pores) and to some extent, in the
meso-pores (the mid-sized pores). However, flood irrigation and heavy rainfall
often leads to soil water occupying the large-sized pores (macro-pores), albeit
for a short while.
Soil water is held around mineral particles
by the force of matric suction, measured in bars. The strength of this force
varies, depending on the amount of water in the soil. Based on this variation, soil
water is classified into three categories:
–
Gravitational water: This
refers to water that has occupied all the pore spaces in the soil. In other
words, this water constitutes 50% of total composition of mineral soils (i.e. 100%
pore spaces). It is of little use to plants because it supplants soil air,
thereby impairing soil aeration. Heavy rainfall and flood irrigation often
leads to gravitational water. However, it drains freely under gravity
especially in soils with a good abundance of macro-pores. Here, suction is less
than 0.1 bars.
–
Capillary water: Otherwise
known as field capacity water, this
water is held in the micro-pores after gravitational water has drained from the
macro-pores. This water behaves according to the laws governing capillarity,
and it is useful to plants as they can absorb this water. Here, suction is
between 0.1 and 31 bars.
–
Hygroscopic water: This water
is held very tenaciously around mineral particles at suction greater than 31
bars. It exists mostly in the vapour form and is similar to water remaining in
the soil after air-drying. Higher plants cannot absorb this water but some
microbial activities have been observed in soils containing only this category
of water.
Biologically, all water held between 0.1 and
15 bars in the soil is termed available water, while that held under suction
greater than 15 bars is termed unavailable water.
From the above, it can be seen that the
strength of matric suction increases with a decrease in volume of soil water.
Soil water has some of the following functions:
–
It is involved in all soil reactions.
–
As a universal solvent, it creates soil
solution by dissolving nutrient elements, thus making them available for plant
use.
– Soil water influences the soil’s response to
soil-modifying treatments such as ploughing etc.
Further Studies in
this theme:
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