In the last article we focused on soil moisture and how it is stored in the soil; adhesion, cohesion and capillarity. But how does this relate to the terms: saturation, field capacity and permanent wilting point?
When soil moisture is stored in the soil it is possible to
measure both the amount (content V%) and the tension. The soil tension forms
the basis of the following soil moisture parameters: saturation, field capacity
and permanent wilting point.
A soil is saturated when all pores (micro and macro) are filled with water and no air remains in the soil. At saturation there is free water in the soil profile. Gravity will cause water to drain from macro pores and saturation is therefore a temporary state.
When a soil is at field capacity, water is held by adhesion
to soil particles and capillarity in micro pores. Field capacity is reached when rapid drainage decreases
(Figure 1).
On your Vantage NZ soil moisture plots the field capacity is determined
for each sensor depth, then summed to determine the l field capacity for the
active root zone. This allows for soil texture changes throughout the profile
and provides you with a field capacity unique to the sensor site.
Evapotranspiration and drainage (to
a much lesser extent) will cause the soil to dry below field capacity. During
this process water is removed from all but the smallest micro pores. The
permanent wilting point (PWP) varies depending on plant conditions, plant type
and soil texture (Figure 2). Nevertheless, the soil water potential at which
permanent wilting occurs is considered to be 1500 cba.
Available water (AW) is the amount of water held in the soil
between field capacity and wilting point for a defined depth of soil and is
expressed as V% or millimetres (mm).
AW = FC - PWP
Not all the available water is equally (readily) available
to plants. Water becomes more difficult for plants to extract the closer the
water potential comes to permanent wilting point. This is because the reminding
water is bound to the soil at increased tension.
Plants need to take up enough water to satisfy their
transpirational demand and sustain optimum growth rates. For every kilogram of
dry matter (DM) produced, a plant must transpire between 200 – 500 litres of
water.[1]
For plants to obtain this quantity of water from the soil, water needs to be
readily available. Water is said to be readily available when plant growth is
not restricted by water availability. Stress point is the point at which plants
can no longer extract water at potential rates. On a soil moisture plot this
will be demonstrated by a change in water use, i.e. a change in slope of the
soil moisture trace (Figure 3).
[1] McLaren, R.G. and Cameron, K. C. (2000). ‘Soil Science’, Sustainable production and environmental protection. Second edition, Oxford University Press. Page 99.