se Jongboer

Maximise
Neil du Sautoy
Agricultural Services
your soil potential
Dr Neil du Sautoy, Natural Resources Specialist of Senwes Agricultural Services, discusses
the challenges and opportunities that modern technology and research bring
to soil assessments.
Plant growth is driven by trapping
energy from the sun and using it to
convert carbon dioxide into plant
material through the process of photosynthesis.
A plant acquires carbon
dioxide by opening the stomata in the
leaf (Figure 1). When the stomata are
open, water is lost from the plant in a
process known as transpiration. Loss
of water is the natural tax that a plant
must pay to get its basic requirements!
Based on this principle, Tanner
and Sinclair showed in 1983 that daily
plant growth is directly proportional to
the amount of water a plant transpires
38 Senwester, Julie 2005
per day and inversely proportional to
the dryness of the atmosphere (i.e.
vapour pressure deficit), thus:
Daily plant growth = k*(T/V)
Where k is a plant constant known
as the dry matter water ratio, T is transpiration
in mm and V is the vapour
pressure deficit.
Put numbers to
intuitive observations
This principle, coupled with a
series of other algorithms describing
Figure 1: Diagrammatic representation of transpiration – open stomata in a leaf
allow carbon dioxide intake, whilst losing water.
crop growth processes is the basis of
research done by the University of
Pretoria (funded by the Water
Research Commission) to establish
the Soil Water Balance Model. This
research has shown that by following
the water balance in the soil-plantatmosphere-continuum,
a very good
indication of the production potential
of a soil-climate unit can be derived.
Research done by the University of
the Free State has also shown the
value of including climate in assessments,
i.e. the SWAMP model.
The value of these models are
that, with the use of modern technology
such as desktop computers and
automated climate monitoring systems,
it is possible to put numbers to
intuitive observations. For example
we know maize yields in the
Bethlehem district are lower than
other regions due to lower heat units
while the Schweizer-Reneke district
has ample heat units, but rainfall is a
limitation.
New opportunities and
challenges for soil
cartographers
These models have opened new
opportunities, as well as challenges,
to soil cartographers when assessing
lands. The opportunity is that by
adopting these techniques soil cartographers
can derive more accurate
and scientifically based estimates on
yield potential of soils. This is essential
for effective precision farming and
sound management of the natural
resources.
The challenge is to numerically