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MADE POSSIBLE BY
THE MAIZE TRUST
14
O
NE COLUMN IN YOUR SOIL TEST REPORT HEAD-
ED CATION EXCHANGE CAPACITY (CEC) IS
OFTEN OVERLOOKED OR NOT UNDERSTOOD.
THIS IS A VERY IMPORTANT ASPECT OF SOILS
TO TAKE NOTE OF IN ORDER TO MONITOR SOIL
FERTILITY, EFFECT OF LIME APPLICATIONS OVER THREE
YEARS OR MORE, THE BALANCE OF MINERALS HELD IN
THE SOIL AND THE AVAILABILITY OF THE FERTILISERS AP-
PLIED AS WELL AS GENERAL SOIL FERTILITY LEVELS.
SOILS
Soils are made up of four basic components – minerals, air, water
and organic matter. A typical soil might be made up of 45% minerals,
25% water, 25% air and organic matter from 2% to 5%. The mineral
portion consists of three kinds of particle sizes classified as sand, silt
or clay. The representative portion of these is used to identify a soil
as sand, loamy sand, sandy loam, fine sandy loam, loam, silty loam,
silt, silty clay loam, clay loam, and clay.
SOIL ORGANISMS
Within the complex structure shown above the soil is teeming with
many different organisms including bacteria, actinomycetes, moulds,
algae, protozoa, nematodes, insects, worms, and plant roots. The
mass of all these organisms in the top 175 millimetres of topsoil can
be about 7 000 kg/ha. The soil is thus a dynamic living environment
into which we plant our crops together with chemical fertilisers.
ORGANIC MATTER AND HUMUS
Organic matter is the fraction of soil that is made up of both the living
organisms above and then once living plant residues from a previous crop
or grass ley in various stages of decomposition. Humus is a very complex
long chained molecule that can only be examined under a microscope that
gets built up from the final decomposition of organic matter. Humus can be
made up of brown or grey portions and can be seen appearing in soils after
minimum tillage or conservation tillage methods.
Look at a soil that has been abused by too much disc tillage with no
plant residues and very little organic matter and compare it, if possible,
to a neighbour or land nearby under conservation tillage. Compare the
two by digging profile holes. The humus layers can be seen. Keep in
mind it takes several years of conservation tillage to achieve a constant
presence of humus. Soil containing 4% organic matter together with the
humus can supply up to 200 kg of nitrogen to a crop. What a potential
saving in fertilser nitrogen (N) costs.
If you can research Dr William Albrecht’s papers on soils it will
be very enlightening and rewarding and help in your knowledge of
how to improve the soils on your farm. Soils and soil management is
indeed a complex subject.
WHAT ARE CATIONS?
Plant nutrients are held in the soil sand, clay and silt complex or colloid.
In a well-balanced soil the colloid complex can hold the main and other
What does CEC on
your SOIL TEST mean?
Richard McPherson, Pula Imvula
contributor. Send an email to
richard@agrimetrix.co.za
The CEC is thus a measure of how good your
soil is at holding the essential plant nutrients.