Page 56 - Agriculture_Form_Three

Agriculture for Secondary Schools

Therefore, this method is not elaborated here. It is much simpler to check the C/N
ratio of your organic residues in existing databases. If you cannot be able to find
your specific organic residue in the database, you can just try to find which of the
existing organic residues in the database is most similar to yours and consider the
given values. Another more rough approximation of C/N ratio of organic residues
is to categorise the materials between carbon-rich and nitrogen-rich materials. In
most cases, carbon-rich materials tend to dry and become brownish in colour, for
example, straws, stovers and stalks whereas nitrogen-rich materials tend to be wetter,
for example, haulms. It is assumed that carbon-rich materials have a C/N ratio higher

than 20 whereas nitrogen-rich materials have a C/N ratio lower than 20.

The C/N ratio is of great agricultural importance because it determines the rate of
decomposition of organic matter and the availability of nitrogen to plants. Organic
matter is broken down by microorganisms which use carbon for energy. These
microorganisms also have a high requirement for nitrogen. Microorganisms have
a requirement of about one nitrogen atom for each 25 carbon atoms. This is a C/N
ratio of 25. If the organic matter has a higher C/N ratio, microorganisms will need
more nitrogen than it is in the organic matter and they will take it from the soil.
Microorganisms are more efficient than crops in obtaining nitrogen from the soil.
If there is no enough nitrogen for both microorganisms and crop, the crop will not
obtain what it needs thus it can die.

If the organic matter with a high C/N ratio is applied to the soil shortly before
planting a crop, additional nitrogen may be needed to ensure that the needs of both
microorganisms and crop are met. The organic matter with a C/N ratio of less than
25 is not a problem and in some cases can contribute nitrogen for crop use. Residues
of leguminous crops have C/N ratio ranges from 15 - 19 while that of cereal crops
ranges from 44 - 80. This makes residues from leguminous crops more suitable for
adding nitrogen to crop rather than cereal crops. Generally, the decomposition of
residues from crops with high C/N ratio may reduce the quantity of nitrogen present
in the soil as microorganisms will use it in order to decompose them.

The C/N ratio of organic material added to the soil has to be considered due to its
influence on decomposition of organic matter in the soil. The completely decomposed
organic matter or humus has several effects. It improves the physical properties
of the soil. It increases the CEC and water-holding capacity. It also contributes to
the structural stability of sandy soils by helping to bind particles into aggregates.
Soil organic matter, of which carbon is a major part, holds a great proportion of
plant nutrients including cations and micro-elements that are of importance to plant

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