Page 55 - Agriculture_Form_Three

Agriculture for Secondary Schools

Management of soil acidity and alkalinity

The ideal soil pH for growth of most plants as well as biological and chemical
reactions is between 6.5 to 7.5 Any deviation from these pH ranges should be
managed. The management can be either by raising the pH values or lowering it.
pH values are raised by liming and lowered by acidification. Liming is the process
of adding any material to the acid soil for the purpose of neutralising it in order to
create a favourable condition for plant growth and microbial activities. Substances
which are commonly used for liming are calcium and magnesium salts of weak
acids and basic compounds such as oxides of calcium and magnesium. Oxides and
carbonates of calcium and magnesium are called agricultural limes. When such
substances are added to the acidic soil, they raise the exchangeable cation level in
the soil. Acidification is the build-up of hydrogen cations which reduces the soil
pH. This is done by adding ground sulphur, sulphuric acid, phosphoric acid, ferrous
sulphate or aluminium sulphate. These materials are, however, always expensive
and unpleasant to handle.

Note: Where it is not possible to correct soil pH at all due to various reasons, for
example, economic constraints, crops tolerant to an existing soil reaction
may be grown with advantage. To implement this, you should have to seek

advice from your local extension worker or other agricultural experts.

Activity 2.1
Perform the following tasks.
1. Collect soil samples from different areas at your school farm and determine
their pH using different methods by following procedures you have learnt.

2. Outline the lessons you have learnt from this activity.

Carbon to nitrogen ratio
Carbon to nitrogen ratio is closely related to breakdown or decomposition of organic
matters in the soil. It is defined as a ratio of the mass of carbon to the mass of
nitrogen in organic residues. Carbon to nitrogen ratio is abbreviated as C/N or C:N
ratio. The C/N ratio of a material is obtained by dividing the mass of carbon by that
of nitrogen of a particular material. The C/N ratio is generally greater than one,
that is, organic residues have more carbon than nitrogen. So, it is often written as a

number; not as a ratio.

The mass of carbon and nitrogen in a material is determined by using an elemental
analyser machine. However, the machine method is quite sophisticated and expensive.

Student’s Book Form Three
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AGRICULTURE FORM 3 9.11.2022.indd 44 10/01/2025 12:31