Page 49 - Agriculture_Form_Three

Agriculture for Secondary Schools

The importance of soil pH
Soil pH is important for the following reasons:
(a) It influences the suitability of the soil for the growth of plants and
microorganisms
The suitability of the soil as a medium for growth of microorganisms depends on its

pH. The most favourable range for the majority of crop plants is from slightly acidic
to slightly alkaline pH. However, acidic pH is sometimes desired for controlling
diseases. Low pH restricts the growth of organisms that cause diseases. Some crops,
for example, tea, cucumbers, broccoli, squash, onions and blueberries, require acidic
soil conditions.
Since crop plants vary in their preference or tolerance to acidity or alkalinity, soil pH
helps in selecting a suitable crop. As it is for crop plants, soil organisms too live and
perform well in a certain pH value. However, most soil organisms live and function
well when the pH value of the soil is neutral or slightly acidic, that is, it is between
pH 6.5 and 7.5. Therefore, most crop plants and microorganisms grow best in soil
with a fairly neutral pH, that is, between 6.5 and 7.5. When the pH level is lower
than 5.0 or higher than 8.0, many plants and microorganisms would not grow well

as expected.
(b) It affects the decomposition of organic matter in soil
The decomposition of organic matter slows down under extreme acidic or alkaline

soil conditions. This is due to decreased microbial activities in such conditions.
(c) It affects the solubility and availability of nutrient elements in soil
When the soil pH is extremely low, solubility of iron, manganese, boron and
aluminium is increased to the extent that they become toxic to some plants. When
the pH is raised, they become less soluble. Similarly, solubility of calcium and
magnesium is reduced hence they are deficient in extremely acidic soils while in

alkaline soils their solubility increases and they are plenty. Likewise, at very low
or high soil pH values, phosphorus reacts with other substances in the soil to form
insoluble compounds. As a result, phosphorus cannot be absorbed by plant roots.
When this occurs, the phosphorus is said to be fixed and the process is termed as
phosphorus fixation. Phosphorus fixation occurs in two main ways. At low pH, the
soluble iron, manganese and aluminium react with phosphorus to form complex
substances which are insoluble. At high soil pH values, calcium in the soil reacts
with phosphorus to form insoluble substances. Therefore, these reactions make
phosphorus unavailable to plants.

Student’s Book Form Three
38

10/01/2025 12:31
AGRICULTURE FORM 3 9.11.2022.indd 38
AGRICULTURE FORM 3 9.11.2022.indd 38 10/01/2025 12:31