Page 44 - Agriculture_Form_Three

Agriculture for Secondary Schools
Agriculture for Secondary Schools

Since they are very small, they have a very large total surface area per unit volume
on which chemical reactions can take place. Furthermore, clay and humus particles
usually remain scattered and move over a large area in the soil solution. Clay and
humus particles are therefore called soil colloids. Clay colloids are also called
mineral colloids while humus colloids are called organic colloids. Soil colloids have

two major chemical characteristics: They are negatively charged and exhibit ion
exchange. Most chemical reactions in the soil occur on colloid surfaces because of
their charged surfaces. Due to their chemical make-up and large surface area, they
can attract ions (charged particles) within the soil solution.

Ion exchange in soil

Ions are atoms or groups of atoms, that is, molecules which have an electrical charge.
The ions with positive (+) charge are termed as ‘cations’ while those with a negative
(-) charge are called ‘anions’. Plants take up nutrients from the soil either as cations
or anions. Many of the nutrient elements are cations. These include ammonium
(NH ), calcium (Ca ), magnesium (Mg ), potassium (K ), iron (Fe ), manganese
2+
+
2+
3+
+
4
(Mn ), zinc (Zn ) and copper (Cu ) ions. Other cations of importance are hydrogen
2+
2+
2+
(H ) and aluminium (Al ). Cations are attracted to negatively charged surfaces of
+
3+
clay and organic (humus) colloids. This attraction is called adsorption and the sites
where cations attach to particles are cation-exchange sites.
Cations can move from the adsorption sites on colloids into the soil solution and
vice versa where they are available for root uptake and are also subject to leaching.
Figure 2.1 (a) shows the cation exchange in soil while Figure 2.1 (b) shows how
plants obtain mineral nutrients from soil through cation exchange. Observing the
figures, you can notice that cations dominate the exchange sites because the colloid is
primarily negatively charged. You can also notice that cation exchange is important
for maintaining adequate quantities of, among other cations found in the soil, plant-
available calcium (Ca ), magnesium (Mg ) and potassium (K ). In addition, it can
+
2+
2+
be noted in Figure 2.1 (b) that hydrogen ions (H ) help to make nutrients available
+
by displacing the cations.

Student’s Book Form Twos Book Form Three
Student’
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AGRICULTURE FORM 3 9.11.2022.indd 33 10/01/2025 12:31