Page 48 - Agriculture_Form_Three

Agriculture for Secondary Schools

a large amount of hydrogen ions is adsorbed on the soil colloids, the amount of
hydrogen ions in the soil solution also becomes large and vice versa. In this way,
adsorbed hydrogen ions act as a source of hydrogen ions to the soil solution.
This usually happens when soil reaction is moderate to slight acid.

(d) Minor sources of hydrogen ions in soil: Roots release of hydrogen ions during
the absorption of cations, nitrification of ammonium from fertilisers, organic
matter mineralisation, reaction of sulphur compounds as well as rainwater and
acid rain are among other sources of hydrogen ions in soil.

Soil acidity occurs naturally very slowly as soil is weathered, but it is accelerated by
some agronomic practices. Among the causes of soil acidity are:
(a) Heavy rains or irrigation may render a soil acid due to leaching of basic cations,
for example, Ca , Mg , K , and Na .
2+
+
+
2+
(b) Acids formed in soil through various means, for example, carbon dioxide
evolved during root respiration and microbial activities form carbonic acid.
Also, some acids are produced during decomposition of soil organic matter. In
addition, small amounts of anions such as NO , NO , H PO , HPO , SO ,
2-
-
-
2-
-
3
4
2
2
4
4
and Cl , which are naturally present in soil solution, contribute to soil acidity as
-
they form acids when they react with ionised water.
(c) Prolonged use of acid-forming fertilisers, for example, ammonium sulphate and
ammonium chloride. This is because ammonium nitrogen is readily converted
into nitrate and hydrogen ions in the soil. If nitrate is not taken-up by plants, it
can leach away from the root zone leaving behind hydrogen ions thus increasing
soil acidity. Each ammonium ion in these fertilisers, when nitrified, releases
hydrogen ions in the soil as shown in the following equation.
2NH + 3O Enzymatic 2NO + H O + 4H + Energy
−
+
+
4 2 2 2
oxidation
Sources of hydroxyl ions in soil and causes of soil alkalinity
Hydroxyl ions originate from many chemical reactions which take place into the soil
and release hydroxyl ions, for example,
H O + Ca 2+ Soil colloidal ! ⇀! H + Soil colloidal Ca + 2OH −
2+
2 particle ↽ !! + particle
Water H Hyrdroxyl
ions
When such reactions take place, the hydroxyl ions which are formed, are added to
the soil solution. Generally, when the amount of basic cations (for example, Ca ,
2+
Mg , K , and Na ) that are adsorbed on the soil colloids is larger compared to the
+
+
2+
amount of hydrogen ions adsorbed on such colloids, there is a tendency for many
hydroxyl ions to be formed. As a result, the soil becomes alkaline in reaction.
Student’s Book Form Twos Book Form Three
Student’
37

AGRICULTURE FORM 3 9.11.2022.indd 37 10/01/2025 12:31
AGRICULTURE FORM 3 9.11.2022.indd 37
10/01/2025 12:31