Page 118 - Enzymes in Tropical Soils

106 Enzymes In Tropical Soils

+
-
This decomposition reaction consumes H ions and produces OH ions; therefore, it
+
increases the soil pH. In general, the higher the concentrations of H ions or the
lower the soil pH, the more intensive the decomposition of calcium phosphates.

4.6

Soil pH 4.4

4.2

4
0 200 400 600 800
Addition of KH PO
2 4
-1
(kg P ha )

Fig. 7.1. The effect of P-fertilizer on soil pH (Adapted from Salam, 1997b).

The effect of liming on soil pH is different depending upon some soil
properties. Among these factors, the soil buffering capacity and all factors
affecting this chemical property such as soil texture are of great importance and
must be understood. Clay soils in most cases show greater buffering capacity
towards pH. Therefore, clay soils need more lime materials that do sand or sandy
soils to increase the soil pH. For example, to increase soil pH 1 unit higher from 4.5
-1 -1
to 5.5 only 1 ton ha lime is needed for sand soil, but as high as 3 ton ha is needed
by a sandy loam soil to satisfy the increase. Organic soils need more lime to
increase 1 unit pH from 4.5 to 5.8 (Table 7.3). Soil organic matter is another soil
3+
factor important to consider, since it can chelate Al to be exchangeable Al
(Hargrove and Thomas, 1981).
The shifting in soil pH as affected by any of the above reactions and many
other reactions will drive the changes in the soil enzymatic activities. The effects of
pH on the activities of soil enzymes are clearly demonstrated by soil phosphatase
(Salam et al., 1998d). The change is generally attributed to the reversible reaction

Abdul Kadir Salam - 2014