Page 117 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 105

Table 7.2. The decrease in soil pH as affected by long-term use of Urea *.

-1
Urea N (kg ha )
Soil Tillage
0 100 200
Intensive Soil Tillage 5.03 4.83 4.44
Minimum Soil Tillage 5.18 4.96 4.78
No Tillage 5.06 4.89 4.75

Average 5.09 4.89 4.66
*Adapted from Salam et al. (1998c)

Soil acidification may also happen intentionally in response to the need of
agriculture management. For example, soils in the pineapple plantations owned by
PT Great Giant Pineapple in Central Lampung, Indonesia, are acidified by using
sulfur powder to manage particular plant deseases at low pH. This practice has
acidified the soils to relatively low values (Salam et al., 1999e).
One example of alkalination is liming, which may significantly increase the
soil pH (Barber, 1981; 1984; Kamprath, 1984; Thomas and Hargrove, 1984; Liu et
al., 2004). The liming materials will encounter a series of chemical reactions in soil
water and will end with the increase in soil pH. For example, CaCO 3 in soil water
2+ 2- 2+
may dissosiate to form Ca and CO 3 ions. Ca ions will displace exchangeable H
2-
and Al attached to the soil colloid surfaces, while CO 3 ions will react with water
- - - + 3+
molecules to form HCO 3 and OH . OH ions will neutralize H and Al ions to form
+ 3+
water and Al(OH) 3 precipitates. The decrease in H and Al ions will then increase
the soil pH.
To a lesser extent, the use of P-fertilizer may also gradually increase the soil
pH as shown in Fig. 7.1. The effect of P-fertilizer on soil pH can be explained by the
dissolution of calcium phosphates, the minerals used to make commercial P-
+
fertilizers. In the present of water and H ions, this mineral will decompose to
-
2+
release Ca ions, orthophosphates, and OH ions as shown in the following reaction
(Eq. 7.4):

-
-
2+
+
Ca 3 (PO 4 ) 2 + 2 H 2 O + 2 H  3 Ca + 2 H 2 PO 4 + 2 OH ....... Eq. 7.4

Abdul Kadir Salam - 2014