Page 116 - Enzymes in Tropical Soils

104 Enzymes In Tropical Soils

Table 7.1. The soil reaction and phosphatase activities in
the root-zones of some vegetations.

Soil Property IC AT AS CK

Soil pH 6.25 5.03 5.01 5.64
Acid phosphatase* 179 170 112 155
Alkaline
62.5 45.8 32.3 41.0
Phosphatase*
Adapted from Salam et al. (1997e), IC = I. cylindrica = , AT = A. tricolour, AS =
-1
-1
A. Spinosus, CK = C. Kyllingia; *in mg p-Nitrophenol g h

The Urea molecule is hydrolyzed to produce carbondioxyde and ammonia
molecules. The carbondioxide molecules will then react with water molecules to
+
produce H ion as indicated by Eq. 7.1. Ammonia molecules after being protonized
+
to be an ammonium ion will encounter nitrification to produce H ions also (Eq.
7.3) as follows:

+ - +
2 NH 4 + 2 O 2  NO 3 + 2 H + H 2 O ....... Eq. 7.3.

The above two processes may eventually decrease the soil pH. A long term effect
of the use of Urea on soil pH is shown in Table 7.2. The soil pH is shown to
decrease with the increase in the amounts of Urea treatments. For example, the
-1
long-term use of Urea at 200 kg Urea N ha lowers the soil pH from 5.09 to 4.66.
Greater decrease occurs when the Urea treatment is done in the field with
intensive soil tillage.
According to Boulman et al. (1995), the decrease in soil pH with Urea
+
treatment is attributed to two reasons: (a) the production of H ions due to Urea
hydrolysis and (b) the decrease in exchangeable Ca and Mg due to replacement by
+
NH 4 . The decrease in soil pH due to long-term application of Urea has been
reported to increase the soil exchangeable Al and Al saturation and also to
3+
decrease the exchangeable Ca and Mg (Schroder et al., 2011). Because Al is a
potential source of acidity, this condition will worsen the soil pH. However,
Boulman et al. (1995) reported that the decrease in pH by Urea treatment was
lower than that by the use of ammonia (NH 3 ).

Abdul Kadir Salam - 2014