Page 89 - Ilmu Tanah

76 The Chemistry and Fertility of Soils under Tropical Weeds

Amaranth (Amaranthus tricolor L.). The pH in the root zones of these vegetation
were 6.25, 5.03, 5.01, and 5.64, respectively.
One good example of anthropogenic acidification is related to the intensive
use of Urea in agriculture. Urea is an important commercial source of N commonly
used, particularly in Indonesia. Since it contains one of the major plant nutrients,
the use of Urea is huge, and in greater amount than those of other major nutrients
such as P and K. However, the use of Urea may cause acidification as seen
chemically in Eq. 5.2 as follows:

NH 2 CONH 2 + H 2 O  CO 2 + 2 NH 3 ....... Eq. 5.2.

The Urea molecule is hydrolyzed to produce carbondioxide and ammonia
molecules. The carbondioxide molecules may then react with water molecules to
+
produce H ion as indicated by Eq. 5.1. Ammonia molecules after being protonated
+
to be an ammonium ion will encounter nitrification to produce H ions (Eq. 5.3) as
follows:

+ - +
2 NH 4 + 2 O 2  NO 3 + 2 H + H 2 O ....... Eq. 5.3.

The above two processes (Eq. 5.2 and Eq. 5.3) may eventually decrease the soil pH.
A long term effect of the use of Urea on soil pH is shown in Table 5.2. The soil pH is
shown to decrease with the increase in the amounts of Urea treatments. For
-1
example, the long-term use of Urea at 200 kg Urea N ha lowered the soil pH from
5.09 to 4.66, about 0.43 units. 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) report that the decrease in pH by Urea treatment was lower
than that by the use of ammonia (NH 3 ).
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

Abdul Kadir Salam and Nanik Sriyani – 2019