Agriculture Reference
In-Depth Information
sulfur have become more widespread as rice yields and the intensity of cropping
have increased (Dobermann
et al
., 1998). However, while deficiencies of sulfur
in crops in Europe have increased in recent decades as deposition from industrial
pollution has decreased, deficiencies are being offset in rice in large parts of Asia
by increasing emissions from industry and the large-scale burning of forest.
Though it is not strictly an essential nutrient, rice plants accumulate very large
amounts of silicon-shoot contents typically exceeding 5%-and this has various
beneficial effects on the plant (Savant
et al
., 1997). Silicon adds to the mechan-
ical strength of cell walls, confers resistance to certain pests and diseases, and
is thought to offset abnormalities in the supply of certain other nutrients. Defi-
ciencies occur in highly weathered soils from which the soluble silicon has been
leached, and in organic soils with low mineral reserves. More widespread inci-
dences are expected as rice cropping continues to intensify (Dobermann and
Fairhurst, 2000).
Of the essential micronutrients (Fe, Mn, Zn, Cu, B and Cl), deficiency of
zinc is the most commonly reported (Quijano-Guerta
et al
., 2002). Generally the
amounts of zinc and other micronutrients brought in with irrigation water, rainfall
and sediments are more than sufficient to offset crop removals. Deficiencies
arise as a result of particular changes in the soil following submergence causing
immobilization (Section 7.2). Such problems become more acute the greater the
rate of removal in cropping, and increases in incidences of Zn deficiency are
expected with the advent of Zn-dense rice for improved human nutrition (Welch
and Graham, 1999).
7.1.2 ACIDITY BALANCES IN RICEFIELDS
A further reason for the long-term sustainability of wetland rice farming is that
the soils tend not to become acid after continuous, intensive cultivation. In culti-
vated upland soils, acidification occurs because of the leaching of the NO
3
−
ion.
Nitrification of NH
4
+
added in mineral fertilizers produces 2mol of H
+
per mol
of NH
4
+
nitrified (Table 7.1, Process 3). If some of the NO
3
−
is subsequently
leached from the soil accompanied by an exchangeable cation, 2H
+
are left behind
per mol of NO
3
−
leached, acidifying the soil. Because in general little or no NO
3
−
is leached through submerged rice soils, any NO
3
−
entering the soil or formed
in it being either absorbed by the crop or denitrified, this process does not occur.
The removal of fertilizer N in the crop as NH
4
+
does not lead to acidification.
Hydrolysis of urea fertilizer—by far the main form of N fertilizer used in wetland
rice, together with ammonium bicarbonate in some countries—consumes 1mol
of H
+
per mol of NH
4
+
formed (Table 7.1, Process 1). So although absorption
of N as NH
4
+
leads to a net export of H
+
from the roots to balance the resulting
excess intake of cations over anions (Table 7.1, Process 5), this acidity is matched
by the H
+
consumed in urea hydrolysis. Likewise there is no net generation of
acidity as a result of NH
3
volatilization, although 1 mol of H
+
is left behind per
mol of NH
4
+
converted to NH
3
(Table 7.1, Process 3).
