Agriculture Reference
In-Depth Information
1.8
180
4.0
6
3.6
1.6
160
3.2
1.4
140
5
2.8
1.2
120
4
arable area
2.4
1.0
100
2.0
3
rice
0.8
80
1.6
0.6
60
2
1.2
fertilizer
N
0.4
40
wheat
0.8
population
1
0.2
20
0.4
irrigated
area
0
0.0
0
0.0
1860
1880
1900
1920
1940
1960
1980
2000
Year AD
Figure 7.1
Increases in world population, arable area, average yields of rice and wheat,
amount of fertilizer N used, and the irrigated area of the world (Evans, 1997). Reproduced
by permission of the Royal Society
rice grain and straw and legumes,
C
, seepage and percolation,
S
, and gaseous
emission,
G
. Therefore the balance,
B
,is
B
=
(R
+
F
+
S
+
N
+
M)
−
(C
+
P
+
G)
(
7
.
1
)
Macronutrients
The macronutrient balances for irrigated rice are shown in Figure 7.2. Pre-1960,
there were no inputs from mineral fertilizers though manures were applied, but
there were substantial inputs from sediments borne by the irrigation water, which
was obtained by stream diversion. The sediment supplied the bulk of the annual
P and K inputs, and the bulk of the N was derived from biological fixation in
the soil and floodwater during the rice crop and in legume crops following the
rice. The calculation is for a single rice crop of 2 t ha
−
1
of grain and 3 t ha
−
1
of straw and 0
.
5tha
−
1
of legume, all removed from the field. The figure shows
that the balances of N and K are positive but that of P substantially negative. A
positive balance for P is attained with a crop of 1 t ha
−
1
of rice grain if the straw
is retained.
