Agriculture Reference
In-Depth Information
Irrigated rice before 1960
140
14
180
R
R
160
120
12
F
140
Cg
F
S
100
10
120
Cg
80
8
R
100
Cs
F
Cg
80
60
6
S
Cs
N
60
S
Cl
Cs
40
4
40
Cl
Cl
20
2
20
P
M
P
P
M
M
G
0
0
0
In
Out
In
Out
In
Out
Irrigated rice after 1960
400
50
300
R
R
F
250
40
F
Cg
300
200
N
Cg
30
Cg
200
150
Cs
Cs
M
20
100
Cl
Cs
M
100
P
10
Cl
50
Cl
R
G
P
F
P
0
0
0
In
Out
In
Out
In
Out
Figure 7.2
Typical annual nutrient balances for irrigated rice soils pre- and post-1960
calculated from probable inputs (left side of each graph) and outputs (right side) (data
from Greenland, 1997). Inputs come from R
=
rainfall, F
=
floodwater and irrigation,
S
=
sediments, N
=
nitrogen fixation and M
=
manures and fertilizers. Outputs are due
to removals in Cg
=
rice grain, Cs
=
rice straw, Cl
=
legume crop, S
=
seepage and per-
colation and G
=
gaseous emission
Post-1960 and the green revolution, much larger quantities of nutrients are
removed in intensive double and sometimes triple rice cropping. The irrigation
water is now often obtained by storage in reservoirs behind high dams and deliv-
ered through artificial channels so the sediment settles out of the water and no
longer reaches the ricefields with its load of nutrients. However there are now
much larger additions of nutrients in mineral fertilizers. A greater proportion of
the mineral fertilizer N is lost through gaseous emissions, and, because under
multiple cropping the soil is flooded for more of the year, percolation losses are
greater, particularly of K.
