Biology Reference
In-Depth Information
Soil quality of rice soils in rainfed lowlands of Asia
Table 5.1.
Total area
Very poor soils
Poor soils
Problem soils
Good soils
Country
'000 ha
'000 ha
% of total
'000 ha
% of total
'000 ha
% of total
'000 ha
% of total
India
16,139
2,187
14
6,541
41
1,218
8
6,193
38
Thailand
8,202
5,195
63
1,658
20
396
5
920
11
Bangladesh
5,093
539
11
558
11
627
12
3,388
67
Indonesia
4,006
1,604
40
1,023
26
275
7
1,108
28
Vietnam
2,911
764
26
327
11
869
30
945
32
Myanmar
2,411
577
24
234
10
354
15
1,246
52
China
1,746
716
41
262
15
45
3
716
41
Cambodia
1,573
748
48
192
12
146
9
483
31
Philippines
1,323
559
42
340
26
0
0
423
32
Nepal
760
209
27
253
33
0
0
299
39
Laos
438
342
78
31
7
18
4
48
11
South Korea
279
120
43
42
15
0
0
117
42
Sri Lanka
218
95
44
79
37
12
6
30
14
North Korea
115
3
3
23
20
2
2
86
75
Buthan
17
8
46
6
37
0
0
3
17
Timor-Leste
7
2
23
0
4
0
1
5
72
total
45237
13,668
11,569
3,962
16,010
Data based on Haefele and Hijmans 2009.
Since weeds cannot be controlled by flood-
ing the field, weeds are a major competi-
tor for resources in rain-fed systems, espe-
cially during crop establishment.
grain. Maintenance P application rates are
expected to be higher in irrigated rice, a
measure aimed at balancing this depletion.
II. Rain-fed upland rice. In addition to the fac-
tors listed above, several upland-specific fac-
tors affect P availability, which consequently
tends to be at the opposite (low) end of the P
availability spectrum:
Breeding Targets Related to P
Efficiency in Rice
Typically, soils in uplands are older and
more weathered than lowland soils, espe-
cially compared with alluvial soils in the
vicinity of rivers. As a result, the soil con-
centration of P and other nutrients can be
low;
Selection of modern rice cultivars has typically
been performed under high-nutrient conditions
with a primary focus on yield. Although some of
these modern varieties may perform well under
moderate P deficiency, it has been pointed out
that this is most likely a result of their supe-
rior harvest index (HI), which allows them to
turn a higher proportion of their total biomass
into grain yield, and not due to specific toler-
ance mechanisms that increase P uptake or total
biomass (Wissuwa et al. 2009). Selection for
yield conducted in target environments, such as
generally poor or P-deficient soils, may therefore
offer advantages. However, conclusive data sets
comparing the effect of selection under differ-
ent P levels on subsequent yield performance of
rice in different environments do not exist. Yet
Rates of P fixation by soil constituents can
be extremely high as a result of weathering.
With the tendency for P to be immobilized
rapidly, only a small fraction (
10-20%)
of fertilizer P may be available to a crop in
the year of P-fertilizer application;
<
The lack of water control in rain-fed upland
systems increases the risk of droughts -
drying of the topsoil where most of the P
is concentrated thus temporarily reduces P
availability further;
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