Agriculture Reference
In-Depth Information
The minimum quantity of nitrogen that the crop must accumulate to produce
1 t of rough rice is of the order of 16-18 kg, though efficiencies are lower at
low yield levels (Cassman
et al
., 1998; Dobermann
et al
., 2002). Thus 8 t of
rice per ha requires 128-144 kg N ha
−
1
. Experiments without additions of N
in a wide range of rice environments over many years have shown that from
50-80kgNha
−
1
can be accumulated from soil reserves that are replenished by
biological N fixation and crop residues. For higher yields, additional N must be
provided from sources outside the ricefield, either as organic manures or mineral
fertilizers.
The calculations in Figure 7.2 are for a rice-rice-legume cropping system
removing 8 t ha
−
1
of rice grain (3 in the wet season, 5 in the dry), 8 t ha
−
1
of
straw (i.e. a harvest index of 0.5 compared with 0.4 in the pre-1960 varieties) and
1tha
−
1
of legume. A positive balance is maintained for N and P, but because
there are no additions of K fertilizer and no additions with sediment, there is a
steady depletion of soil K reserves: 195 kg ha
−
1
year
−
1
if straw is removed or
35 kg ha
−
1
year
−
1
if retained. Such a picture is not unrealistic, and yield limi-
tations due to K deficiency in irrigated rice systems are becoming increasingly
common across Asia (Dobermann
et al
., 1996). The importance of mineral fer-
tilizers in sustaining intensive rice production is evident.
Figure 7.3 shows corresponding balances for rainfed rice systems. Here the
yields are more modest: 2, 3 and 0
.
5tha
−
1
of rice grain, straw and grain legume,
respectively, for rainfed lowland rice; 2 and 8 t ha
−
1
of grain and straw for flood-
prone rice; and 1, 2 and 0
.
5tha
−
1
of grain, straw and grain legume for upland
rice. The nutrient budgets are correspondingly smaller. In rainfed lowland rice,
as for the pre-1960 irrigated rice, positive balances are maintained for N and K
but a negative balance for P; a positive P balance is maintained if only 1 t ha
−
1
of rice is harvested and the straw retained in the fields. The supplies of P and
K in sediment are critical. In flood-prone rice the balances of all three nutrients
are negative if straw is removed, but roughly balanced if it is retained. Nitrogen
fixation in the deep, low P water is diminished, and the supplies of N, P and
K in sediment are crucial. In upland rice, the N, P and K balances are again
all negative, even for yields of only 1 t ha
−
1
. Few soils are sufficiently fertile to
withstand such withdrawals for very long, hence the importance of the restorative
fallow for several years in shifting cultivation systems based on upland rice.
Secondary and Micronutrients
In the colluvial and alluvial soils of the main rice producing areas, the amounts of
calcium and magnesium deposited in irrigation and floodwaters, especially if they
carry sediment, usually far exceed crop removals (Greenland, 1997; Dobermann
and Fairhurst, 2000). Deficiencies do however occur in rainfed lowland rice,
especially on highly weathered soils where they are compounded by deficiencies
of other nutrients. The only secondary nutrient that is commonly removed in
greater amounts than it is supplied in inflowing water is sulfur. Deficiencies of
