Yoshida - 1981 - Fundamentals of Rice Crop Science

PHYSIOLOGICAL ANALYSIS OF RICE YIELD 245
Table 7.4. Comparison of rice yields in dry and wet seasons
with ammonium sulfate alone and with partial substitution of
nitrogen with compost (average from the 10th crop, 1969 dry
season, to the 23rd crop, 1975 dry season, IRRI). a
Paddy yield
Season Nitrogen application (kg/ha) (t/ha)
Dry 140 b
Compost (10 t/ha = 24) + 116 b 7.43
7.23
Wet 60 b c 4.74
Compost (10 t/ha = 24) + 36 b 4.80
a Tanaka (1978). b Asammonium sulfate. c 140 kg N/ha only in 1973.
supply of other nutrients is favorable (Table 7.4). When aiming at high yields,
however, it is advantageous to build soil fertility. Compost or other kinds of
organic matter supply nitrogen to rice plants throughout growth. This continuous
nitrogen supply favors high yields by preventing excessive vegetative growth and
lodging. Furthermore, it is difficult to simulate such a condition by manipulating
fertilizer nitrogen without the help of soil organic matter (Tanaka 1978).
d. Adequate percolation and drainage. High yields can be better achieved on
well-drained fields. Although high rates of percolation cause nutrient loss, adequate
rates may remove toxic substances from the rooting zone and prevent
excessive soil reduction. A percolation rate of 15–25 mm/day is considered
adequate in Japan. In Figure 7.9, the lower line indicates that grain yield decreases
with increasing percolation rates. The upper line indicates that high yields are
obtained at a percolation rate of about 15 mm/day. Percolation rates faster than 15
mm/day decrease yield. Why this is true is not well understood. Most likely
percolation removes some toxic substances such as hydrogen sulfide, ferrous iron,
and organic acids (Takijima and KOnnO 1959, Takijima 1960). If the above
explanation is correct, it follows that a certain amount of percolation would benefit
rice growth only in soils where injurious substances accumulate to levels toxic to
rice.
e. Drainage and intermittent irrigation. Drainage and intermittent irrigation,
commonly practiced by the prize winners, implies that water is drained until the
soil surface is exposed to air, after which irrigation water is reintroduced. The rate
of drainage and the interval between the cycles vary with soil characteristics and
weather conditions. This practice is believed to remove toxic substances and
maintain healthy roots under reductive soil conditions (Agricultural Policy Study
Commission 1971, Togari 1966). Table 7.5 shows that drainage and intermittent
irrigation treatment increases the root-shoot ratio and improves root growth.
f. Soil dressing. Incorporation of clayey hill soils into paddy fields is also a
common practice to improve soil fertility. Hill soils at early stages of weathering
supply various bases, silica, and iron that have been depleted by intensive,
continuous rice cropping. Soil dressing increases the depth of the surface soil