optimum temperature is 30–35 C. The seed can germinate under water, but more
slowly than when planted into moist soil.
Soil: heavy soils, especially alluvial soils of river valleys and deltas, are better than
light soils—they can be “puddled”, and also they lose less water and nutrients
through percolation. The optimum pH is 5.5–6.5 when dry, which becomes pH 7.0–
7.2 on flooding. Some varieties are classified as “tolerant” to acidity; other varieties
can grow in soils with pH 8–9, and these can be used to reclaim saline or sodic soils.
Phosphate is often a major factor in limiting yields.
30–50 kg/ha in 50 cm rows. There are about 30–35,000 seeds per kg.
Seed spacing: (between rows) Paddy: 20–30 cm for later varieties, 10–20 cm for
earlier varieties. Upland: 20–50 cm. The plant population is normally about 100–120
per square metre.
Depth: Paddy: 5–6 cm in light loams, 2–3 cm in heavy clays. Upland: 1–3 cm.
Intercropping: not suitable for paddy rice. Upland rice is sometimes mixed with
other crops, especially in shifting cultivation systems.
Rotation: rice is often grown in soils unsuitable for other crops, so monocropping is
common. Green manure crops such as Berseem (Trifolium alexandrinum) are often
planted between rice crops. The green manure should be ploughed in just before
flooding. Or 2–3 years of rice can be rotated with 2–3 years of grazing, either with
volunteer rice plants, or better still with pasture mixtures including legumes. There
are often fish in paddy fields, which are sometimes introduced; even prawns and
crayfish have been used. In these cases great care must be taken with pesticides.
GROWTH CONDITIONS
Day length: most rice types are short-day plants, but varieties differ in their
response. There are two main groups of rice varieties:
planted. For example, the growth period of the variety Siam 29 varies from 162 to
313 days depending on when it is planted. Many of the tropical indica rices are
season-limited.
matter when they are planted. Many of the temperate japonica rices are time-limited.
Most modern varieties are insensitive to photoperiod and so can be grown in a
wide range of latitudes.
Growth period: see above. If early (ie short growth period) varieties which are
Temperature: an average temperature of 20C during the entire growth period is
required, with 22–38 C being optimum during the main growth period, and a
minimum of about 25 C during flowering. The plant requires long periods of
sunshine, especially for the final six weeks or so. Irrigation water ideally should be
between 21 and 30C.
Rainfall: water is normally a more limiting factor than soil. Rice grows mainly in
the humid tropics and is one of the few crops that can be grown in the evergreen
forest belt.
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Seed rate: Paddy: 90–110 kg/ha drilled in 15–20 cm rows, 135–230 kg/ha broadcast,
160–170 kg/ha broadcast from a plane. Upland: 100–120 kg/ha in 20–25 cm rows,
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2.Time-limited (period fixed)—their growth period is more or less the same no
1.Season-limited (date fixed)—they flower on a certain date whenever they are
insensitive to photoperiod are grown, 3 crops per year are possible.