Agriculture Reference
In-Depth Information
irrigated rice when adequate water is supplied, but the problem is one of supplying
enough water for the crop at critical stages. In most areas, upland rice is rain fed.
The level of water maintained in a paddy is not constant but varies depending on
the stage of crop development. If direct seeded, only 2 to 3 cm of water is sufficient to
keep the soil moist and permit emergence of seedlings. Transplanted plants can tolerate
higher water levels, 10 to 12 cm being ideal. Water levels may be reduced to 4 to 5 cm
when the plants start to tiller (forming additional stems). This ensures an adequate
oxygen supply near the growing point of the plant. As the crop nears flowering, an ade-
quate water supply is key to maximizing yields. Any water stress at this stage will
directly reduce yields. Keeping water levels at least 15 cm deep also aids with weed
control. The paddy is drained 15 to 20 days before harvest so that the soil is not
muddy. This facilitates the use of combine harvesters and manual harvest of the crop.
3.4.8 Fertilization
The most limiting nutrient in rice production is nitrogen. Because paddy rice is grown
in standing water, the soil around the roots is under anaerobic conditions. This creates
different problems for application of chemical nitrogen fertilizers. When nitrate forms
of nitrogen are applied to soils under anaerobic conditions, soil bacteria transform this
into the gaseous form of nitrogen that escapes into the atmosphere. As much as 70
percent of nitrate fertilizer may be lost to this denitrification process in rice paddies.
Remaining nitrate forms are also subject to leaching down through the soil profile.
This increases environmental contamination and reduces nutrients available for plant
use. To reduce denitrification problems, ammonium forms of nitrogen are applied to
rice paddies. Upland rice is not grown under water and so does not have the problem
of anaerobic denitrification losses.
As much as 70 percent of the nitrogen fertilizers are applied before planting and
flooding. Later applications are done by inserting the fertilizer below the flooded soil
to prevent volatilization losses. Upland rice can receive applications of fertilizer
after the seed is planted. In some areas, application of midseason fertilizer is done
by airplane. Other systems use the irrigation water to apply any remaining nitrogen.
3.4.9 Weeds, Insects, and Diseases
Certain weeds cause major problems in flooded rice paddies. One that causes problems
in the United States is the so-called wild rice, Zinzania aquatica. While it is not a
member of the Oryza genus, it is one of the related families found in North America.
Because it grows in standing water, it can compete with cultivated rice. Herbicides
are less effective against this species since any chemical that can kill Zinzania can
also kill cultivated rice. Active research is underway to develop allelopathic character-
istics in rice varieties. Allelopathy is the characteristic of a plant that discourages, or
prevents, growth of competing plants of other species nearby. The means by which
this is accomplished varies among plants, but seems to involve release of organic com-
pounds that prevent germination of plants belonging to other species.
Rice blast is the serious disease facing most rice producers in the world. It is caused
by the fungus Pyricularia grisea. It stops development of the grain in the floret,
