Cereals processing technology

Rice production 91
caused by the various climatic conditions, fertilizer application, soil types, rice
cultivars, availability of water, rice species, and cultural practices. Through
diversification accelerated by climatic changes, human dispersal and selection
over a wide range of latitude and altitude, plus manipulation for cultural
adaptation, many different cultivars have been developed. The dispersal of O.
sativa has led to the development of three ecogeographical races: indica,
japonica, and javanica, each grown with cultural practices ranging from upland
to lowland and to deepwater cultures.
The main area of rice production is found on the Asian continent and adjacent
islands which lie in the tropical and subtropical regions. Much of the Asian rice
is produced in the monsoon area where abundant rainfall, plus some
supplementary irrigation, provides an edaphic advantage. Rice production is
concentrated in areas where water management is convenient on flat lowlands,
river basins, and delta areas. Within the Asian area, regional monsoons, winds,
typhoons, and tropical depressions create a pattern of distinct wet and dry
seasons and add variability to the climatic features from year to year.
5.3.1 Climatic factors affecting rice production
Rice production is controlled primarily by climatic variables that ideally should
provide adequate water during the entire growing season, relatively high air and
soil temperatures, adequate solar radiation, a moderately long growing season,
and relatively rain-free conditions during the ripening period. Important edaphic
factors include the need for relatively level land with poor internal drainage and
favorable chemical and physical properties in the soil itself.
5.3.2 Precipitation and water
Water, precipitation is the most important factor influencing the distribution of
rice in the world, and also its growth and yield potential. Consequently, rice is
preferentially grown only in the rainy season except where water storage and
irrigation facilities are available. The intensity and distribution of the rainfall, the
water retention characteristics of the soil, and water conserving cultural practices
all affect water availability. Irrigated lowland rice covers about 50% of the ricegrowing
area of the world. Irrigation allows more complete control of water
application and depth and is becoming the major system of world rice culture.
5.3.3 Temperature
In temperate regions, temperature is a limiting factor in rice culture. In the main
rice-growing season of the Asian tropics, temperature is more or less constant
and within safe limits. Rice is adaptable to areas with abundant sunshine and
average temperatures above 20 to 38ºC. Temperatures below 15ºC retard
seedling development, delay transplanting, and consequently reduce grain
yields.