Among prevailing agricultural systems, rice cultivation is predominant for densely populated areas with the Monsoon climate. Rice is a unique crop for flooded soils. Submergence of soil makes it possible to cultivate rice every year without any fallow land, because many pathogenic fungi do not survive the anaerobic condition. Light power is enough for cultivation. Because the weight of the soil block is decreased in water, a single cattle power, usually a water buffalo, can easily plow and paddle the soil, which is otherwise very heavy. Levees keep irrigation water in terraced farms; thus, rice cultivation is protected against soil erosion.
Electrochemical changes that occur in submerged soils were earlier discussed in detail by Ponnamperuma.1 According to his paper, in normal tropical soils a set of soil conditions are achieved by submergence, where availability of nitrogen, phosphorus, potassium, calcium, magnesium, iron, manganese and silicon is high, while the supply of copper, zinc and molybdenum is adequate. The decrease in redox potential (Eh) increases pH of acid soils and decreases pH of alkaline soils.
Soil fertility can be conserved better under anaerobic conditions, because ammonium in soil from crop residues or other organic matter is not easily oxidized into nitrate compounds which are carried away by water or volatilized into air. Mineral nutrition for rice is also supplied through irrigation water. This is the reason for the traditional no-input rice farming, which consistently yields about 1.5-2.0 tons per hectare (t/ha). Nitrogen can be supplied through fixation by algae and other microorganisms.
Weeds are controlled by irrigation, because many kinds are not adapted to submergence. No other crop is planted to flooded soil except rice. Besides, there are some additional merits in irrigated rice farming. Cattle can be fed with the weeds on levees. Fish culture in canals or swamps provides protein resources. The nutritional balance of rice proteins is one of the best among the staple cereals. Farms in arid regions are desalinized by a regular rotation of rice planting.
For introduction of rice cultivation to new areas where rice has not been cultivated, there are a set of problems. Supply of water is a limiting factor. Accumulation of salts can be a problem in arid areas, where salts are carried from underground to the surface and water evaporates, leaving salts.
The advantages of rice farming can be better understood in contrast with other farming systems like those in Europe, which are characterized by the need of fallow land. No crop can be planted in consecutive years in a field plot, due to disease buildup; therefore, fallow land, pasture or rotation of crops is essential. In European types of agriculture, a major part of the nutritional proteins are supplied through animal husbandry, which requires forage crops. Uptake of proteins through animal products is estimated to require seven times more land area than relying on proteins of plant products. There is also potential soil erosion through overgrazing and rainfall. Similar problems can be seen in dryland farming, and also in nomad agriculture.
In conclusion, the ecological stability of rice farming is excellent, being comparable only to some plantation agriculture of perennial plants such as tea, coconut, oil palm, rubber tree, cacao bean and so forth. Rice farming will be one of the best models for agricultural systems in the future.
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