Paddy field management

Rice grows on a wide range of landforms affected by an equally wide range of hydrologic conditions. The term upland rice in rice cultivation is used in the sense that rice is grown on nonbunded, nonleveled fields that are prepared and seeded dry and depend on rainfall for moisture. The crop is grown in nonflooded fields although may be transplanted, but still be considered upland. Upland rice grows as a dryland crop like any other cereal crop, or it can grow in contrast to paddy rice where the soil is kept under water during most of its growing cycle.

Any field that is used for growing aquatic rice can be called a paddy field. The field is suited to grow rice where sufficient water is available to submerge the soil for the necessary length of time needed by the crop (Kyuma, 2004). Paddy soil in this definition is related directly to land use, but not to any particular type of soil in a pedological sense. Paddy soils occur basically in lands with an aquic (this is a technical term used in U.S. Soil Taxonomy and widely recognized as a term to denote hydromorphic conditions) moisture regime, in floodplains, deltaic plains, fans, and terraces. Paddy soils commonly connote a class of soils occurring on land brought under wetland rice cultivation that is used for the crop growing and of which the surface is submerged during all or part of the crop-growing season. Therefore, a soil moisture regime is present naturally or imposed artificially on the upper part of the profile, but the lower horizons remain largely unchanged, reflecting the free drainage of the original profile. Modification of the natural water regime, if required for soil-water management, towards a more aquatic regime basically involves two management practices:

• The leveling and bunding of individual fields, which leads to retention or ponding of water. Such leveled and bunded rice fields are designated as paddy fields.

2-14

• Irrigation of rice fields, either by water brought in from elsewhere or by overflow from a higher to a lower paddy field.

The advantages of growing rice in an aquatic milieu are several, and their relative importance is locale specific in that each advantage operates to a varying degree depending on soil, climate, hydrology, soil fertility, biotic factors, etc. A few advantages, however, seem to operate in most circumstances (Moormann and van Breemen, 1978):

• Sufficient water supply for the rice plant, which among food crops is one of the least drought-resistant plants.

• Ease of land preparation in moist to wet soils, especially when only hand tools or simple animal-operated equipment is used. This factor is important in the dominantly clay paddy fields of Asia.

• Simplified weed control by wetland preparation and flooding. Only a limited number of weed species can grow and compete with rice under flooding. Weed control is a major problem on nonflooded rice lands, e.g. on the phreatic rice lands in West Africa.

• Greater availability of plant nutrients. Under flooding more nitrogen is supplied to the soil, mainly because of biological nitrogen fixation. This is the main reason that rice grows year after year in paddy fields without application of fertilizer nitrogen.

Phosphorus becomes more available after a soil is flooded as do several minor elements with the noticeable exception of zinc. While this enhanced availability of plant nutrients is generally an advantage, other phenomena in flooded and reduced soils may negate the positive effects of an aquatic milieu in this respect.

The Asian technique of bunding and leveling land for rice cultivation increases the capacity of the land to retain water by limiting runoff and by storing surface water. An artificially induced aquatic regime on pluvial and phreatic rice lands can be indicated by the addition of the word anthraquic to the terms pluvial and phreatic. This term is a contraction of anthropic indicating the man-made aspect, and aquic related to aqua, the Latin word for water. Adding anthraquic to the term fluxial makes it redundant because fluxial rice lands are by definition aquatic for part or most of the growing season.

2-15

Bunding of rice lands, and its collateral leveling of sloping land, bring about a considerable change in the water regime of that land (De Datta, 1981). The overall effect is that runoff water is diminished and more water, whether from a natural source or from irrigation, or both, is retained on or in the soil. Bunding and leveling of land for rice make a near perfect measure of water conservation. In areas of marginal rainfall, with soils that have a moderate or even low water-holding capacity, rice can grow on rainfed bunded paddy lands.

The practice of bunding and leveling to create an aquatic environment in rice fields in Asia has permitted rice growing in areas beyond the crop’s natural ecological boundaries. In most wetland rice soils only a 0.10 to 0.30 m thick surface layer is saturated during the growing season. However, hydrologic conditions often vary within a single paddy field. Flooding or inundation (shallow ponding of water) easily occurs in soils with a slow natural hydraulic conductivity (e.g., in many clayey soils), in soils with a low porosity, and in soils with an impervious layer at a shallow depth. Of even greater importance, however, is the diminished permeability of the surface and subsurface horizons due to the soil and crop management under wet conditions. Therefore, puddling is practiced so that the loss by percolation is slower than the accumulation of water from whatever source in the paddy fields. The expectation from a puddling practice is the formation of a less permeable subsurface horizon, plough pan, which reduces percolation and makes the ponding of water possible, even on soils that are naturally well-drained and permeable.

Fluvisols, Gleysols, and Cambisols are the most common pedological members belonging to paddy cultivation around the world, but some other members such as Acrisols and Luvisols, to some lesser extent, are also under paddy farming on older land surfaces (Kyuma, 2005). Eswaran et al., (2001) has mentioned Inceptisols and Ultisols as the commonly used ones for aquatic rice cultivation. Irrespective of their pedological nature, all paddy soils are submerged for at least a few months a year, either naturally or artificially. As cultivated rice (O. sativa) originated from a semi-aquatic ancestor, it is very sensitive to water shortage (Kögel-Knabner et al., 2010). Managing paddy soils under water is entirely different from managing other soils used for upland crops, and this produces important differences, particularly in chemical and biochemical or microbiological processes. However, paddy soils are also kept dry or flood-fallowed during the turnaround period between two crops or for the rest of the year, again naturally or artificially. This cyclic change in micro-environmental conditions exhibits properties not encountered in other soils and differentiates paddy soils from most other soil systems.

2-16