How Is Rice Grown? Farming in Standing Water on Purpose

Rice is a semi-aquatic grass with aerenchyma, a tissue of air spaces running from the leaves down through the stem into the roots, which supplies oxygen to roots sitting in waterlogged soil where oxygen has been exhausted. Almost no weed shares that adaptation, so standing water suppresses competing plants continuously without any herbicide, which is the principal reason for flooding. The water also stabilises soil temperature, releases phosphorus and other nutrients by changing the soil's chemistry under low oxygen, and suppresses several soil-borne diseases. The field is prepared by puddling, ploughing the soil while wet to break its structure deliberately and create a compacted layer beneath that slows percolation, so the water stays where it is put. That destroys soil structure in a way that would be disastrous for any other crop and is exactly what a paddy needs, which is why fields alternated between rice and dryland crops need careful management.

Rice is grown in conditions ranging from permanently submerged to entirely rainfed:

• •Irrigated lowland, bunded fields with controlled water, which is about half the rice area and produces around three quarters of the harvest because yields are highest and two or three crops a year are possible
• •Rainfed lowland, bunded but dependent on the monsoon, which is vulnerable to both drought and flooding and supports a large share of the world's poorest farmers
• •Upland rice, grown on unflooded land like any other cereal, low yielding and important in hilly regions
• •Deepwater or floating rice, grown where seasonal flooding is metres deep, using varieties whose stems elongate rapidly to keep the leaves above rising water
• •Terraced systems on steep slopes, where retaining walls create level flooded steps, of which the Philippine Cordilleras and the terraces of Yunnan and Bali are the best known and are maintained by communal labour
• •Transplanting seedlings raised in a nursery is traditional and gives the crop a head start against weeds, while direct seeding is faster and cheaper and is displacing it as labour becomes expensive

Harvested rice is a grain enclosed in a tough inedible husk, and what happens next determines what the consumer gets. Drying comes first and is urgent, since grain stored damp moulds and can develop toxins. Hulling removes the husk, leaving brown rice, which is the whole grain complete with bran and germ. Milling then abrades the bran layer away to produce white rice, which stores far longer because the oil in the bran goes rancid, cooks faster, and loses much of the fibre, B vitamins and minerals in the process. That loss caused epidemics of beriberi, a thiamine deficiency disease, once machine milling spread in the late nineteenth century, and the connection between polished rice and the disease was one of the discoveries that established the concept of vitamins. Parboiling, steaming the grain in its husk before milling, drives nutrients from the bran into the endosperm and is traditional across South Asia, which is why parboiled rice retains more nutrition than ordinary white rice.

Flooded fields are a major source of methane, because the oxygen-free soil suits archaea that produce it, and rice cultivation accounts for a significant share of agricultural greenhouse emissions. Rice also consumes an enormous share of the world's irrigation water. The practice with the clearest evidence behind it is alternate wetting and drying, allowing the field to dry until the water table falls a set distance below the surface before reflooding, which cuts water use substantially and reduces methane sharply without lowering yield when managed correctly. The system of rice intensification, using younger seedlings, wider spacing and intermittent wetting, reports large yield gains that remain contested in the literature. A separate hazard is arsenic, which is more available to plants in flooded soil and accumulates in the grain, a serious problem where groundwater is naturally arsenic-rich, and one that alternate wetting also reduces. Breeding has delivered submergence-tolerant varieties that survive being completely underwater for two weeks, which matters greatly in flood-prone deltas.

Rice is flooded because it can breathe through internal air channels and almost no weed can, making standing water a weed control method rather than a tradition. Fields are puddled deliberately to destroy soil structure and hold water. Irrigated lowland is half the area and three quarters of the crop, alongside rainfed, upland, deepwater and terraced systems. Milling away the bran to make white rice extends storage and removed enough thiamine to cause beriberi epidemics. Flooding drives methane emissions that intermittent wetting substantially reduces.