Managing Soils for Rice

M. B. Sturgis.

Most of the rice planted in the United States is grown in fields that are flooded to a depth of 3 to 9 inches through most of the growing period. The rice does not naturally need it, but flooding insures that the moisture requirement is met and controls weeds.

The practical benefits of flooding are associated with the development of such chemical conditions as poor aeration, low oxygen tension, and the loss or accumulation of reduced products. The management of soils planted to rice and crops rotated with rice therefore demands special consideration.

Rice is grown mostly in silt, loam, clay loam, silty clay, and clay soils that are nearly level or flat. The runoff is slow. The soils are slowly permeable after they have been saturated.

They have been derived from Recent alluvial deposits or from Pleistocene sediments on terraces adjoining the newer alluvial deposits. The most widely used soils are of the Crowley, Lake Charles, Beaumont, Sharkey, Mhoon, Portland, Miller, Stockton, Sacramento, Yolo, Willows, and Capay series.

The Crowley, Lake Charles, and Beaumont soils are Planosols developed on the Pleistocene Prairie terrace. They differ from the others in being older or in having more pronounced differences between the surface and subsoil layers. They are low in available phosphorus and total phosphorus.

The rice areas have an abundance of good irrigation water.

Most of the water used on rice in Arkansas comes from wells owned by individual farmers. It is pumped from water levels 25 to 60 feet or more below the surface.

Most of the rice in Louisiana and Texas is irrigated by water pumped from rivers and bayous that pass through or are adjacent to the fields. The water is pumped in short lifts from the streams and distributed in surface canals by privately owned canal companies to the farms. Some farmers pump or siphon water from the larger streams.

About 35 percent of the rice grown in Louisiana and 20 percent of the rice in Texas is irrigated from wells on the farms. Ten percent of the rice acreage in California and a large part of the acreage in Mississippi are irrigated from wells.

The amount of water required to produce a rice crop in Arkansas is about 30 acre-inches, including the rain during the growing season. The crop in Louisiana and Texas requires about 48 acre-inches, of which usually 20 inches comes from rainfall during the growing season.

In California, which has the highest water requirement, 48 acre-inches or more are used. Ten feet may be used on some types of soil, but the selection of other crops for those soils should be considered. The value of irrigation water as a regulator for maintaining uniform temperature for better crop growth also is a factor in California.

The quality of the irrigation water generally is good and contains less than 600 parts per million (p.p.m.) of soluble salts. Water that contains more than that amount of sodium chloride should not be used to irrigate dry soils and cannot be used throughout the growing season and year after year without injury to both crop and soil.

In years of less than normal rainfall, brackish water intrudes from the Gulf of Mexico into the streams and waterways that supply irrigation water for the Gulf Coastal Prairie soils of Louisiana and Texas. In those areas, wells are often drilled into water-bearing strata that touch nearby salt domes. They may become salty in periods of heavy drawdown. When the concentration of the water reaches 1,200 p.p.m. of sodium chloride, two or more floodings seriously damage the crop and in a few years may produce harmful conditions in the soil. Sometimes the electrical conductivity values of saturation extracts of the soils have risen above 4.0 millimhos per centimeter at 23 C., the pH above 7.5, the contents of soluble salts above 1,000 p.p.m., and the sodium saturations to nearly 15 percent.

The effects of the continued use of salt water include increases in compactness or bulk density of the soil and loss of permeability of the soil to air and water.

Rice is more tolerant of water containing a mixture of soluble salts that contain relatively little sodium and chlorides. When the dissolved salts in the irrigation water is lower in concentration than the soluble salts in the soil, rice has been grown during the time that the saline and saline-alkali soils are being reclaimed. If the damage to soil from the use of salt water is not of long standing, it can be corrected by flushing and increasing the surface drainage.

In the Gulf Coastal region, where damage frequently occurs from the intrusion of sea water or from salt water let loose in drilling oil wells, the salts can be removed from the soil by smoothing the surface of the field with land leveling equipment and by opening surface ditches, so that the rain water will flush the surface of the soil and move to the drainage canals as quickly as possible. By leaving the land fallow after smoothing and establishing surface drainage, the high rainfall will remove the excess salts in 2 years or more.

FIVE MAJOR PROBLEMS are involved in managing soils planted to rice: The maintenance of soil organic matter and nitrogen and the aggregation necessary for desirable structure; the development of drainage that permits the rotation of rice with other crops; the development of more efficient methods for the application of fertilizers to rice and to crops rotated with rice; crop rotations; and the control of weeds.

Planting a field to rice year after year or in 2-year rotations with fallow or other crops brings rapid losses of nitrogen and organic matter. The losses commonly are associated with declines in the aggregation of the soil particles and the development of poor physical conditions, such as crusting and compaction.

The content of soil nitrogen in a field of Crowley silt loam in Louisiana, which had been planted to rice 40 years, dropped from 0.20 to 0.08 percent. The soil organic matter also declined to 1.6 percent, which was approximately 40 percent of the amount present in the virgin soil or at the time the soil was first planted to rice. The water-stable aggregates of sizes between 0.1 and 2.0 millimeters in diameter in the virgin soil was 68 percent. The soil that had been planted to rice 40 years had 11 percent. This particular area of deflocculated soil is an example of one that breaks up to large clods when it is plowed, and it crusts badly when rained on after the seedbed is prepared.

The rice growers recognize the importance of increasing and maintaining nitrogen and organic matter in the soils. To do that, the land must be kept out of rice for longer periods. It also has to be better drained to allow for the development of pasture sods and the growth of other grains and deeper rooted crops. The lower demand for rice in the mid-1950's encouraged the use of pastures and longer rotations with rice.

In Arkansas, 4-year rotations have produced larger yields of rice. In a rotation of soybeans (cultivated in rows), fall oats, soybeans (turned under for soil improvement), and 2 years of rice, the increase in yields of rice was 21 bushels an acre for the first crop and 10 bushels for the second crop. Weeds were controlled while; the arable crops grew.

In Gulf Coastal prairie areas, the rotation of 3 years of improved clover-grass-lespedeza pastures with 2 years in rice has become an established practice. The increase in yield of the first crop of rice, due to turning under the pasture sod, has been more than 30 bushels an acre. The increase in yield of the second crop of rice may be only 5 bushels.

Estimates in Louisiana indicate that turning the pasture sods from 3 years of improved pastures increased the soil organic matter by about 5 thousand pounds an acre and gained about 180 pounds of soil nitrogen.

The problem of the small effects of the sod residue on increasing the yield of the second crop of rice may be related to the loss of nitrogen by biochemical reduction of nitrates or to the loss of soluble forms of nitrogen by surface washing. The rotation of rice with pastures also combats the weed infestations and improves structure.

The questions of how stable is the improvement in soil structure and how deep and what methods to use in turning pasture sods and other crop residues before seeding rice need more practical answers than can be given now.

The tendency has been to plow shallow, but earlier research in Arkansas showed that plowing to a depth of 9 inches was more effective than either the 6-inch or 3-inch depths.

The soil drainage in most of the rice areas is over the surface. Many of the terrace areas originally had low hummocks or mounds. The continued culture of rice has tended to reduce the small variations in relief, but only with the newer land planes could the small elevations and depressions in ricefields be smoothed practically.

Rather wide variations exist, however, in the productivity of some fields, which are due to the original natural relief. The smoothing and the better drainage prevent the development of alkaline conditions on the mounds or in depressions. The smoothing or leveling is a particular requirement before seeding in water. In topdressing with fertilizers when the floodwater is removed or allowed to evaporate before fertilizers are applied, the water should be removed uniformly from the surface so that most of the feeder roots, which are in the immediate surface of the soil, can absorb the nutrients quickly.

If nitrogen fertilizers are thrown into alkaline water, some nitrates will be lost by reduction to gaseous nitrogen and some ammonia will be lost by volatilization. Good drainage is necessary for the development of pastures and their utilization. Drainage is important if other fall-sown grains and row crops are to be rotated with rice.

THE FERTILIZER recommendations vary for the different areas. Nearly all of the soils are deficient in nitrogen. The prairie or terrace areas are low in available phosphorus and available potassium. Rice on the clays and clay loams of this group does not generally respond to applications of potassium. Relatively few areas of the alluvial soils are deficient in either phosphorus or potassium. Phosphorus, however, is more likely to be deficient than potassium. Nitrogen is commonly the only nutrient element applied to rice on the Recent alluvial soils.

The amount of nitrogen applied in fertilizers is 30 to 90 pounds an acre. If phosphoric oxide is applied, the amount is 20 to 50 pounds an acre. If potash is added, the amount is 15 to 50 pounds an acre.

The applied fertilizers have increased yields generally from 9 bushels to more than 35 bushels an acre.