When large amounts of straw are produced, there is a tendency to burn it to make it easier to prepare the seedbed. The straw should be spread and returned to the soil to conserve organic matter and plant nutrients. The removal or burning of straw increases markedly the loss of potassium.

Experiments in rice-growing States indicate a low efficiency in use of fertilizers on rice particularly nitrogen. Some of it is due to the reduction of part of the nitrates to gaseous nitrogen, a relatively smaller amount to the loss of ammonia by volatilization, and some to the competition from weeds.

Normal, well-aerated soils have an oxidation-reduction potential above 0.5 volt at pH 5. Reducing conditions in waterlogged or flooded soils occur below an oxidation-reduction potential of 0.3 volt. At pH 7, these values would be 0.4 volt and 0.2 volt, respectively. Oxidation-reduction potentials below 0.2 volt commonly occur in soils planted to rice within 4 to 10 days after flooding. At oxidation-reduction potentials below 0.2 volt in the soil, nitrates will be reduced to nitrogen gas or ammonia and sulfates will be reduced to sulfides. The nitrates that have accumulated in the soil from nitrification or directly from the application of fertilizer to be fully effective must be absorbed before flooding or within a few days after the rice is flooded.

Ammonia or the ammonium forms of nitrogen are more desirable for the fertilization of rice.

The grower, when he places the fertilizer, should take into account the benefits derived from keeping the nitrogen in the ammonium form until it can be absorbed by the roots. Within 10 days or less after flooding, a reducing zone develops immediately below the surface of the soil. The free oxygen in the zone is used faster than it can diffuse from the soil-water interface. The process of nitrification is stopped. The ammonium form of nitrogen remains unchanged and is held by the colloidal complexes, from which it is available to the roots. Nitrates that occur in the reducing zone may be reduced to gaseous nitrogen and lost.

If nitrogen fertilizers are broadcast on the surface before flooding or broadcast into the floodwater, the oxidative changes of ammonia to gaseous nitrogen may also occur to some extent.

Some evidence is at hand that the availability of soil phosphorus is increased by flooding with nonalkaline water. The availability of soil potassium is increased by flooding-- particularly as the water temperatures increase. Benefits from potassium applied to rice is greatest in the early stages of growth.

Complete and mixed fertilizers can be applied best by drilling or disking them into the soil at seeding or just before.

Ammonia or the ammonium forms of nitrogen drilled into the soil to a depth of 3 to 4 inches at seeding or just before will be held by the soil in the more available ammonium state within easy contact by the rice roots and more removed from the weeds sprouting on the surface.

The application of topdressing by broadcasting the fertilizer into floodwater is not so effective as applying the fertilizer after the fields have been drained so that the roots at the immediate surface of the soil can quickly absorb the nutrients. Some ammonia may be lost by volatilization if the floodwater is alkaline and hot.

A rather common practice in the terrace areas is to apply 300 to 400 pounds an acre of 5-10-5 or 3-12-12 fertilizers or their equivalents in higher grades by drilling or disking at or just before seeding and then topdressing not later than 8 weeks after emergence with 30 to 50 pounds of nitrogen as ammonium sulfate, urea, or ammonium nitrate.

The application of anhydrous ammonia to a depth of 3 to 4 inches and drilling mixtures of phosphorus and potassium at or just before planting the rice has been tried. Some growers inject complete liquid fertilizers to depths of 2 inches or more before planting. If only nitrogen fertilizers are used, all or part of the nitrogen as ammonia may be applied before seeding, or all or part of it may be applied in the solid forms.

If split applications as topdressing are used, nitrogen in the solid forms are broadcast on the rice by airplane from just after emergence to not later than 8 weeks after emergence.

RICE GROWS BEST in slightly acid or neutral soils.

The development of improved grass-clover pastures and their rotation with rice is particularly important in the cattle-rice type of farming that has been prevalent in Louisiana and Texas and has been increasing in Arkansas and Mississippi. Many of the soils of the cattle-rice areas are acid and need to be limed. The soil should be limed to pH 6.5 before seeding clovers.

If liming is necessary, liberal applications of phosphorus and potassium usually are necessary for the establishment of pastures that contain legumes. Small amounts of nitrogen also are applied before seeding the grass-legume pastures. After the establishment of the clover-grass pastures, top-dressing with nitrogen fertilizer may be desirable to stimulate grass growth, and annual applications of small amounts of nitrogen and lesser amounts of phosphorus and potassium are used than was applied before seeding.

The need for nitrogen varies inversely with the legume growth. The demand for potassium declines after establishment. The other grain crops grown in rotation with rice have about the same nitrogen requirements as rice, but have much higher phosphorus requirements. The annual legumes, such as soybeans, vetch, and lespedeza, need more phosphorus and potassium than rice.

The rapid deterioration of soil organic matter and desirable soil structure that are associated more or less with continuous cropping to rice are experiences in all rice-growing localities. Intermittent flooding and draining increases the hydrolytic decomposition processes in the soil and the removal of the decomposition products over the surface.

Leaving a badly deteriorated land to idle fallow is the simplest if not the most effective way to stop deterioration and improve soil productivity.

All rotations have proved to be better than continuous rice, but the longer rotations, which allow the land to remain out of rice for at least 2 years in succession, are more desirable.

Some 2-year rotations, in which a legume is turned under, however, are effective in supplying nitrogen and maintaining or increasing soil organic matter. The most effective rotations for soil improvement are those that include 2 years or more in grass-clover pastures; the sods are turned under before rice or a row crop.

The most inclusive rotations have been developed in Arkansas. They include: A rotation of soybeans, followed by a winter legume with rice; a rotation of rice, followed by lespedeza with the first crop of lespedeza seeded in the rice stubble and followed by a second crop of lespedeza; a 4-year rotation of cotton and a winter legume, soybeans, winter oats, and lespedeza, followed by rice, and a 5-year rotation of soybeans, winter oats, lespedeza, and 2 years in rice.

A 6-year, rice-pasture-row crop rotation has been used in the rice-livestock areas. It consists of 2 years of rice with a permanent pasture mixture of whiteclover, alsike clover, redtop, and ryegrass, which is seeded in the second rice crop by plane after the water is removed for harvesting. Lespedeza is overseeded in the spring. The pasture is grazed 3 years, and then is followed by a row crop, which could be corn, grain sorghum, or soybeans.

Two-year rotations in the Gulf Coastal areas include rice and lespedeza; rice and cotton with or without winter legume; rice and winter oats or wheat; rice and oats followed by lespedeza; rice and sorghum.

The rotation of 2 or 3 years of clover-grass-lespedeza pastures with 2 years of rice has gained in importance in the Gulf Coastal areas. A relatively large acreage is left in idle fallow for 1 year or more after each rice crop, however. The fallow land is grazed and produces about 50 pounds of beef an acre.

In California, rice is rotated with wheat, barley, beans, milo, and Ladino clover. One rotation includes rice, beans, wheat, and beans. Green-manuring crops, such as purple vetch, have become more popular. The vetch or other legume is sown in the standing rice a few days before the water is drained off before harvesting. The gains in nitrogen and the physical improvements in the soil from turning under the legume are effective.

THE CONTROL OF WEEDS in rice fields is an unsolved problem. Crop rotations that involve cultivated row crops and the use of pasture rotations where the land remains out of rice for 2 years or more tend to reduce weed infestations greatly. The use of clean seed is important.