RICE FERTILIZATION

Research on rice fertilization has continued throughout most of the 16 years of the rice research marketing order. Studies in the UC Davis Department of Agronomy and Range Science have included work on nitrogen, phosphorus, and zinc, the most frequent limiting nutrient elements in California. Potassium, iron and sulfur needs also have been studied.

Nitrogen management

Research has shown the superiority of preflood nitrogen fertilization over split applications in terms of yield, fertilizer efficiency, and grower economics. Sixty to 70 percent use efficiency is achieved by preflood application compared to 40 to 60 percent with split application.

Nitrogen incorporation reduces losses from ammonia volatilization and denitrification. Nitrogen top dressings should be applied if the initial fertilizer rates were too low. The need for top dressing is best determined by plant analysis of the most recently mature "y" leaf. Critical nutrient values have been developed for all growth stages for nitrogen, phosphorus, potassium and zinc. Plant analysis and soil tests developed for rice are widely used in California.

Phosphorus needs

Phosphorus deficiencies were found on many California rice lands, particularly in the reddish-brown terrace and foothill soils. Preflood applications of phosphorus are equally effective whether broadcast or banded, but preflood broadcast treatments are more effective in years when cold weather persists. Soil test values can accurately predict the need for phosphate fertilizer. Upland crops following rice are often critically deficient in phosphorus because chemical fixation ties up available supplies. Crops grown after rice should be fertilized with banded phosphorus. Mono-ammonium phosphate is the best phosphorus source.

Widespread zinc deficiency

Zinc deficiency was found to be quite widespread in California rice. Some soils lack sufficient amounts of this element, and soil with pH above 7.0 causes soil zinc to have low plant availability. High levels of bicarbonates in well water also cause zinc deficiency. Yellowing and poor rice seedling performance for about 3 weeks after planting are frequently the results of zinc deficiency. These conditions can be corrected with zinc sulfate or oxide, which are about equally effective, or with certain chelates. Soil analysis using DTPA as a zinc extractant does a good job identifying zinc needs.

Potassium fertilization is not required except on a few California soils.

Sulfur is not generally deficient. Excess sulfur incorporated with crop residues can cause sulfide toxicity.

Figure 17. Donald B. Crosby explains molinate dissipation rates to growers attending a rice field day.

RESIDUE MANAGEMENT

Incorporation of large amounts of organic matter such as rice straw and weeds, followed by flooding of the soil for sowing rice sometimes produces gases and organic acids that harm rice seedlings. The organic acids — formic, acetic, propionic and butyric — persist for about 30 days after fields are flooded. A midseason problem of a similar nature is caused by the formation of hydrogen sulfide in the soil. This problem occurs when large amounts of organic matter are incorporated into soils that are high in sulfate sulfur. Field experiments show that sulfide toxicity can be prevented or alleviated by avoiding straw incorporation, not using sulfate bearing fertilizers, and by temporarily draining affected fields.

SEED COATINGS

Research on coating rice seed with zinc and with calcium peroxide has led to the use of this practice by some seed suppliers. Coating stimulates seedling development, eliminates the need for field fertilization with zinc, and eliminates the seed soaking procedure. Seeds coated with as little as 1 to 2 pounds actual zinc per acre correct the zinc deficiency problem. Calcium peroxide coatings permit drill seeding before the soil is flooded. This treatment reduces the amount of seed required per acre. The practice is in limited use in California.

LOW TEMPERATURE INDUCED FLORET STERILITY

A field survey of floret sterility (blanking) was conducted in 1971 and 1972 by the UC Davis Department of Agronomy and Range Science. Average floret sterility was 12 to 13 percent, with a range from 2.7 to 34.8 percent. The amount of floret sterility differed according to variety, location and management. It is caused by night temperatures below 60°F for several consecutive nights 10 to 15 days before heading.

The developing panicles enclosed within the sheath are 3 to 12 inches above the soil surface during the low temperature sensitive stage of panicle development. Water temperatures are safely above the low night air temperatures and provide some protection against floret sterility. Even greater protection is provided by increasing water depth 2 to 3 weeks before heading. The shortstatured varieties are better protected by the warmer water temperatures than tall varieties because more of the panicles are still below water level during the low temperature sensitive stage of development. Very high nitrogen levels increase floret sterility.

Yields in the cooler rice growing areas are affected by floret sterility. Solutions to the problem include breeding varieties that are less sensitive to low temperatures, early planting, use of early maturing varieties, deep water prior to heading, and avoiding excessively high nitrogen rates.