|Fertilization - 72
Project Leader and Principal UC Investigators
D.S. Mikkelsen,Fertilization of Rice and Diagnosis of Nutirtional Problems
The so-called "alkali disease" problem, widespread in sodic and high-pH soils in California, has been identified as zinc deficiency.
Affected plants usually die within 4-6 weeks of planting unless remedial top dressing is done 21-28 days after seeding. First symptoms are a yellowing of leaves, starting at the base and gradually spreading over the entire leaf. Later, dark necrotic spots develop on the leaves, which are naturally weak and lie on the water.
Zinc fertilizer, less costly and more effective than remedies used before this discovery of the research program, are expected to save California rice growers $225,000-375,000 annually - another instance where a single finding pays big dividends.
Before this discovery, various commercial products have been used despite undependable benefits and uncertainty as to the actual mechanism. Ferric sulfate, iron oxide, and similar products were effective when they contained zinc as a contaminant. Now, however, the most economical and effective procedure is to use one of the zinc fertilizers commercially available, such as zinc oxide or zinc sulfate. Results are about equally good with either, and the cost is only about $5 per acre (contrasted to $20-30 for bulky iron sources containing zinc as a contaminant). (RB1)
Over $4 million is spent annually on fertilizer by California rice growers. An increase in fertilizer efficiency of only 10% would save about $400,000. Proper fertilization (all that is needed, but no more) should yield substantial economies while providing yields that are optimum in quality and quality.
In a statewide survey, 23% of rice fields sampled had applied insufficient nitrogen for optimum yields, 11% were deficient in phosphorus, and 1% were deficient in potassium. Yields can be greatly increased by identifying such deficiencies and remedying them by timely fertilizer applications. The survey was conducted cooperatively by growers, the California Fertilizer Association, and Agricultural Extension.
Plant analysis is a newly developed and valuable tool for evaluating the nutritional status of rice. It provides a sure way of making fertilization efficient and optimum in effect. Recently matured leaves are sampled, preferably at three stages of plant development to allow a continuing check on fertilization adequacy. Then, when nitrogen, phosphorus, potassium, or zinc falls below the minimum for optimum plant growth, fertilizers can be applied.
This precise method replaces the traditional dependence on personal judgment and previous cropping history to determine fertilizer needs. It outmodes the considerable guesswork consequent to differences in soils and cropping history and climatic variables from year to year. (RB1)
Fertilizer effectiveness can be boosted 25-50% by proper placement. Incorporating preplant nitrogen 2-4 inches deep may give 35% more efficiency than surface application. The effectiveness of phosphorus and potassium was improved only slightly by incorporation, though they should be incorporated or drilled simultaneously with any nitrogen applied.
Zinc, in contrast, is better when broadcast on the surface before planting. Since zinc is critical to seedling requirements during the first four weeks, it must be available near the roots. Zinc should not be incorporated unless rates are heavy enough that, despite incorporation, it remains adequate near the roots of developing seedlings. (RB1)
Nitrogen is needed in relatively large amounts for satisfactory yields and returns from rice. High nitrogen needs of rice plants occur at two peaks: 1) in early vegetative growth, to promote optimum tillering, expand leaf area, and maximize numbers of heads per unit land area; and 2) when plants begin reproductive growth, to promote head development (including maximum seed differentiation and proper filling of the seed on each head).
Preflood nitrogen promotes the development of head-bearing tillers and supplies extra nitrogen for increasing the number of differentiated seed and preventing degeneration of the seed formed, thus increasing percentage of ripened grain and final grain yields.
When nitrogen top-dressings are needed, optimum application time is 14-21 days after head differentiation (about 60-80 days after seeding of 150-day-maturing varieties like Caloro and Calrose, and 50-65 days after seeding of early varieties like Colusa and Earlirose). Plant analysis can determine whether top-dressing is desirable, and suggests the amounts needed. Yields have been increased while fertilizer was saved, thanks to the new knowledge on timely top-dressings to promote plant growth and grain formation. (RB1)
CONTROLLED NITROGEN RELEASE
Nitrogen fertilizer may be lost when preplant flooding is delayed or flooded fields are drained. Drainage or delayed flooding allows ammonium nitrogen to be transformed to nitrate nitrogen, which is lost in gaseous form upon flooding. As much as 70% of the nitrogen fertilizer applied can be lost in this way.
Reduction of this type of loss would appear possible through controlled-release fertilizers. Wide tests in California show promise in new materials such as sulfur-coated urea, which releases nitrogen only after the sulfur coating deteriorates. Sulfur-coated urea performed well both under delayed flooding and under a drainage-flooding sequence. As expected, no advantage was apparent under the ideal conditions of timely and continuous flooding. (RB1)