|Nutritional and Environmental Factors
Project Leader and Principal UC Investigators
Dr. Duane S. Mikkelsen,Department of Agronomy and Range Science, UC Davis
Using plant analyses to diagnose the nutritional status of rice helps insure fertilizer practices that optimize rice yields. Recent adoption of short-statured rice varieties in California has made it necessary to revise the critical nutrient levels. The newest fully-developed leaf on the rice plant, the "Y-leaf," is the most sensitive and convenient plant part for N-P-K evaluations. Field test correlations between grain yields, fertilizer application, and the total N and 2% HAc extractable PO, -P and K content of the "Y-leaf" suggest that optimum rice yields can be obtained if plant nutrient levels are kept above critical values at each stage of plant development.
Nitrogen fertilizer use efficiency by the rice crop varies between 30 and 60 percent, depending upon the time and method of fertilizer application. Sulfur-coated urea (SCU) developed by the National Fertilizer Development Center is an experimental fertilizer that has slow release characteristics. SCU dissolves slowly, releasing ammonium nitrogen for rice over a period of 12 to 16 weeks, whereas, urea nitrogen is dissipated after 4 to 6 weeks. SCU or other slow-release nitrogen fertilizers can help conserve fertilizer nitrogen and increase fertilizer use efficiency where fertilizers are not incorporated into the soil and promptly flooded. SCU with slow release characteristics retains from 6 to 21 percent of its nitrogen after 16 weeks, depending upon formulation, even if flooding is delayed up to 4 weeks after the fertilizer is applied. SCU has significantly increased grain yields over those obtained using urea.
The incorporation of rice straw in a rice-rice rotation presents some serious practical and economical problems, including possible development of gases and organic acids that may be toxic to rice and a possible increase of stem rot. However, straw incorporation does increase the availability of certain plant nutrients. The release of ammonium-nitrogen from the organic matter increases availability of soil phosphorus, iron, manganese, and zinc in many typical California soils used in rice production.
Green manure crops such as vetch or horse beans can provide the equivalent of 100 pounds per acre of fertilizer nitrogen when they are incorporated into the soil preceeding rice plantings. Comparison of vetch and ammonium sulfate fertilizer in field experiments indicates that the green manure crop did not provide as much nitrogen to rice as did the fertilizer when applied at the same N-rates in the first year after application. Fertilizer nitrogen was more available than vetch nitrogen for the first 45 days of plant growth, and the final grain yields were significantly better with fertilizer nitrogen. Vetch nitrogen at 53, 106, and 159 pounds per acre gave respectively 5,560, 6,157, and 6,700 pounds rough rice per acre. Ammonium sulfate at the same rates of application gave 6,548, 8,081, and 8,090 pounds rough rice per acre.
Research on the effects of air temperature on the growth of California rice varieties indicates that seedling growth is favored most by air temperatures between 77 and 86°F, but root anchorage is best at about 77°F. High air temperatures increase the rate of tiller production, but the duration of tilling is shortened. With the seeding rates used in California, high tillering is not usually essential for high grain yields. The rate of leaf emergence increased with temperature over the range of 60 to 95°F, but photosynthesis is relatively insensitive to temperature. The rate of nutrient accumulation in the rice plant increases with temperature up to about 86°F. Grain development is delayed in the low temperature range of 52 to 65°F, but the ripening period is extended.
Water stress is not usually a problem in the production of flooded rice, but the question arises as to what stage of plant growth does water stress have the most effect on rice yield. Experiments reveal that yield reduction is greatest when water stress occurs from mid-tillering to the panicle initiation stage. Late season water stress, during grain filling, has the least detrimental effect on yield, but may adversely affect grain quality.