Project Leader and Principal UC Investigators
D.S. Mikkelsen, Department of Agronomy and Range Science, University of California, Davis
There is an increase in the availability of phosphorus from the soil following flooding largely because of the increased solubility of iron phosphate compounds. After a flooded soil is drained and exposed to air, a reverse process occurs in which ferrous compounds fix the phosphorus into an unavailable iron complex form. Organic matter such as rice straw incorporated into the soil increases phosphorus fixation. As soil temperatures increase in the spring, phosphorus fixation increases. For each 10°C soil temperature increase, phosphorus fixation increases three-fold. Application of phosphate fertilizer temporarily increases soluble phosphorus, but new insoluble phosphate compounds are soon formed. Banding of phosphate fertilizer near the seed of crops grown in rotation with rice provides enough phosphorus for satisfactory yields.
New chemical and biological techniques for predicting nitrogen fertilizer requirements have been evaluated that correlate well with plant nitrogen uptake, dry matter production and rice grain yields. An anaerobic soil incubation technique measures the availability of nitrogen after two weeks at 25°C. Correlation with yield and nitrogen uptake has been excellent. A second procedure involving chemical oxidation of soil with alkaline permanganate or acid dichromate also gave good results and has the advantage of a shorter diagnostic period. These methods provide the possibility of predicting in advance of rice seeding, the amount of pre-planting nitrogen to apply and, thereby, avoiding over- or underfertilization.
Recovery of nitrogen from urea is slightly less than from aqua ammonia or ammonium sulfate banded in the same manner. Nitrogen recovery by plants is reduced to a small extent by direct ammonia volatilization from the flood water, 3 to 9 percent is fixed by clay minerals and about one-third of the fertilizer nitrogen is temporarily immobilized in the soil organic matter fraction. A small amount of ammonia is volatilized directly from plants when high nitrogen rates are used and when plants grow in air temperatures exceeding 90°F after flowering.
These studies were conducted from other than Rice Research Board funds.