Objective

Development of a better understanding of the fertilizer needs of rice plants and methods for correcting deficiencies in rice plant nutrition.

Nitrogen transformation in flooded soil

Usually, only 30 to 40 percent of the nitrogen applied to the rice crop is directly utilized for plant growth. The remainder is lost in a variety of ways, with one of the largest losses coming from volatilization of the nitrogen into the atmosphere. Degree of this volatilization loss depends on a variety of things: properties of the soil, air and water temperature, changes in water pH, and atmospheric conditions. Agronomic practices, including fertilizer source and rate, time, method and depth of application also influence volatilization.

Studies of the effects that soil type and rate and method of nitrogen application have on volatilization losses indicate that these losses vary from 0.25 to nearly 23 percent of the total nitrogen applied. Best results were obtained when the fertilizer was placed four inches below the soil.

Ammonia volatilization losses from the test soils were not as closely correlated with the initial pH of the soil as they were with the amount of algae and aquatic weeds which developed on the soils.

 

Effect of rate and method of nitrogen application of ammonia volatilization losses
		Percent of Added Fertilizer Nitrogen Lost as Ammonia
Soil type	N Applied lbs/ac	Broadcast Incorporated	Banded
Stockton Clay (pH 8.2)	0	--	--
	40	9.35	0.25
	80	10.33	0.37
	160	18.27	0.56
Meyers Clay (pH 5.7)	0	--	--
	40	1.40	NIL
	80	4.18	NIL
	160	5.03	NIL
Sacramento Clay (pH 6.9)	0	--	--
	40	13.45	0.56
	80	18.79	0.73
	160	22.59	0.72

Effect of varying the depth of nitrogen fertilizer

It was generally found that shallow incorporation, or drill placement,  of nitrogen fertilizer was superior to deep incorporation or broadcast application. Nitrogen recovery by plants at different sampling dates  indicated that drilled nitrogen fertilizer was the most efficient.

Foliar fertilization of rice

Foliar fertilization of field crops has received considerable attention in recent years with reports that solutions containing nitrogen, phosphorus, potassium and sulfur are increasing the yields of crops such as corn and soybeans.

Three experiments evaluating the effect of a commercially available foliar fertilization product on the growth and yield of rice suggest that foliar application does not increase rice yield. One exception was a treatment using the foliar fertilizer at a rate of 10 gallons per acre on M9 rice at the heading stage. Yields for this treatment were significantly better than when the product was applied at the panicle initiation stage. Foliar application of plant nutrients will not substitute for preplant fertilization of rice and does not appear to be a method for significant yield augmentation.

Sulfide toxicity to rice

The reduction of sulfate to sulfide under certain conditions is a major characteristic of flooded soils, but the water soluble sulfide content in most flooded soils is usually sufficiently low so that it does not limit rice production. However, in some California soils where proper reducing conditions occur, largely controlled by the presence of decomposing organic matter or crop residues, enough hydrogen sulfide can accumulate to cause a "physiological disease." This disease may blacken rice roots and, ultimately, arrest growth and kill plants. Soil pH and temperature also influence this condition. High soil temperatures favor acid volatile sulfide formation and rice plant toxicity, while cool soil temperatures favor the accumulation of water soluble sulfides.

The hydrogen sulfide toxicity problem can be corrected by liming to adjust soil pH and by avoiding the addition of straw of sulfate-sulfur (ammonium sulfate fertilizer) to the soil. Drainage of rice fields to temporarily aerate the soil also helps. Further research is needed before specific guidelines for proper soil and crop management to overcome this toxicity problem can be developed.