Cover Crop Interaction with
Rice Straw-90



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Project Leader and Principal UC Investigators

G. Stuart Pettygrove, Extension Siols Specialist, Dept. of Land, Air and Water Resources, UC Davis

Deng Jiayou, postgraduate researcher, Dept. of Land, Air and Water Resources, UC Davis

S. Upadhyaya, Dept. of Agricultural Engineering, UC Davis

R.K. Webster, Dept. of Plant Pathology, UC Davis

C.M. Wick, UC Cooperative Extension, Butte Co.

J.F. Williams, UC Cooperative Extension, Sutter Co.


 A special grant from the Rice Research Board established this multi-year field study of the interaction of green manure and straw disposal. Two sites were chosen in commercial rice fields, one in Butte County (Emmett and Bill Skinner Farm) and another in Sutter County (Sills Farms).

Rice growers may some day routinely rely on cover crops to enhance the nitrogen content of the soil. This is a photo of white lupine from a study two years ago.

Legume cover crops were grown on plots at both sites; nitrogen rate trials were conducted for the 1989 and 1990 rice crops. Rice straw treatments-fall burning, fall discing or fall rolling, with all plots disced in the spring-were conducted only at the Suttee County site. The objectives of the study were to:

  • Determine the effects of winter green manure cropping on rice straw decomposition.
  • Determine the effects of straw incorporation and green manuring on soil tilth.
  • Determine the effects of straw incorporation and to develop a soil test for estimating potentially available nitrogen.
  • Determine the effects of green manuring on stem rot disease where rice straw had been incorporated.
  • Evaluate lime and phosphate applications on purple vetch grown on acid soil; investigate pelleting of these materials.

Field measurements and a greenhouse experiment showed no obvious evidence that a green manure crop will hasten rice straw decomposition. It appears possible that fall discing resulted in some reduction in straw but not to the levels found on burned plots. Further experimentation is needed.

There is no single measure of soil filth, but researchers are using resistance to a tractor-pulled chisel as a reflection of soil strength related to filth. They are also monitoring soil organic carbon; penetrometer resistance and bulk density.

"Field and greenhouse observations show no indication that a green manure crop will hasten rice straw decomposition."

So far, little or no difference has been observed in measurements of chisel draft or bulk density. However, data from fall 1990 was still being analyzed as this report was being written. Measurements will be made in the spring and fall of 1991, and finally in the spring of 1992-after four green manure crops and three rice crops in a continuous rotation.

At both sites in both years, the fertilizer replacement value of green manure has exceeded the above-ground nitrogen content of the cover crop just before spring discing. At the Sutter County site in 1990, for instance, the purple vetch containing 16 to 38 pounds per acre nitrogen provided a nitrogen fertilizer replacement value of 60 to 75 pounds per acre. This is a striking illustration of the greater efficiency of nitrogen recovery from a cover crop than that of a preplant fertilizer.

Vetch stand uniformity was greater on fall-burned plots than on fall-disced or fall-rolled plots, apparently compensating for the loss of nitrogen that occurs due to rice burning.

"Purple vetch showed greater nitrogen recovery efficiency than preplant fertilizer. "

On non-green-manured plots, spring-discing of rice straw resulted in an increased nitrogen requirement, apparently due to immobilization of nitrogen by soil microorganisms. The nitrogen requirements of spring-disced plots were no higher than those for burned or fall-disced plots that were green manured. Apparently, purple vetch enhanced nitrogen mobility.

Work is continuing on development of a nitrogen mineralization soil test. The most promising method relies on anaerobic incubation of soil ammonium. Both stem rot and aggregate sheath spot appeared at the Sutter County site in 1990. Plant samples were collected for disease rating in September 1990. Data are being analyzed.

A number of small plots were established in the fall of 1990 at the Sutter County site to determine the best method for establishing cover crops in unburned rice residue. Factors being tested include cover crop species, pre-harvest versus post-harvest planting, phosphate fertilizer requirements and response to pelleting of rock phosphate with seed. Purple vetch and "Lana" woollypod vetch survived the drought, but clovers included in the preharvest sewing did not.

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