An aerial view of the 15-acre research plot on a Sutter County farm to study the relationship between straw incorporation and green manuring.

Begun in 1988, the project involves a continuous rotation of rice and purple vetch on a Sutter County farm. Three different residue management strategies followed each rice harvest: fall burning, fall , incorporation and'spring incorporation. The treatments were replicated on one-acre plots. ,

Each fall half of each plot is seeded with a cover crop of purple vetch. Researchers examined how each of these different cropping systems influences nitrogen nutrition, vetch cultural practices, rice diseases and soil tilth. Additionally, nitrogen fertilizer experiments were conducted at six other farms in Butte, Sutter and Colusa counties during 1992 to provide information on the influence of straw disposal method on fertilizer nitrogen requirements and to evaluate potential indices, of soil mineralizable nitrogen.

Straw incorporation

As in past years, researchers observed no direct effect of the purple vetch cover crop on rice straw decomposition. Straw incorporation and green manuring practiced over three years has not resulted in any measurable change in soil tilth.

Neither does a green manure cover crop hasten rice straw decomposition between harvest and the following season's seedbed preparation. Researchers have not investigated whether a high. nitrogen cover crop would modify straw decomposition after spring incorporation and flooding.

Nitrogen nutrition

Researchers continue to find evidence that green manure cover crops add considerable nitrogen to the rice crop - replacing between 60 pounds and 90 pounds per acre of fertilizer, nitrogen. Also, preharvest, water-sown vetch resulted in nearly double the amount of nitrogen than vetch planted after harvest on fallburned or fall-disced plots.

Scientists working on this project are also studying at other locations nitrogen management in unburned fields. This hand-held chlorophyll meter gives researchers an immediate analysis of leaf greenness, an indication of nitrogen availability.

Grain yields in the six nitrogen fertilizer experiments were very high in 1992, ranging from approximately 9,500 pounds per acre. to a remarkable 13,600 pounds per acre. Stem rot disease limited maximum yields at straw-incorporated sites. On the burned plots nitrogen rates were not reduced below. that required to achieve high yields.

At the long-term Sutter County project site maximum yields for the green manure-straw management treatments ranged from 10,200 to 10,500 pounds per acre. For the third year in a row researchers observed higher grain yield on fall incorporated, green manured plots than on the burned plots. Researchers surmise that burning rice straw results in a loss of nitrogen that may be contributing to lower yields.

Disease incidence

In tracking the incidence of rice diseases in this study, researchers found stem rot severity was often greater on straw-incorporated plots than on burned plots. Stem rot severity is apparently unaffected, however, by the use of purple vetch. They also observed an inverse relationship between stem rot and aggregate sheath spot.

Other developments in the straw incorporation study include:

• A broadcast application of lime and/or superphosphate fertilizer on strongly acidic soil only marginally improved purple vetch growth but not enough to justify its cost.
• Researchers continued work on a soil test to estimate mineralizable nitrogen. Further refinements in experimental design are needed.