This project has been taking an in-depth look at the complex nutrient cycling changes, particularly with nitrogen and potassium, taking place under various straw management practices.  It also is examining how changes in soil fertility are affecting soil pathogens. This project has been supported by more than $1 million in research funds from a variety of sources (in addition to the Rice Research Board), including the California Energy Commission, Ducks Unlimited, the UC Sustainable Agriculture Research and Education Program, the Rice Foundation and CDFA’s Fertilizer Research and Education Program.

Fertility Impacts

A field experiment to determine maximum yield potential following prolonged straw incorporation has continued for nearly a decade.  With standard fertilizer practices, grain yield has not declined compared to the historical practice of burning rice straw.  It is clear that available soil nitrogen increases after three years of residue incorporation and winter flooding.  Under continuous straw incorporation and winter flooding, researchers determined, fertilizer nitrogen application could be reduced at their research sites 50 to 90 pounds per acre without sacrificing yield.

Researchers caution against extrapolating their findings universally to all rice fields.  For instance, the fertilizer nitrogen results apply to heavy clay soils and would need to be adjusted for different soil types. Variability from research findings may also exist because actual grower field practices are affected by weather, pest management and other factors.  Additionally, without winter flooding, no soil fertility benefit was derived and normal fertilizer practices must be continued to maintain yields.

Yields in various straw management and winter flooding treatments with no N fertilizer addition

In 2002 soil nutrient status, plant nutrient uptake and yield were monitored in 10 grower fields in Butte and Colusa counties.  These sites include a range of soil types and management practices.  Data from these fields — and other locations from throughout the Sacramento Valley that scientists hope to expand to — are being examined and will be reported on at a later time.

Concern over a perception that increasing amounts of organic matter in the soil may bind calcium and make it less available to rice plants has prompted growers to add lime.  There have been numerous anecdotal reports that liming fields may increase yields, improve soil tilth and bring about changes in weed pressure.  However, there is little scientific evidence to support these claims and a great deal of skepticism whether it is economically viable.  In 2002 two grower-managed sites were established for study.  These trials will run for three years and will be reported on later.

Soil pathogens

Scientists also examined how straw incorporation affects the presence of soil pathogens.  A number of root pathogens have been isolated from the roots of both rice and weeds.  As might be expected, burned plots showed the lowest level of pathogenic organisms, while incorporated, unflooded winter plots showed the highest level.  Soil incorporation seems to exacerbate the monoculture effect and potential chronic pathogenicty to rice.

Whole plot yields of all straw treatments from final year of the Maxwell experiment

In one laboratory experiment researchers demonstrated that in sterile soil, rice grew about twice as fast as in non-sterile soil.  This was not a soil nutrient effect. The most viable solution to reducing the potential effect of these root pathogens is to change monoculture management by introducing new and different crops into a rotation or growing a winter cover crop.

Field Guide

Work on a soil fertility guide for growers based on the results of long-term field plots and observations from individual grower fields has progressed.  A shortened version of a field guide was published in California Agriculture (56:69-75).  This article can be downloaded from here.

Work on a comprehensive fertility guide is ongoing.  Researchers have confidence in what they’ve learned about nitrogen and potassium.  A draft guide for nitrogen and potassium fertility is under review by UC farm advisors.  However, additional information on other macro- and micronutrients is needed to interpret differences among grower fields.