Soil Fertility and
Fertilizer Use - 2005



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Project Leader and Principal UC Investigators

Chris van Kessel, professor and chair, Dept. of Plant Sciences, UC Davis

Bruce Linquist, post-doctoral researcher, Dept. of Plant Sciences, UC Davis


This continuing project is evaluating the impact of grower field management practices on nutrient cycling. Its chief goal is to improve fertility management guidelines for rice growers.

Fertility Management

The 2005 season was the final year of a three-year aqua-ammonia nitrogen (N) rate fertility experiment involving field trials with 20 growers and 44 fields in locations throughout the major rice growing areas of the Sacramento Valley.

The experiment examined the impact of increasing or decreasing pre-plant, aqua-ammonia fertilizer application rates in 25-pound N/acre increments on mid-season fertility indicators such as color, height, vigor and tissue nitrogen levels, as well as yields at harvest. A zero nitrogen plot was included to determine nitrogen use efficiency at different fertility rates. Researchers took extensive soil and plant samples to help identify overall fertility trends across a variety of soils under different straw management practices.

Grain yield response to either a 25 lb N/ac reduction or increase to the standard practice (SP) aqua ammonia rate in fields where the straw was burned or incorporated.

Data from 2003 through 2005 show a yield response with increased N rates. In 2003 and 2005, years when statewide yields were down, however, the response was small and not significant. In more typical years such as 2004, though, the response to N was greater.

The 2005 research showed that when straw was incorporated in the fall the N fertility rates could have been reduced by 25 lb N/ac without reducing yields. In fields where straw was burned, however, the standard practice provided optimal yields. This suggests that growers are applying excessive N in straw-incorporated fields. These results from a large number of fields are consistent with other research results showing that where straw is incorporated N fertility rates can be reduced by 25 lb N/ac.

Starter Fertilizer Use

A 2003 fertility management survey revealed that starter fertilizers are used by 95 percent of growers. Yet no standard recommendations exist for the selection, timing or rate of application for starter fertilizer. Rice growers have asked for better information on starter nitrogen management, particularly under different straw and water management practices.

Starter nitrogen is either surface applied or lightly incorporated, which makes it readily available to newly emerging rice plants. Rapid early rice growth improves competitiveness against weeds. Strong early uptake of nitrogen through the use of a starter fertilizer may also increase overall nitrogen uptake from aqua-ammonia and indigenous soil sources.

Ten fields at five locations in the Richvale and Princeton rice-growing areas were chosen for a starter fertilizer experiment. An additional trial took place near Arbuckle in a field where rice is rotated with other crops. Each location had side-by-side straw-burned and straw-incorporated fields or checks, allowing researchers to compare the interaction between starter nitrogen and straw management. The effects of different starter nitrogen rates were also examined under a range of aqua-ammonia rates within a straw-burned, straw-incorporation or crop-rotation management regime. Plants were sampled at mid-tillering, panicle initiation and harvest to measure nitrogen accumulation, biomass and grain yield.

Rice biomass (left) and rice N (right) accumulation up to PI. Data is grouped by straw / crop management regime and is pooled across starter N treatments and regions.

Differences in rice biomass and nitrogen accumulation were noticeable by mid-tillering and panicle initiation. The highest nitrogen accumulation occurred in the crop rotation site. Nitrogen uptake and biomass at panicle initiation were significantly greater in fields where straw had been incorporated for three to four years than those where straw was burned.

Furthermore, at panicle initiation more starter nitrogen was taken up in fields where straw was incorporated or where rice was rotated with other crops than where straw was burned. Yield data taken at harvest, however, showed little response to treatment differences. The most likely reason for this is that 2005 was generally a poor year for rice with yields being about 10 to 15 percent lower than average. Lower yields result in lower nitrogen demand, so even the lowest rates in this study may have been adequate for optimal rice growth. Studies will continue in 2006.

Rice biomass (top) and rice N (bottom) accumulation up to PI. Two treatments, one with and one without starter N, were compared within straw / crop management group, across regions.


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