Improving Fertilizer Guidelines for California's Changing Rice Climate, 2014


Project Leader

Bruce Linquist, UCCE rice specialist, Dept. of Plant Sciences, UC Davis

The goal of this project is to develop fertilizer management guidelines that are economically viable and environmentally sound. Research objectives in 2014 were to determine the potassium status of rice soils, develop management practices for growing rice under alternating wet/dry conditions, and quantify rice yield variability in the Sacramento Valley.

Potassium status

Scientists assessed the potassium status of 55 rice fields in the Sacramento Valley in 2012 and 2013. Samples were taken from the soil, water, and rice flag leaves in all fields and analyzed for potassium. In addition, growers were asked about yields from fields, straw management, and winter flooding practices during the past five years.

Soil potassium values ranged from 35 to 350 ppm. Based on the field history information, there was no relationship between soil potassium values and the amount of potassium that had been added and removed. The primary observed pattern was that soil potassium values were lowest in the southeast part of the valley, followed by the northeast and northwest. Highest values were in the southwest. All fields below the threshold of 60 ppm were on the east side.

When soil potassium values were below 60 ppm, half the flag leaves sampled had values below the critical level of 1.2%. Based on this data, potassium fertilizer should be considered when soil potassium levels are below 120 ppm.

There was a significant difference in the concentration of potassium in irrigation water. The Sacramento River had the highest potassium values (1.18 ppm), while the Feather River averaged 0.79 ppm. Well water had the highest overall potassium concentration (2.3 ppm), but it was also highly variable. Recycled irrigation water averaged 1.4 ppm and also was variable.

Findings from this research will be developed into a format that is useful for growers and made available for online use, as well as distributed through other outlets to ensure that growers can further improve potassium management.

Alternating wet/dry conditions

Current water management practices keep California rice fields continuously flooded through the majority of the growing season. This strategy helps provide high yields, good weed control, and efficient nitrogen use. Nonetheless, there is interest in exploring alternative production practices should continuous flooding become restricted.

One strategy under study alternates flooding with periods of dry soils, which may help manage certain weeds. Experiments continued in 2014 to determine whether this approach is viable or not.

A research site at the Rice Experiment Station was established to evaluate three different water management practices—conventional water seeded, water seeded with alternating wet and dry conditions, and drill seeding with alternating wet and dry conditions. These were large plots, more than a half-acre each. Nitrogen rate trials were established in each main treatment plot. Greenhouse gas emissions also were measured. At the end of the season, plots were harvested, yields were determined, and samples were analyzed for nitrogen uptake.

As in the 2013 growing season, grain yields were similar among all three water management treatments. Water management had no significant effect on yield. Nitrogen response was similar among the different water treatments in the 2013 trial; 2014 data were not yet available.

In regards to greenhouse gas emissions, nitrous oxide emissions were low for all three treatments. Methane emissions were lower in the alternating wet-dry, water-seeded treatment compared to the conventional treatment. Methane emissions were lower still in the drill-seeded treatment. These results indicate that alternating wet and dry conditions can maintain rice grain yields and high nitrogen use efficiency, while significantly reducing the greenhouse gas emissions associated with rice cultivation. While these results are encouraging, it is not clear how easy this method would be to implement at the field scale.

Rice yield variability

California rice yields are among the highest in the world. However, over the past 15 to 20 years, yields have stagnated. Could there be ways to further increase yield through improved management to optimize the yield potential of varieties being developed? To find an answer to that question, work began on a database that includes:

• Statewide variety trial data from 1999 through 2013

• Yields linked to site-specific daily climate and soils data

• A procedure to calculate climate variables specific to statewide variety trials

Preliminary analysis shows that yields from the statewide variety trials will be a suitable comparison for statewide trends. Year-to-year variability in average yields for each site is relatively high, though some sites consistently yield greater than others. Temperature—specifically during flowering—appears to affect yields across sites. This will be the focus of future work.