Improving Fertilizer Guidelines for California's Changing Rice Climate, 2013


Project Leader

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

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

James E. Hill, 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 2013 were to: determine the potassium status of rice soils, quantify nitrogen losses from leaching, and develop management practices for growing rice under alternating flooded/dry conditions.

Potassium status

Scientists assessed the potassium status of 24 rice fields in the Sacramento Valley in 2013. In the previous year, 31 rice fields were examined. Grower surveys were conducted to determine the history of each field—fertilizer and straw management, yields, and water source. Soil samples were taken from three checks within each field. Water samples were taken twice during the season. Flag leaf samples were taken at heading. Data from both years of this study are being analyzed together. In 2012, 14 of 31 fields had a recent history of potassium fertilization, while in 2013 six of the 24 fields received potassium fertilizer. Average rate was 33 pounds/acre. Two fields, both in 2013, had rice straw baled regularly.  Eleven fields used well water for irrigation.

Soil potassium levels ranged from 35 ppm to 350 ppm. Potassium levels, however, were not related to fertilizer history. Soil potassium values were lowest in the southeast part of the Sacramento Valley, followed by the northeast and northwest. The highest values were in the southwest. All fields below the critical threshold of 60 ppm were located on the east side of the valley.

The critical flag-leaf potassium value is 1.2%. Where the soil potassium values were below the critical threshold, half the flag-leaf samples had potassium values below the critical level as well. However, where soil potassium ranged from 60 ppm to 120 ppm, eight flag-leaf samples had potassium levels below the critical range. This suggests that potassium fertility may need to be considered at levels below 120 ppm.

Potassium concentrations varied significantly in different irrigation waters. Of the two primary rivers, the Sacramento River had the highest potassium values at 1.18 ppm, while the Feather River averaged 0.79 ppm. Well water had the highest overall potassium concentration at 2.3 ppm, but it was also highly variable. Recycled irrigation water averaged 1.4 ppm and also was variable. (See graph at right.)

Nitrate leaching

California’s irrigated lands program may begin putting water quality restrictions on agricultural management practices that allow nitrate to enter surface and ground waters. Previous research addressed nitrate in surface water. This project in 2013 focused on groundwater and nitrate leaching. Little data had been available quantifying nitrate leaching in flooded rice systems.

Rice soils in California are relatively impermeable. The potential for nitrate leaching is believed to be minimal because of slow percolation of water and anaerobic conditions in flooded soils that cause nitrate to be lost to the atmosphere.

To quantify nitrate leaching losses in rice fields, researchers:

Measured nitrate in soil samples to a depth of 7 feet

• Directly measured nitrate in water below the rice root zone for a full year at four locations

• Measured labeled fertilizer nitrate to determine the source of nitrate below the root zone

• Analyzed data from shallow wells in the rice-growing region for nitrate

Average potassium (K) concentration from various sources of irrigation water. Feather River includes the Yuba and Bear Rivers.

Soil nitrate concentrations below the root zone were usually less than  1 ppm.  Nitrate was observed in water samples below the root zone only during the first two weeks after flooding—winter or summer. Fertilizer nitrogen accounted for zero to 12% of nitrate in water samples below the root zone. After one year, 2.5% of fertilizer nitrogen was recovered below the root zone. Wells associated with rice all had nitrate levels below 10 ppm, the standard for water quality.

The results all indicate that nitrate leaching is not a concern for the majority of California rice soils under current crop management practices. This research has been completed and made available to various stakeholders, including growers, the California Rice Commission, and legislators. It will also be published in the Journal of Environmental Quality.

Alternating wet/dry rice

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. Furthermore, recent research has shown that on the highly impermeable soils, where a majority of California rice is grown, water is used very efficiently. Nonetheless, for a variety of reasons there is interest in exploring alternative production practices.

One strategy under study alternates flooding with periods of dry soils, which may help manage certain weeds. However, rice yields can be reduced substantially with this practice. Experiments were conducted to determine whether this approach is viable or not.

A research site was established to evaluate three different water management practices—conventional water seeded, water seeded with a “flush” irrigation, and drill seeding with continuous flushing. These were large plots, more than a half-acre each. Greenhouse gas emissions were also measured.

Rice grain yields were good, averaging 9,400 pounds/acre across all treatments. Water management had no significant effect on yield. Nitrogen response was similar among the different water treatments. Greenhouse gas emissions varied considerably among treatments. The conventional treatment had much higher methane emissions than the other treatments. Nitrous oxide emissions were low for all treatments. More research is needed in this area and is planned for 2014.