Study Helps Steer Organic Rice Producers to New & Effective Fertilizers

Project title: Evaluation of alternative nitrogen sources for California rice systems

Investigator: Bruce Linquist, Dept. of Plant Sciences, University of California, Davis, CA
Project location: Sacramento Valley, California
Project links: Complete project report (20pp) by co-investigator Paula Wild

Researchers evaluating fertilizer products take soil samples in a drained rice field prior to reflooding.

In 2008, Lundberg Family Farms initiated a project to explore new sources of nitrogen fertilizers for organic rice systems. Family member Jessica Lundberg contacted Bruce Linquist at the University of California, Davis and developed a plan to evaluate various pelletized fertilizers in comparison with poultry litter, a traditional fertilizer resource among organic rice producers. Graduate student Paula Wild conducted the field research at Lundberg producer Aaron Scheidel’s fields in Nicolaus, California. This one-year trial provided valuable data to help guide organic rice producers to new fertilizer resources that work well in flooded rice production conditions.

Key results: Study results showed that pelletized fertilizer products provided higher rice yields and economic return than traditionally used poultry litter products under continuously flooded field conditions. Under drained field conditions, fertility products may not be warranted.

Other outcomes: Project co-investigator Paula Wild presented the results of this project at the annual American Society of Agronomy meeting, November 1, 2010. An article on project results is published in Agronomy Journal, July 2011, Volume 1033, Issue 4.

SUMMARY
California is the largest single producer of organic rice worldwide with 15,000 acres, most of which is located in the Sacramento Valley. Nutrient management is a major challenge for organic rice growers. In the Northern Sacramento Valley where this study was conducted, soils have high clay content and nitrogen (N) fertility has traditionally been applied in the form of poultry litter (PL) due to the difficulty of growing cover crops under these heavy soil conditions. For over a decade, PL from California’s poultry industry has served as a primary source of nutrients for organic rice producers.

Therefore, the objectives of the present study were to (1) to determine the effectiveness of organic fertilizer N mineralization in meeting rice crop N demand, as reflected by plant N uptake, NRE, and improved grain yield; (2) to determine N mineralization rates of organic fertilizers, i.e., poultry litter, blood and bone meal, feather meal, and poultry litter plus feather meal under anaerobic conditions; and (3) to compare the returns on investment in the pelletized organic materials with those of poultry litter.

A UC-Davis student hand-applies fertilizers to test plots in rice field.

Field trials were conducted at Scheidel Ranch in Nicolaus, California, in two adjacent fields in 2009. The two fields were managed differently by the grower. One field was continuously flooded over the course of the growing season (henceforth referred to as CF field). The other field was drained at 25 days after sowing (DAS) and remained dry for one month as a weed control measure, a common practice in organic rice production (henceforth referred to as drained field). The drained field was then re-flooded at 55 DAS, however the drain period slowed the rice growth and resulted in a delayed harvest. The difference in field management between the CF and the drained field influenced project results.

Trials were planted in a randomized complete block design with an N-responsive rice variety (S-102) and fertilized at a rate of 134 kg ha-1 (120 lb ac-1) with four sources of N fertilizer as treatments (N rates were based on manufacturer’s stated N-concentration as listed below):
           
            Poultry litter (PL) 3% N; 
            Pelleted PL and feather meal 6% N (6-3-2);
            Pelleted feather meal 12% N (12-0-0);
            Pelleted bone and blood meal 13% N (13-0-0).

An unfertilized control (no N) was also used.

The four fertilizers used in the project: poultry litter, above left, was compared with three pelletized products.

Organic phosphorus and potassium were applied to all plots to ensure that these nutrients were not limiting. Aboveground biomass, grain yield, plant N-uptake, and soil ammonium-N (NH4+-N) were measured at different stages of crop growth. An anaerobic laboratory incubation with the aforementioned fertilizers was conducted to quantify mineralization rates of the organic fertilizers.

Project Results
Relative to poultry litter, the pelletized fertilizers resulted in higher yields (9,392 vs. 8,708 kg ha-1), N uptake (138 vs. 115 kg ha-1), and N recovery efficiency (36 vs. 20%) in both fields.

Yields. Among the field sites, grain yields (reported at 14% moisture) ranged from 7,055 kg/ha (7,912 lbs/ac) to 10,072 kg/ha (11,299 lbs/ac). Grain yield in control treatment averaged 7,166 kg/ha (8,039 lbs/ac) and was always significantly lower than the yields in fertilized plots.

Pelletized fertilizer response varied across fields, but on average pelletized fertilizers increased yield by 2.5 Mg ha-1 over unfertilized control plots, compared to PL, which increased it by 1.8 Mg ha-1. When the variation was partitioned using contrasts, PL resulted in significantly lower grain yield than pelletized fertilizers in the CF field; however this difference was not significant in the drained field.

Nitrogen uptake and recovery efficiency. N uptake increased over the course of the growing season, and was similar in both fields at harvest. However, in the CF field the bulk of N uptake (84%) occurred before panicle initiation, while in the drained field roughly half of seasonal N uptake occurred after panicle initiation.

Low N uptake before panicle initiation as compared to CF field may have been due to drought stress created by the extended drain. Comparing the organic fertilizers, poultry litter had lower total N uptake (101 to129 kg ha-1) than the pelletized fertilizers (126 to 143 kg ha-1) in both fields although differences were not always significant. Pelletized fertilizers performed similarly to each other with no consistent differences in plant N uptake between them.

Fertilizer N-recovery efficiency (NRE) was calculated as the difference in N uptake between the fertilizer treatment and the control, divided by the rate of N applied.

In the CF field, the average NRE of pelletized materials ranged from 37-50% and was higher than in the drained field where it ranged from 25 to 31%. The NRE of PL was similar in CF and drained fields, ranging from 19 to 21%.

These data indicate that PL, at the rates applied, does not provide sufficient N to meet crop demand, resulting in lower rice yields.  The pelletized materials mineralize at a faster rate and thus provide higher yields at similar N rates.

A comparison of fields under different management practices (drained on left, continuously flooded on right). The top photo was taken at 53 days after seeding. The bottom photo taken at harvest. Note high weed density in drained field.

Economics. The returns on investment in fertility were high in the continuously flooded field (between 57% and 76%), with poultry litter having the lowest increase in profits from fertilizer. These results suggest that an investment in pelletized materials with higher N content may be warranted for continuously flooded fields.

The returns on investment in fertility were lower and more variable in the drained field (between -21% and 80%) suggesting that investment in fertility may not be worthwhile in the event of an extended drain.

It is difficult for growers to predict at the beginning of the season whether they will need to drain a field due to weed pressure. Further work is needed to determine the appropriate timing and rate of application of fertilizers in fields under both types of management practices. This will lead to a more refined economic analysis.

A final report describing the results of this project was submitted in March 2010.