Improving Fertilizer Guide-
lines for California’s
Changing Rice Climate - 2008



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Project Leader and Principal  Investigators

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

Bruce Linquist, project scientist, Dept. of Plant Sciences, UC Davis

James Hill, UC Cooperative Extension specialist, Dept. of Plant Sciences, UC Davis


This project’s primary goal is to develop economically viable and environmentally sound fertilizer guidelines for California rice growers. Specific objectives include:

  • Improved nitrogen fertilizer guidelines for alternative water management strategies, such as early season drains
  • Improved nitrogen management strategies for conventionally managed fields and analysis of economic tradeoffs involved in fertility management
  • Evaluation of fall- or spring-applied phosphorous in relation to its availability for rice growth

Later surface nitrogen best

Effective weed management continues to be one of the greatest challenges facing California rice growers. If inadequately controlled, weeds can inflict major yield losses and lead to exorbitant herbicide costs. Herbicide-resistant weeds, combined with increased restrictions on herbicide applications, are limiting the effectiveness of traditional weed control strategies. Alternative water management strategies to control weeds are promising. However, this has profound implications for nitrogen management, since early-season drains can lead to significant soil nitrogen loss. Growers need up-to-date nitrogen fertilizer recommendations based on current water- and weed-management strategies.

Research in 2006 and 2007 improved knowledge of nitrogen dynamics in these systems. Improved nitrogen management will likely require changes to the timing or placement of nitrogen fertilizer to achieve necessary use efficiency.

In 2008 a study addressing this issue was conducted in a field between Biggs and Richvale. Subsurface nitrogen was applied by the grower as aqua ammonia to a depth of 3-4 inches using a commercial rig. Four rates were evaluated – 0, 80, 120 and 160 pounds/acre. In each main plot, three treatments were evaluated – no surface N, 40 pounds N/acre applied to the surface before planting, or 40 pounds N/acre applied 41 days after sowing. The field was planted on May 13 and drained for a Clincher® application on May 18. It was then reflooded on May 29.  When the soil was reflooded, it was dry enough to have large cracks. At harvest, the plots were sampled for total above-ground biomass and yield. Grain and straw samples were analyzed for nitrogen content in order to determine N uptake in each treatment.

When no nitrogen was applied, yields were 3,715 pounds/acre and there was a significant response to aqua ammonia. The highest yields were more than 12,000 pounds/acre in response to the 160 pounds N/acre treatment. There was also a significant response to surface-applied nitrogen. When surface nitrogen was applied preplant, yields increased by about 600 pounds/acre. When the surface nitrogen was applied in mid-season, yields increased by more than 1,000 pounds/acre.

These results indicate that surface nitrogen is better utilized when applied later in the season. Applying nitrogen before planting to the soil surface may have resulted in significant nitrogen losses due to nitrification. The data also suggest that the best nitrogen strategy in systems with an early dry-down is to apply nitrogen as ammonia. Applying fertilizer nitrogen deep in the soil helps to protect the nitrogen from denitrification losses.

Starter nitrogen unnecessary

Research has been conducted since 2005 to evaluate the necessity of surface-applied nitrogen just before planting. It was commonly believed that starter N improves early season vigor and is necessary to achieve high yields. However, starter N applied to the surface is more subject to loss than deep-placed aqua nitrogen.

In 2008 a large-scale experiment on three grower fields included two treatments: (1) growers’ standard practice of aqua and starter nitrogen as preplant nitrogen and (2) all preplant nitrogen as aqua nitrogen. Results indicated:

  • Surface applications of nitrogen increase early season biomass in some locations.

  • At equivalent nitrogen rates, yields for aqua N alone treatments were higher than those with both aqua and starter nitrogen.

  • Results confirm the findings of small-plot studies and effectively demonstrate that starter nitrogen fertilizer is not necessary. Furthermore, there is an opportunity to reduce a tractor pass across rice fields.

Fall or early spring phosphorous applications supported

Understanding the effects of application timing on phosphorous availability is important for a number of reasons. First, black algae is an increasingly common and phosphorous-sensitive problem for many growers. Applying phosphorous in the fall would allow for better mixing into the soil and result in less phosphorous in flood irrigation water. Second, research is showing that surface phosphorous applications in the spring may encourage weed growth. Third, in no-till or stale seedbed systems, phosphorous must be applied in the fall, since the soil is not tilled and fertilizer needs time to percolate into soil. Finally, if growers adopt the strategy of applying all nitrogen as aqua ammonia, they can reduce a tractor pass during the planting season and opt for a fall application of phosphorous (and potassium).

Applying phosphorous in the fall or early spring (before the first tillage) is not a new practice, but it has not been tested or widely used in California. In 2008, research examined phosphorous applied in the fall or early spring in three minimum-tillage fields and six conventional-tillage fields across Glenn, Colusa, Butte, and Sutter counties. Based on rice leaf-tissue samples taken 35 days after sowing and by yield at harvest in minimum tillage, this research showed that phosphorous applied in the fall is as available as phosphorous applied in spring preplant in conventional fields. In conventional fields, phosphorous concentrations varied in leaf samples, but there were no significant differences in grain yields at harvest. Because phosphorous does not appear to be limiting in the fields studied, it is important to examine whether changing the timing of fertilization will affect phosphorous availability. Research in 2009 will examine phosphorous fertilization timing in fields that have demonstrated a yield response in the past.


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