How Much "N" to Sidedress: Try a PSNT

Henry Krusekopf, Jeff Mitchell, Tim Hartz, Don May, Gene Miyao & Michael Cahn

Excessive nitrogen (N) application is an economic loss to growers in terms of unnecessary input costs, and may also result in greater pest management problems. From an environmental perspective, overuse of chemical nitrogen fertilizer has been associated with increased levels of nitrate-nitrogen (NO3-N) in ground and surface water. For these reasons, development of a better system for recommending fertilizer rates is a major goal of agricultural research.

The largest N fertilizer input for processing tomatoes occurs at sidedressing when plants are 4 to 6 inches tall. Recommended sidedress N application rates are 120 to 180 lbs N/acre, but to ensure maximum yield growers typically apply 130 to 225 lbs.

PSNT: Research has shown a correlation between NO3-N concentration in the top 12 inches of soil prior to sidedressing and crop yield response to sidedress N. The evidence suggests that a Pre-sidedress Soil Nitrate Test (PSNT) can indicate a critical level of soil NO3-N above which crop yield will not be increased by subsequent sidedress N application. Although the PSNT method has not been widely used to determine specific sidedress N application rates for fields testing below a critical level, it has been found helpful at identifying fields where no sidedress N fertilizer is required to maintain yields.

Research Goal: The main objective of our research was to determine if the PSNT technique was useful for predicting the necessity of sidedress N fertilizer on a field-by-field basis in conventional processing tomato production in California.

Ten Tomato Fields: The project was carried out at 8 com-mercial farms and 2 research stations in 1998-99. Tomato plantings followed standard crop rotations for the region and the individual grower's cultural practices including pre-plant and/or pre-sidedress N fertilization. Common hybrid tomato varieties were grown at all locations. All fields received a single sidedress application of urea at rates between 0 to 250 lbs N/acre in six increments (0, 50, 100, 150, 200, 250 lbs N/acre) when plant height was approximately 4 to 6 inches. Fertilizer was banded using a standard applicator to a depth of 6 inches, and at a distance of 6 inches from the plant row. All fields were furrow-irrigated and grown under typical practices.

Prior to sidedress N application, a PSNT was conducted at all sites to a depth of 12 inches. Soil cores were taken from shoulders of beds then immediately stored on ice to inhibit N mineralization until processed and analyzed for NO3-N concentration. At harvest, fruit yields were determined by mechanically harvesting plots into a scale-equipped wagon. Samples of unsorted fruit were collected from the harvester from each plot for determination of fruit maturity and percent defects. Fifty red fruit from each plot were evaluated for soluble solids content - which included the degrees brix - and blended juice color. Relative fruit yield for each treatment was calculated by dividing the mean yield for each treatment by the mean of the highest yielding treatment in that field.

Yield Results: Of the 10 fields utilized in this study, only four fields had any significant yield response to sidedress N, and none of these fields demonstrated yield response to sidedress N application above 100 lbs N/acre. In fields eight, nine and ten the application of any sidedress N increased yield compared to unfertilized plots, but yields at 50 lbs N/acre were not significantly different to those achieved with higher fertilization rates. In field four, a significant yield increase was observed up to 100 lbs N/acre. There were no fields with yield response to sidedress N application that had pre-sidedress soil NO3-N concentrations above 15.7 ppm.

Fruit Quality Results: Fruit maturity and quality parameters (% red, % rotten fruit, blended fruit color, and soluble solids content were generally unaffected by N treatment in most fields. In field four, the unfertilized and the 250 lbs N/acre treatments showed the lowest color score - most intense red color - while intermediate N rate treatments had lower soluble solids content. In field eight, fruit soluble solids content decreased with increasing N rate.

Excessive N Rates: This study showed that both university recommended and industry sidedress N application rates for processing tomato production in CA are excessive and could be substantially reduced without loss of yield or fruit quality.

Critical Levels: The lack of yield response to sidedress N application in fields with NO3-N at greater than 16 ppm prior to sidedressing was not surprising, since these soil nitrate nitrogen levels represented more than 60 percent of seasonal total N uptake (180 lbs N/acre) for high-yield tomato production. Pre-sidedress residual soil N in project fields was augmented by in-season N mineralization of soil organic matter, which could have provided an additional 35 to 70 lbs N/acre to plants during the growing season. Therefore, in-season mineralization of organic N, coupled with existing soil NO3-N estimated by PSNT, are likely factors in the overall weak crop response to sidedress N. A PSNT level greater than 16 ppm of NO3-N in the top 1 foot of soil could represent a conservative threshold level for determining whether sidedress fertilization is required. This suggested PSNT threshold level for processing tomatoes is slightly lower than those determined for corn production in the Northeastern and Midwestern USA, and CA coastal valley lettuce and celery production. These studies generally set PSNT thresholds between 20 to 25 ppm NO3-N.

Summary: These results support using a PSNT to identify processing tomato fields that are unlikely to respond to sidedress N application. Fields with concentrations greater than 16 ppm of NO3-N in the top foot of soil have low prob-ability of increased yields with sidedress N application. Furthermore, the limited response to sidedress N application, even in fields with minimal residual NO3-N levels, suggested that sidedress N rates currently used by the commercial tomato industry could be substantially reduced with no loss of yield or fruit quality.

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