Nutrient Management Research Database

Overuse of chemical N fertilizers has been linked to nitrate contamination of both surface and ground water. Excessive use of fertilizer also is an economic loss to the farmer. Typical N application rates are 150 to 250 kg/ha. The contributions of residual soil NO3-N and in-season N mineralization to plant nutrient are not generally included in fertilizer input calculations, often resulting in overuse of fertilizer. The primary goal of this research was to determine if the pre-sidedress soil nitrate test (PSNT) could identify fields not requiring sidedress N application to achieve maximum tomato yield; a secondary goal was to evaluate tissue N testing currently used for identifying post-sidedress plant N deficiencies. Field experiments were conducted during 1998 and 1999. Pre-sidedress soil nitrate concentrations were determined to a depth of 60 cm at 10 field sites. N mineralization rate was estimated by aerobic incubation test. Sidedress fertilizer was applied at six incremental rates from 0 to 280 kg N/ha, with six replications per field. At harvest, only four fields showed a fruit yield response to fertilizer application. Within the responsive fields, fruit yields were not increased with sidedress N application above 112 kg/ha. Yield response to sidedress N did not occur in fields with pre-sidedress soil NO3-N levels > 16 mg/kg. Soil sample NO3-N levels from 30 cm and 60 cm sampling depth were strongly correlated. Mineralization was estimated to contribute an average of 60 kg/ha N between sidedressing and harvest. Plant tissue NO3-N concentration was found to be most strongly correlated to plant N deficiency at fruit set growth stage. Dry petiole NO3-N was determined to be a more accurate indicator of plant N status that petiole sap NO3-N measured by a nitrate-selective electrode. The results from this suggest suggested that N fertilizer inputs could be reduced substantially below current industry norms with reducing yields in field identified by the PSNT as having residual pre-sidedress soil NO3-N levels >16 mg/kg in the top 60 cm.

• Avoiding excess N fertilization allows growers to save money, while also helping to protect the environment.
• Sidedress N application accounts for the majority of N fertilization for processing tomatoes.
• Pre-sidedress soil nitrate testing (PSNT) allows for on the fly adjustments to nutrient budgets during the season.
• Threshold levels of soil nitrate-N had previously been developed for other crops, this paper investigated the usefulness of PSNT to identify tomato fields that do not require sidedress N fertilization.

• PSNT results did not differ between 0-12 and 12-24 inches.
• Only 4 of 10 fields showed yield increases with sidedress N applications, many fields did not show increases above the lowest rate studied (50 lbs/ac).
• Fields with PSNT results of 16 ppm or higher should not show yield increase with sidedress N applications.