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Overuse of chemical N fertilizers has been linked to nitrate contamination of both surface and ground water. Excessive fertilizer use is also an economic loss to the farmer. Typical N application rates for processing tomato production in California's Central Valley are 150-250 kg·ha-1, and growers generally fail to fully consider the field-specific effects of residual soil NO3-N concentration, or N mineralization potential of the soil. The purpose of this research was to determine the effects of sidedress N fertilizer application, residual soil NO3-N, and in-season N mineralization, on processing tomato yield. Research was conducted during the 1998 and 1999 growing seasons at 16 field sites. Pre-sidedress soil nitrate concentration was determined at each trial site to a depth of 1 m, and aerobic incubation tests were conducted on these soils (top 0.3 m depth) to estimate N mineralization rate. Sidedress fertilizer was applied at six incremental rates from 0 to 280 kg N/ha, with six replications of each treatment per field. Only five fields showed yield response to fertilizer application; yield response to fertilizer was associated with lower pre-sidedress soil nitrate levels. In most fields with fertilizer response, yield was not increased with sidedress N application above 56 kg·ha-1. Mineralization was estimated to contribute an average of ≈60 kg N/ha between sidedressing and harvest. These results suggest that N fertilizer inputs could be reduced substantially below current industry norms without lowering yields, especially in fields with higher residual soil nitrate levels.
In Fall 1995, 12 row crop farmers in conjunction with Univ. of California, NRCS and private agency advisors established the West Side On-Farm Demonstration Project to conduct demonstrations of soil and pest management options aimed at sustained profitability and environmental stewardship in the western San Joaquin Valley of California. Monitoring of soil physical, chemical, and biological properties is done in side-by-side on-farm comparisons of plots amended with organic inputs and unamended plots. Intensive monitoring of beneficial and pest insects is carried out within each comparison block, and the data generated is used to guide pest management decision-making at each site. Yields and soil characteristics of the amended plots did not differ from those of unamended plots after the first year. The on-farm context and the cooperative farmer–scientist interactions of this project facilitate the development of timely and relevant research directions to be pursued beyond the core set of monitoring activities.