For tomato growers, nitrogen management has been relatively straightforward.
The norm has been to apply 120 to 150 pounds of nitrogen as a sidedress application at the layby growth stage. The consequence of a luxuriant supply of N has not resulted in a yield reduction, delayed harvest or out-of-control vine growth in canning tomatoes.
With an increase in cost of N, some growers may consider reducing nitrogen rates. Field tests conducted in the late 1990s by UC Vegetable Crop Specialist Jeff Mitchell's graduate student Henry Krusekopf indicated that soil residual nitrogen at or above 16 ppm nitrate-N in the top foot prior to sidedressing was sufficient to produce maximum yields. No supplemental N was required.
Most of the fields were in Fresno County's Westside, but a couple of the tests were in northern California. Timing of the soil sample was in the spring sufficiently ahead of sidedress period to allow completion of lab work.
In N-depleted soils, an application of 75 pounds of N per acre provides the bulk of the yield gain for tomatoes. Higher rates of nitrogen provide incremental increases, but at a diminishing rate of return. Thus as N cost increases, the rate of N would tend to be reduced.
A practical grower-approach would be to sample a couple of fields to obtain soil lab reports specific to the field. If the soil tests are above 25 ppm, cut back from the normal N application rate to 75 pounds of applied N or so on part of your field and compare that to your normal application. If the levels are extremely high, 35 ppm or more, reduce the N to zero in some of the acres, half rate on the majority of the field, and the remainder will be a full normal rate. Of course, all of this involves extra adjustments and effort to assess the results in each particular field.
In fields where well water is used, checking nitrate-N levels may provide additional information on N availability. The conversion factor for calculating N applications from irrigation water is 2.7 times nitrate-N in parts per million equals pounds of N per acre-foot of water. For example if lab result is five ppm of nitrate-N and three acre feet are applied per cropping season, then 40 pounds of N were delivered in the irrigation. It is unlikely all 40 pounds would be available to the crop because of runoff and perhaps leaching.
In 2005, we conducted two trials in commercial fields at a grower request. The previous crop at one site was following heavily fertilized bell peppers with 15-ppm nitrate-nitrogen in the springtime sample. The second site was following wheat in the crop rotation with 9 ppm. As part of our test, nitrogen was sidedressed with either 75 or 150 pounds of N and included a non-fertilized control.
At the site with high residual N, additional N boosted yields from 49.9 to 51.8 tons at the 75-pound N rate, a slight, but economic gain. Following the wheat crop, with 9 ppm nitrate-N, sidedressing at the 75-pound rate was sufficient to maximize yields.
In the non-fertilized controls at the wheat site, plants appeared chlorotic and vine cover was marginal resulting in a high level of sun-damaged fruit from our hot 2005 summer.
The pre-sidedressed soil sample program slightly underestimated the N application rate in one test and was on target in the second site.
Upshot: The pre-sidedressed nitrate soil sampling program provides some guidance on sidedressed N. application rates. Except for high residual N fields, fine-tuning N input is not precise nor economically without risk of sacrificing tomato crop yield.