The Value of Presidedress Soil Nitrate Testing as a Nitrogen Management Tool in Irrigated Vegetable Production

The utility of presidedress soil nitrate testing (PSNT) in irrigated lettuce ( Lactuca sativa L.) and celery ( Apium graveolens L.) production was evaluated in 15 commercial fields in California from 1996 to 1997. Fields were selected in which soil NO 3 -N (5- to 30-cm depth) was >20 mg·kg –1 at the time the cooperating grower made the first sidedress N application. The grower's N regime was compared with reduced N treatments established by reducing or eliminating one or more sidedress applications. All fields were sprinkler and/or furrow irrigated, with minimal in-season precipitation. Reductions in seasonal N application averaging 143 and 209 kg·ha –1 N in lettuce and celery trials, respectively, had no effect on marketable yield in any field. Crop biomass N at harvest in the lowest N treatment in each field averaged 94% (lettuce) and 88% (celery) of that in plots receiving the full grower N program. Based on controlled-environment aerobic incubation of soil from 30 fields in long-term vegetable rotations, in-season N mineralization averaged 1% to 2% of soil organic N. A soil NO 3 -N “quick test” procedure utilizing a volumetric extraction of field-moist soil and measurement by nitrate-sensitive colorimetric test strips was evaluated and proved to be a practical on-farm method to estimate soil NO 3 -N concentration. Lettuce midrib NO 3 -N concentration at cupping stage was poorly correlated with current soil NO 3 -N level. We conclude that PSNT can reliably identify fields in which sidedress N application can be delayed or eliminated without affecting crop performance.