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 lettuc e (Lactuca sativa L,) and celery (Apium graveolens L,) production was evalu ated in 15 commercial fields in California from 1996 to 1997, Fields were s elected in which soil NO3-N (5- to 30-cm depth) was >20 mg.kg(-1) at the ti me the cooperating grower made the first sidedress N application. The growe r's N regime was compared with reduced N treatments established by reducing or eliminating one or more sidedress applications. All fields were sprinkl er and/or furrow irrigated, with minimal in-season precipitation. Reduction s 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 fiel d, Crop biomass N at harvest in the lowest N treatment in each field averag ed 94 % (lettuce) and 88 % (celery) of that in plots receiving the full gro wer N program. Based on controlled-environment aerobic incubation of soil f rom 30 fields in long-term vegetable rotations, in-season N mineralization averaged 1% to 2% of soil organic N, A soil NO3-N "quick test" procedure ut ilizing a volumetric extraction of field-moist soil and measurement by nitr ate-sensitive colorinetric test strips was evaluated and proved to be a pra ctical on-farm method to estimate soil NO3-N concentration. Lettuce midrib NO3-N concentration at cupping stage was poorly correlated with current soi l NO3-N level. We conclude that PSNT can reliably identify fields in which sidedress N application can be delayed or eliminated without affecting crop performance.