The use of tissue testing of crop nitrogen status is widespread in vegetable production; given the high value of vegetable crops the interest in nutrient testing has historically been to ensure maximum production and cosmetic quality rather than to achieve high fertilizer use efficiency. In drip-irrigated culture, growers have the ability to inject N throughout the season, and the use of tissue testing to ‘fine tune’ the N fertility program is common. However, increased awareness of the environmental impact of nutrient loss from vegetable fields has introduced an environmental perspective on nutrient ‘best management practices’ (BMP), and caused a reevaluation of tissue N testing. Recent California research with lettuce and tomato has confirmed the validity of leaf total N concentration as a measure of crop N status. In lettuce and tomato, leaf N is strongly correlated with whole plant N across fields, growth stages and cultivars; current leaf N sufficiency guidelines are generally consistent with those predicted from the theoretical whole plant critical N concentration. Leaf N changes slowly over time, allowing the grower to project crop N status into the future. Leaf reflectance has been explored as a more easily measured surrogate for leaf N concentration, but has proven problematic. Cultivar differences in leaf reflectance characteristics, and the confounding effects of factors unrelated to N status (soil moisture status, foliar disease, etc.) confound the relationship between leaf reflectance and leaf N concentration, and limit the practical value of reflectance measurements.

Petiole NO3-N analysis is popular in the California vegetable industry because it is more easily measured than leaf N (even measured on-farm by sap analysis), and is widely believed to represent recent soil N uptake. Unfortunately, petiole NO3-N testing has proven to be an unreliable measure of plant N status. Petiole NO3-N is poorly correlated to current soil N availability or crop N uptake rate. The effects of environmental factors on NO3-N assimilation rate can be profound; coupled with the relatively small amount of NO3-N usually contained in whole plant biomass (typically the equivalent of less than 3 days of crop N uptake), petiole NO3-N concentration can be highly volatile, and does not dependably predict future fertilization requirement. Current petiole NO3-N guidelines are unrealistically high, and fertilizing to maintain those levels will often result in unnecessary fertilization.

Increased regulatory activity designed to protect groundwater from degradation by nitrate-nitrogen (NO₃-N) is focusing attention on the efficiency of agricultural use of nitrogen (N). One area drawing scrutiny is the way in which growers consider the NO₃-N concentration of irrigation water when determining N fertilizer rates. Four drip-irrigated field studies were conducted in the Salinas Valley evaluating the impact of irrigation water NO₃-N concentration and irrigation efficiency on the N uptake efficiency of lettuce and broccoli crops. Irrigation with water NO₃-N concentrations from 2 to 45 milligrams per liter were compared with periodic fertigation of N fertilizer. The effect of irrigation efficiency was determined by comparing an efficient (110% to 120% of crop evapotranspiration, ET_c) and an inefficient (160% to 200% of ET_c) irrigation treatment. Across these trials, NO₃-N from irrigation water was at least as efficiently used as fertilizer N; the uptake efficiency of irrigation water NO₃-N averaged approximately 80%, and it was not affected by NO₃-N concentration or irrigation efficiency.

Organic cool-season vegetable growers on the Central Coast face challenges in applying nitrogen (N) to balance yields with new environmental regulations. It is hard to time fertilizer applications while calculating N mineralization of soil organic matter and organic fertilizers to plant-available N. Organic fertilizers with high phosphorus (P) to N ratios may elevate P levels and harm surface water quality. In this study, we evaluated (1) mineralization of soil organic matter and fertilizers, (2) effectiveness of residual soil nitrate-N tests and (3) long-term impacts of organic fertilizers on P levels and soil microbial activity. We found that mineralization of N from soil organic matter provided limited N to leafy green vegetables. Soil tests were more reliable in heavier than sandier soils. Application rates of 4-4-2 were calculated to meet N demands, resulting in an oversupply of P. However, only 9% to 17% of fertilizer P solubilized without elevating available soil P levels. While it's difficult for organic vegetable growers to use cover crops, organic fertilizers increased carbon levels, resulting in higher levels of soil microbial activity.

Widespread contamination of surface water with nitrate-nitrogen (NO₃-N) has led to increasing regulatory pressure to minimize NO₃-N release from agricultural operations. We evaluated the use of wood chip denitrification bioreactors to remove NO₃-N from tile drain effluent on two vegetable farms in Monterey County. Across several years of operation, denitrification in the bioreactors reduced NO₃-N concentration by an average of 8 to 10 milligrams per liter (mg L^-1) per day during the summer and approximately 5 mg L^-1 per day in winter. However, due to the high NO₃-N concentration in the tile drainage (60 to 190 mg L^-1), water discharged from the bioreactors still contained NO₃-N far above the regulatory target of < 10 mg L^-1. Carbon enrichment (applying soluble carbon to stimulate denitrifying bacteria) using methanol as the carbon source substantially increased denitrification, both in laboratory experiments and in the on-farm bioreactors. Using a carbon enrichment system in which methanol was proportionally injected based on tile drainage NO₃-N concentration allowed nearly complete NO₃-N removal with minimal adverse environmental effects.