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.

Processing tomato yields have risen sharply in recent years due to cultivar improvement and the widespread adoption of drip irrigation. Efficient fertilization management requires balancing the increasing nutrient requirements of high-yield crops against the adverse environmental impacts of excessive fertilization. Monitoring in California tomato fields has shown that a crop uptake of approximately 2.2 kg N, 0.3 kg P and 3.1 kg K Mg-1 of fresh fruit yield was adequate to support yields in excess of 120 Mg ha(-1). Nutrient removal with harvested fruit typically represented approximately 60% (N) or 70% (P and K) of total crop uptake. High nitrogen-use efficiency is achievable, but efficient N management requires field-specific consideration of residual soil NO3-N, which, in semi-arid production areas, can be substantial. P and K fertilization requirements are best estimated by preplant soil testing. P fertilization at a rate to match expected P removal in harvested fruit should be adequate to maximize yield in fields of moderate soil P status. The likelihood of increasing fruit yield with K fertilization is high in fields with soil exchangeable K < 200 mg kg(-1), but declines with higher soil K availability. While fruit color quality may improve with K application, K fertilization for fruit quality improvement is unlikely to be cost-effective. Plant tissue analysis, while able to document current nutrient sufficiency, provides limited guidance regarding future fertilization requirements.

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 dripirrigated 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, ETc) and an inefficient (160% to 200% of ETc) 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.

The United States Department of Agriculture Natural Resources Conservation Service (NRCS) launched a national Soil health initiative in 2012; as a part of that effort, a soil health index (SHI) has been developed. The SHI is calculated using results of three soil tests: 24-h carbon mineralization following rewetting of air-dried soil (C-min, by the Solvita proprietary method) and water-extractable organic carbon (C) and nitrogen (N). These tests are being promoted both as the inputs into the SHI calculation and as predictors of soil N mineralization potential. Soil was collected from 35 California fields in annual crop rotations; 20 fields were under certified organic management and the other 15 under conventional management, to provide a range of soil properties and management effects. Carbon mineralization was determined by the Solvita method, and by a comparison method utilizing head space carbon dioxide (CO2) monitoring by infrared gas analyzer (IRGA); additionally, two soil wetting protocols were compared, capillary wetting (the Solvita method) and wetting to 50% water-filled pore space (WFPS). Both water-extractable C (WEOC) and N (WEON) were determined using NRCS-recommended protocols. Net N mineralization (N-min) was also determined after a 28-day aerobic incubation at 25 degrees C. Solvita C-min was highly correlated with the IRGA method using capillary wetting (R-2=0.81). However, capillary soil wetting resulted in high gravimetric water content that significantly suppressed C-min compared to the 50% WFPS method. N-min was correlated with Solvita C-min (r=0.54) and with WEOC and WEON (r=0.62 for each comparison); combining these three measurements into the SHI slightly improved the correlation with N-min. The organically managed soils scored higher than the conventional soils on the SHI, with a minority of organic soils and the majority of conventional soils scoring below the NRCS target threshold. SHI and soil organic matter were correlated, suggesting an inherent bias that would complicate the application of a national SHI standard.

Intensive production of cool-season vegetables has contributed to nitrate pollution of groundwater along the central coast of California. Broccoli ( Brassica oleracea L. var. italica ), cabbage ( Brassica oleracea L. var. capitata ), and cauliflower ( Brassica oleracea L. var. botrytis ) are important crops in this region, but few data are available regarding the nitrogen dynamics of these cole crops under current production practices, and whether those practices are protective of groundwater. Monitoring was conducted in 14 commercial broccoli, 8 cabbage, and 8 cauliflower fields evaluating crop growth, rooting depth, N uptake and partitioning, patterns of soil N availability, and current N fertilization and irrigation practices. Aboveground biomass N at harvest averaged 367, 367, and 319 kg·ha −1 for broccoli, cabbage, and cauliflower, respectively, with mean N fertilization rates of 209, 280, and 256 kg·ha −1 . The relatively small fraction of biomass N removed at harvest with cauliflower (23%) and broccoli (31%) resulted in a low partial N balance (PNB) of 30% and 57%, respectively, compared with cabbage (PNB of 70%). Rooting depth increased throughout the growing season, reaching ≈1 m by harvest, with about 70% of roots located in the top 40 cm in all crops. Soil mineral N (SMN; 0- to 30-cm depth) varied among fields, with the early-season median value of 18 mg·kg −1 declining to 5 mg·kg −1 by harvest. Seasonal N application was not correlated with early-season SMN. Irrigation applied, predominately through sprinklers, averaged >200% of estimated crop evapotranspiration. Substantial N mineralization from broccoli residue was observed within 2–3 months following fall incorporation, with potential NO 3 -N leaching losses exceeding 100 kg·ha −1 in both monitored fields. We conclude that improved irrigation management, adjusting N rates based on residual SMN, and employing a remediation practice such as cover cropping to limit winter NO 3 -N leaching losses could substantially improve N efficiency in cole crop production.

Organic processing tomato (Solanum lycopersicum L.) production is a significant industry in California, yet little nitrogen (N) fertility research is available to guide N management. A total of 37 certified organic processing tomato fields in the Sacramento Valley of California were monitored during the 2012 and 2013 production seasons, with two objectives: 1) to document current N management practices and 2) to investigate the utility of early-season soil and plant N monitoring techniques in predicting seasonal crop N sufficiency. Between ≈3 and 11 weeks after transplanting (WAT) soil mineral N (SMN), leaf N and petiole NO3-N were determined every other week. In 22 fields, whole plant N concentration at ≈11 WAT was determined as a measure of crop N sufficiency. Growers were surveyed regarding N management practices used and fruit yields achieved. Net N mineralization (Nmin) was measured for 20 fields soils by aerobic laboratory incubation. Carbon mineralization (Cmin) in 24 hours following rewetting of air-dried soil and water extractable organic nitrogen (WEON) and carbon (WEOC) were also determined and evaluated as predictors of Nmin. Nitrogen management was primarily based on the application of manure or manure compost in the fall. Organic fertilizers were applied mainly in spring (pre- and post-transplanting). SMN in the top 60 cm at 3 WAT ranged from 6 to 32 mg·kg−1. About 30% of fields were N deficient by 11 WAT. Sensitivity analysis showed that SMN (whether measured from 0 to 30 or 0 to 60 cm) and leaf N at 5 WAT correctly predicted late-season plant N status in >60% of the fields. Nmin in 28 days ranged from 8 to 31 mg·kg−1, representing an average of 2% of total soil N. Correlation between Nmin and Cmin was weak (r = 0.44, P = 0.051) while stronger correlations were observed between Nmin and WEOC, WEON and total soil N (r = 0.63, 0.61 and 0.51, respectively, all P < 0.03). A multiple linear regression model that used 3 WAT SMN (0–30 cm) and WEON as independent variables improved Nmin prediction (adj. R2 = 0.67). Significant fruit yield increase with sidedress N application of feather meal at 5–6 WAT was observed in 2 of 4 field trials, demonstrating the ability to remedy a soil N limitation identified by early-season N monitoring.

The annual strawberry (Fragaria x ananassa Duch.) production system used in the coastal valleys of central California is highly productive (often reaching fruit yields of 70 t ha(-1)), and has been widely adopted around the world. Strawberry growers in this region have recently come under regulatory scrutiny for potential nitrate pollution of groundwater resulting from their production practices. In this study irrigation and N fertilization practices were monitored in a total of six commercial strawberry fields during the 2009-10 and 2010-11 production seasons. Irrigation volume and timing were documented using water meters. Crop evapotranspiration (ETc) was estimated from daily reference evapotranspiration (ETo) and crop canopy development as determined by infrared imaging. N fertilization records were obtained from cooperating growers. Crop N uptake was determined by monthly destructive plant sampling. Soil leachate NO3-N was measured weekly using suction lysimetry from May through to August, the period during which most of the seasonal irrigation was applied. Irrigation management varied widely among fields, ranging from deficit irrigation to an estimated 175% of ETc. N fertilization was similarly variable, with the seasonal total ranging from 141-476 kg ha(-1). Total seasonal crop N uptake averaged 163 kg ha(-1), with marketable fruit accounting for 46% of the total. Estimated summer NO3-N leaching loss ranged from 1-67 kg ha(-1), averaging 33 kg ha(-1). Soil NO3-N monitoring in the 2010-11 fields indicated that N loss over the winter may have exceeded summer NO3-N leaching.

The impact of strawberry production on nitrate contamination of groundwater is of major concern in the central coast region of California. Nitrogen (N) fertilization and irrigation management practices were monitored in a total of 26 fall-planted annual strawberry ( Fragaria × ananassa Duch.) fields in 2010 and 2011. Soil mineral N (SMN, top 30 cm depth) was determined monthly. Irrigation applied was monitored, and crop evapotranspiration (ET c ) was estimated. Growers were surveyed regarding their N fertilization practices. Aboveground biomass N accumulation was estimated by monthly plant sampling in seven fields. The effect of preplant controlled-release fertilizer (CRF) rate on fruit yield was investigated in three fields. The growers’ CRF application rate (121 or 86 kg·ha −1 N as 18N–3.5P–10.8K, 7- to 9-month release rating) was compared with a half rate (all fields) and no CRF in one field. The rate of N release from this CRF product was evaluated using a buried bag technique. Median CRF N and total seasonal N application (CRF + in-season fertigation through drip irrigation) were 101 and 260 kg·ha −1 , respectively, with total seasonal N application varying among fields from 141 to 485 kg·ha −1 . Biomass N accumulation was slow through March (less than 25 kg·ha −1 ) and then increased by ≈1.1 kg·ha −1 ·d −1 from April through mid-September. Mean seasonal biomass N accumulation was estimated at 225 kg·ha −1 by 15 Sept. Approximately 70% of CRF N was released before 1 Apr. Biomass N accumulation between planting and April was much lower than the combined amount of CRF N release and SMN decline over that period, suggesting substantial winter N loss. Conversely, N loss during the summer harvest season (May through August) appeared limited in most fields. Median SMN was maintained below 10 mg·kg −1 , and median irrigation was 113% of estimated ET c during this period. Reduction in CRF rate did not affect marketable fruit yield in two of three trials; an 8% yield reduction was observed in the remaining trial when the CRF rate was reduced, but the decline may have been affected by spring irrigation and fertigation practices.

Diagnosis and recommendation integrated system (DRIS) leaf blade and petiole optimum nutrient ranges were developed through tissue sampling in 53 commercial strawberry ( Fragaria × ananassa ) fields in the coastal valleys of central California in 2010 and 2011. All fields were in an annual production system using the day-neutral cultivar Albion. Leaf blades and petioles were sampled five times from early flowering through the fruit harvest period. Data on soil nutrient availability and grower fertilization practices were also collected. DRIS analysis was used to develop nutrient optimum ranges based on nutrient concentrations observed in nutritionally balanced, high-yield fields. Blade nitrogen (N), phosphorus (P), and potassium (K) concentrations declined from the vegetative stage until the main harvest period, and stabilized thereafter. Blade calcium (Ca), boron (B), and iron (Fe) increased over time while magnesium (Mg), sulfur (S), manganese (Mn), zinc (Zn), and copper (Cu) decreased. The blade N optimum range was lower than previously published sufficiency ranges during the fruit harvest period, and the Zn optimum range was lower throughout the season. Other nutrients were in general agreement with previously established sufficiency ranges with the exception of Ca, Mn, and Fe, which were higher. Petiole nitrate-nitrogen (NO 3 -N) was highly variable among high-yield fields, was not correlated with soil NO 3 -N at any growth stage, and was therefore of limited value as an indicator of crop N status. Comparison of soil nutrient availability with grower fertilization practices suggested that significant improvement in fertilizer management was possible.

As concern over NO 3 -N pollution of groundwater increases, California lettuce growers are under pressure to improve nitrogen (N) fertilizer efficiency. Crop growth, N uptake, and the value of soil and plant N diagnostic measures were evaluated in 24 iceberg and romaine lettuce ( Lactuca sativa L. var. capitata L., and longifolia Lam., respectively) field trials from 2007 to 2010. The reliability of presidedressing soil nitrate testing (PSNT) to identify fields in which N application could be reduced or eliminated was evaluated in 16 non-replicated strip trials and five replicated trials on commercial farms. All commercial field sites had greater than 20 mg·kg −1 residual soil NO 3 -N at the time of the first in-season N application. In the strip trials, plots in which the cooperating growers’ initial sidedress N application was eliminated or reduced were compared with the growers’ standard N fertilization program. In the replicated trials, the growers’ N regime was compared with treatments in which one or more N fertigation through drip irrigation was eliminated. Additionally, seasonal N rates from 11 to 336 kg·ha −1 were compared in three replicated drip-irrigated research farm trials. Seasonal N application in the strip trials was reduced by an average of 77 kg·ha −1 (73 kg·ha −1 vs. 150 kg·ha −1 for the grower N regime) with no reduction in fresh biomass produced and only a slight reduction in crop N uptake (151 kg·ha −1 vs. 156 kg·ha −1 for the grower N regime). Similarly, an average seasonal N rate reduction of 88 kg·ha −1 (96 kg·ha −1 vs. 184 kg·ha −1 ) was achieved in the replicated commercial trials with no biomass reduction. Seasonal N rates between 111 and 192 kg·ha −1 maximized fresh biomass in the research farm trials, which were conducted in fields with lower residual soil NO 3 -N than the commercial trials. Across fields, lettuce N uptake was slow in the first 4 weeks after planting, averaging less than 0.5 kg·ha −1 ·d −1 . N uptake then increased linearly until harvest (≈9 weeks after planting), averaging ≈4 kg·ha −1 ·d −1 over that period. Whole plant critical N concentration (N c , the minimum whole plant N concentration required to maximize growth) was estimated by the equation N c (g·kg −1 ) = 42 − 2.8 dry mass (DM, Mg·ha −1 ); on that basis, critical N uptake (crop N uptake required to maintain whole plant N above N c ) in the commercial fields averaged 116 kg·ha −1 compared with the mean uptake of 145 kg·ha −1 with the grower N regime. Soil NO 3 -N greater than 20 mg·kg −1 was a reliable indicator that N application could be reduced or delayed. Neither leaf N nor midrib NO 3 -N was correlated with concurrently measured soil NO 3 -N and therefore of limited value in directing in-season N fertilization.

California has four main chile pepper (Capsicum annuum L.) production areas: the southern desert val leys (Imperial and Riverside Counties), the southern coast (San Diego, Orange, and Ventura Counties), the Central Coast (San Luis Obispo, Monterey, San Benito, and Santa Clara Counties), and the Central Valley (Tulare, Fresno, and San Joaquin Counties).Nearly all fields in the