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Timothy K. Hartz and Thomas G. Bottoms

Soil application of humic acid (HA), generally derived from leonardite shale, is a common practice in California vegetable production. Five commercial HA formulations were evaluated for their effects on soil microbial activity, seedling emergence, crop productivity, and nutrient uptake when applied to representative agricultural soils. Two soils differing in organic matter content (8 and 25 g·kg−1) were wetted to field capacity moisture content with solutions of water, nitrogen and phosphorus (P) fertilizer, HA, or fertilizer + HA and incubated aerobically at 25 °C. In the lower organic matter soil, a synergistic effect of fertilizer and HA was observed after 7 days of incubation on both microbial respiration and the amount of phospholipid fatty acids detected; these stimulatory effects were not observed in the higher organic matter soil. In a greenhouse pot study, the effects of HA on seedling emergence, dry mass accumulation, and P uptake of romaine lettuce (Lactuca sativae L.) were evaluated in four soils of low P availability; HA was applied to the soil at a rate simulating a field application of 2.2 kg·ha−1 a.i. HA had no significant effect on emergence rate or percentage, or P uptake, in any soil; plant dry mass was increased in one soil. Field trials were conducted in 2008 and 2009 evaluating the effects of pre-transplant soil application of HA at 1.1 or 3.4 kg·ha−1 a.i. on growth, nutrient uptake, and fruit yield of processing tomato (Lycopersicon esculentum Mill.). In neither year was macro- or micronutrient uptake increased with HA. Similarly, there was no significant HA effect on plant dry mass accumulation or fruit yield. We conclude that, at typical commercial application rates in representative field soils, HA is unlikely to significantly improve vegetable crop nutrient uptake or productivity.

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Timothy K. Hartz and Thomas G. Bottoms

As growers of processing tomato (Lycopersicon esculentum Mill.) adopt drip irrigation, plant vigor and fruit yield typically increase, suggesting a need for re-evaluation of established nitrogen (N) fertilization practices. Trials were conducted in California in 2007–2008 to evaluate growth and N uptake dynamics of drip-irrigated processing tomatoes across N fertigation regimes ranging from deficient to excessive. Whole plants were collected at 2-week intervals for determination of biomass and N content, recently matured whole leaves for total N and petioles for NO3-N. Additionally, six commercial fields were sampled at 3- to 4-week intervals to document N uptake and crop N status under conditions representative of the industry. A seasonal N rate of ≈200 kg·ha−1 appeared adequate to maximize fruit yield across the range of field conditions encountered. The four highest-yielding fields (143 Mg·ha−1 mean fresh fruit mass) averaged 14 Mg·ha−1 of above-ground biomass with fruit representing 62%; these fields averaged 296 kg·ha−1 biomass N, of which 71% was in fruit. The rate of biomass development and N uptake peaked during the period between early fruit setting and early red fruit development (a period of ≈6 weeks) during which N uptake averaged 4 to 5 kg·ha−1·d−1. Leaf N concentration was highly correlated with whole plant N (r 2 = 0.83) and provided a reliable indicator of plant N sufficiency throughout the season. Petiole NO3-N did not reliably discriminate between crops with adequate or deficient N availability; current petiole NO3-N sufficiency guidelines are unrealistically high.

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Thomas G. Bottoms, Timothy K. Hartz, Michael D. Cahn and Barry F. Farrara

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 (ETc) 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 ETc 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.

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Thomas G. Bottoms, Richard F. Smith, Michael D. Cahn and Timothy K. Hartz

As concern over NO3-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 NO3-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 NO3-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 (Nc, the minimum whole plant N concentration required to maximize growth) was estimated by the equation Nc (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 Nc) 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 NO3-N greater than 20 mg·kg−1 was a reliable indicator that N application could be reduced or delayed. Neither leaf N nor midrib NO3-N was correlated with concurrently measured soil NO3-N and therefore of limited value in directing in-season N fertilization.

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Thomas G. Bottoms, Mark P. Bolda, Mark L. Gaskell and Timothy K. Hartz

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 (NO3-N) was highly variable among high-yield fields, was not correlated with soil NO3-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.