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T.K. Hartz and J.P. Mitchell

The rate of N mineralization from 35 samples of manure or compost was estimated by both aerobic laboratory incubation and lath house pot studies at Davis, Calif., in 1996–97. Each manure and compost sample was mixed at 2% by dry weight with a 1 loam soil: 1 coarse sand blend. The amended soil blends were moisture equilibrated under 0.025-MPa pressure then incubated aerobically at constant moisture at 25 °C for 3 (1996) or 6 months (1997); subsamples were collected monthly (1996) or bimonthly (1997) for mineral N determination. Four-liter pots were also filled with the amended soil blends and seeded with fescue (Festuca arundinacea). The pots were watered but not fertilized for 16 (1996) or 18 (1997) weeks in a lath house at ambient summer conditions. N mineralization from the pot study was calculated from total fescue biomass N plus mineral N from pot leachate, minus those quantities in pots of the unamended soil blend. N mineralization rate estimates from the two techniques were highly correlated (r 2 = 0.79). Green waste composts typically mineralized <5% of total N, manure composts 5% to10%, and manures (poultry, dairy, and feedlot) 7% to 20%. After 4 months of incubation, N mineralization rate (expressed as percent of total N per month) from the composts and manures was similar to that of the unamended soil blend.

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T.K. Hartz and G.J. Hochmuth

Drip irrigation provides an efficient method of fertilizer delivery virtually free of cultural constraints that characterize other production systems. Achieving maximum fertigation efficiency requires knowledge of crop nutrient requirements, soil nutrient supply, fertilizer injection technology, irrigation scheduling, and crop and soil monitoring techniques. If properly managed, fertigation through drip irrigation lines can reduce overall fertilizer application rates and minimize adverse environmental impact of vegetable production.

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S.J. Breschini and T.K. Hartz

Trials in nine commercial celery (Apium graveolens L.) fields were conducted between 1997-99 to evaluate grower drip irrigation management practices and their effects on yield and quality. Surface drip irrigation tapes with flow rates higher and lower than the grower-installed tapes were spliced into the field system; as the cooperating growers irrigated and applied N fertigation according to their routine practices these drip tapes delivered either more or less water and N than the field drip system. Total grower water application during the drip-irrigated portion of the season ranged from 85% to 414% of seasonal reference evapotranspiration (ETo). Water volume per irrigation varied among fields from 1.8 to 3.8 cm, with irrigation frequency varying from an average of every other day to once a week. Grower management of drip irrigation was not consistently successful in maintaining soil water tension (SWT) in a desirable range. SWT was often below -30 kPa, and in some cases below -70 kPa. These transient stresses were more often a result of inappropriate irrigation frequency than applied water volume. In four of the fields plots receiving less water than that delivered by the field system produced equivalent marketable yield and quality, indicating a significant potential for water savings. An economically important incidence of petiole pithiness (collapse of parenchyma tissue) was observed in four fields. Infrequent irrigation under high ETo summer conditions, rather than irrigation volume applied, appeared to be the major factor in pith development. N fertigation amount and crop N status appeared to be unrelated to pithiness severity. We conclude that celery drip irrigation management could be substantially improved by maintaining a closer proportionality between irrigation and crop evapotranspiration (ETc), increasing irrigation frequency, and reducing volume per irrigation.

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P.R. Johnstone and T.K. Hartz*

Heavy P fertilization in the Salinas Valley of California has increased soil P concentration to levels of environmental concern. To determine the correlation of various soil test procedures with P pollution potential from agricultural land in this region, soil was collected from 30 fields, most in long-term vegetable rotations. Soils were analyzed for bicarbonate-extractable P (Pbc), calcium chloride-extractable P (Pcc), bio-available P (Pba, by an anion-resin membrane technique), and %P saturation (Psat, by an enrichment technique). The soils were then exposed to a simulated irrigation event, and soluble P concentration in runoff determined. In a separate experiment the effect of cover cropping on sediment and soluble P concentration in runoff was investigated; containers of six soils were planted with oats (Horteum vulgare L.), and then compared to containers of fallow soil. Pcc, Pba and Psat were all highly correlated (r = 0.86, 0.89 and 0.90, respectively) with Pbc, which ranged from 15-177 mg·kg-1. Soluble P concentration in runoff was highly correlated with all measures of P status (r = 0.98, 0.93, 0.85 and 0.83 for Pcc, Pba, Psat and Pbc, respectively). These results suggest that while Pbc, the standard agronomic measure of soil P status, is a useful indicator of P pollution potential, Pcc (a simple laboratory procedure that could be adapted as an on-farm `quick test' technique) may be superior for that purpose. Across soils, cover cropping reduced soluble P concentration in run-off by 41%, and sediment in the runoff by 85%.

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S.J. Breschini and T.K. Hartz

Trials were conducted in 15 commercial fields in the central coast region of California in 1999 and 2000 to evaluate the use of presidedress soil nitrate testing (PSNT) to determine sidedress N requirements for production of iceberg and romaine lettuce (Lactuca sativa L.). In each field a large plot (0.2-1.2 ha) was established in which sidedress N application was based on presidedress soil NO3-N concentration. Prior to each sidedress N application scheduled by the cooperating growers, a composite soil sample (top 30 cm) was collected and analyzed for NO3-N. No fertilizer was applied in the PSNT plot at that sidedressing if NO3-N was >20 mg·kg-1; if NO3-N was lower than that threshold, only enough N was applied to increase soil available N to ≈20 mg·kg-1. The productivity and N status of PSNT plots were compared to adjacent plots receiving the growers' standard N fertilization. Cooperating growers applied a seasonal average of 257 kg·ha-1 N, including one to three sidedressings containing 194 kg·ha-1 N. Sidedressing based on PSNT decreased total seasonal and sidedress N application by an average of 43% and 57%, respectively. The majority of the N savings achieved with PSNT occurred at the first sidedressing. There was no significant difference between PSNT and grower N management across fields in lettuce yield or postharvest quality, and only small differences in crop N uptake. At harvest, PSNT plots had on average 8 mg·kg-1 lower residual NO3-N in the top 90 cm of soil than the grower fertilization rate plots, indicating a substantial reduction in subsequent NO3-N leaching hazard. We conclude that PSNT is a reliable management tool that can substantially reduce unnecessary N fertilization in lettuce production.

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S. Castro Bustamante and T.K. Hartz

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. R 2 = 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.

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T.K. Hartz and R.F. Smith

Research on controlled-release fertilizers (CRF) in vegetable production has been conducted in California for several decades, and commercial CRF products have been marketed throughout most of that time. CRF remain niche products used on only a small percentage of vegetable fields. The potential advantage of CRF is maximized in production systems in which in-season nitrogen (N) leaching is significant but beyond the control of the grower, and where there are cultural constraints on in-season fertilizer application. Neither of those conditions is typical of the California industry. Annual rainfall in the major vegetable-producing regions averages less than 400 mm, with the majority of that received during winter months when vegetable production is limited; in-season leaching occurs almost exclusively from irrigation. The alluvial soils favored for vegetable production tend to be relatively fine-textured, with high water holding capacity that reduces N leaching potential. The widespread adoption of drip irrigation allows for efficient irrigation and for multiple applications of less expensive N fertilizers in synchrony with crop demand. Under representative California field conditions it has been difficult to show a horticultural benefit from the use of CRF, and the higher cost of these products has therefore limited their use. Future government regulation for water quality protection may require more efficient N fertilization practices, but significant expansion of CRF use is unlikely even under that scenario.

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K.S. Mayberry, T.K. Hartz, and M. Cantwell

Trials were conducted in California to evaluate techniques to extend post-harvest life of Western shipper-type muskmelon cultivars (Cusumis melo L.). The use of .025 mm polyethylene bags, either as individual melon wraps or as liners for 18 kg commercial cartons, minimized water loss and associated softening of the fruit. A three minute dip in 58-60°C water effectively checked surface mold and decay. The combination of hot water dip and polyethylene carton liner maintained high quality marketable fruit for at least 30 days of cold storage at 2-4°C. This technique would require only modest changes in commercial handling practices, with minimal additional per carton cost. Commercial utilization of this technique could stimulate the export of California muskmelons to Pacific Rim countries.

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T.K. Hartz, M. LeStrange, and D.M. May

The response of bell pepper (Capsicum annuum L.) to five rates of N fertigation between 0 and 336 kg N/ha was studied at two drip-irrigated sites [Univ. of California, Davis (UCD) and West Side Field Station, Five Points (WSFS)] in California in 1992. Nitrogen application, in the form of a urea: ammonium nitrate mixture (UN-32), was applied in eight (WSFS) or 10 (UCD) equal weekly increments, beginning after transplant establishment. At both sites, fruit yield and mean fruit size peaked at 252 kg N/ha, with additional N retarding crop productivity. Maximum fruit yield was obtained by fertility treatments that maintained petiole NO3-N concentration >5000 μg·g-1 through the early fruit bulking period. Two techniques for monitoring crop N status, designed for field use, were evaluated. There was a close relationship between the NO3-N concentration of fresh petiole extracts, as measured by a portable, battery-operated nitrate selective electrode, and dry tissue analyzed by conventional laboratory technique (r2 = 0.89). Relative chlorophyll concentration, measured nondestructively by a dual-wavelength leaf absorbance meter, was poorly correlated with whole-leaf N concentration (r2 = 0.55). However, the ratio of such chlorophyll readings for a treatment compared to an in-field reference of known N sufficiency (252 kg·ha-1 treatment) showed promise as a technique for identifying N deficiency.

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T.K. Hartz, A. Baameur, and D.B. Holt

A study was conducted to determine the feasibility of fieldscale CO2 enrichment of vegetable crops grown under tunnel culture. Cucumber, squash and tomato were grown under polyethylene tunnels in a manner similar to commercial practices in southern California. The buried drip irrigation system was used to uniformly deliver an enriched CO2 air stream independent of irrigation. CO2 concentration in the tunnel atmosphere was maintained between 700-1000 ppm during daylight hours. Enrichment began two weeks after planting and continued for four weeks. At the end of the treatment phase, enrichment had significantly increased plant dry weights. This growth advantage continued through harvest, with enriched plots yielding 20%, 30% and 32% more fruit of squash, cucumber and tomato, respectively. As performed in this study, the expense of CO2 enrichment represented less than a 10% increase in total pre-harvest costs. Industrial bottled CO2 was used in this study; since bottled CO2 is captured as a byproduct of industrial processes, this usage represents a recycling of CO2 that would otherwise be vented directly to the atmosphere.