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Michael D. Cahn and Husein A. Ajwa

Agricultural runoff is a source of nutrients and sediments in surface water on the central coast of California. Treating soils with high molecular weight anionic polyacrylamide (PAM) may reduce sediments and P lost from furrow and sprinkler irrigated fields by maintaining infiltration and stabilizing soil aggregates. We conducted column and field studies to quantify the effect of PAM on infiltration rate, run off, and sediment and nutrient (ortho and total P, NO3, K) loss from cool season vegetable fields. Column studies demonstrated a reduction in infiltration for 10 soil types when PAM was continuously applied in the irrigation water at 10 ppm. Recirculating infiltrometer studies showed that in furrow systems, PAM, applied only in the initial water at 10 ppm, had no significant effect on infiltration at four of six sites evaluated. Turbidity and total suspended solids were significantly reduced in the PAM treated water. Across all sites, treatment with PAM reduced suspended solids by 85% compared to the untreated control. Additionally, soluble and total P, and total N were reduced in the PAM treated water. PAM had no effect on nitrate or salt levels in the runoff. PAM applied through sprinklers at a 5 ppm concentration was able to significantly reduce the turbidity and the suspended solids in the tailwater. Similar to the results obtained with the recirculation infiltrometer trials, PAM reduced soluble and total P and total N in the runoff, but had no significant effect on NO3-N. Total sediment loss under sprinklers was reduced by as much as 95% using PAM.

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Aaron Heinrich, Richard Smith, and Michael Cahn

In recent years, vegetable growers on the central coast of California have come under increasing regulatory pressure to improve nutrient management and reduce nitrate losses to ground and surface waters. To achieve this goal, growers must understand the nutrient uptake and water use characteristics of their crops. For fresh market spinach (Spinacia oleracea), production methods and cultivars have greatly changed in the last 10–15 years, and as a result, few publications are available on nutrient uptake by modern spinach production methods. This study evaluated nutrient uptake and water use by spinach to provide strategies to better manage nitrogen (N) fertilizer and irrigation applications. In 2011, four fertilizer trials and a survey of 11 commercial fields of spinach grown on high-density plantings on 80-inch beds were conducted on the central coast of California. During the first 2 weeks of the crop cycle, N, phosphorus (P), and potassium (K) uptake was 7.0, 0.6, and 7.2 lb/acre, respectively. In the subsequent 2–3 weeks before harvest the N, P, and K uptake rate was linear and was 4.3, 0.6, and 7.8 lb/acre per day, respectively. N uptake at harvest for the three commercial size categories baby, teenage, and bunch was 74, 91, and 120 lb/acre N, respectively. Of the N in aboveground biomass at harvest, 41% was left in the field following mechanical or hand harvest. Growers at 14 of 15 study sites applied on average 111% more N than was taken up in aboveground biomass at harvest. Results from four fertility trials showed that first crops of the season had low initial soil nitrate concentrations (≤10 ppm), and an at-planting fertilizer application was necessary for maximum yields. For fields following a previous crop (second- or third-cropped) with initial soil nitrate concentrations >20 ppm, at-planting and midseason fertilizer applications could be greatly reduced or eliminated without jeopardizing yield. Rooting depth and density evaluations at four sites showed that 95% of roots were located in the top 16 inches of soil at harvest. To mitigate environmentally negative N losses, the N use efficiency (NUE) can be increased by the use of soil testing done at two critical time points: at-planting and before the first midseason fertilizer application.

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Aaron Heinrich, Richard Smith, and Michael Cahn

High levels of residual soil nitrate are typically present in cool-season vegetable fields in coastal regions of California in the fall, after the production of multiple crops over the course of the growing season. This nitrate is subject to leaching with winter rains when fields are left fallow. Although the benefits of growing nitrate scavenging cover crops on soil and water quality are well documented, the portion of vegetable production fields planted to winter cover crops in this region is low. Most growers leave their fields unplanted in bare-fallow beds because the risk of having too much cover crop residue to incorporate may delay late winter and early spring planting schedules. A possible strategy to derive benefits of a cover crop yet minimize the amount of residue is to kill the cover crop with an herbicide when biomass of the cover crop is still relatively low. To evaluate whether this strategy would be effective at reducing nitrate leaching, we conducted field studies in Winter 2010–11 (Year 1) and Winter 2011–12 (Year 2) with cereal rye (Secale cereale). Each trial consisted of three treatments: 1) Fallow (bare fallow), 2) Full-season (cover crop allowed to grow to full term), and 3) Partial-season (cover crop killed with herbicide 8 to 9 weeks after emergence). In Year 1, which received 35% more rainfall than the historical average during the trial, the Full-season cover crop reduced nitrate leaching by 64% relative to Fallow, but the Partial-season had no effect relative to Fallow. In Year 2, which received 47% less rainfall than the historical average during the trial, the Full- and Partial-season cover crops reduced nitrate leaching by 75% and 52%, respectively, relative to Fallow. The Full-season cover crop was able to reduce nitrate leaching regardless of yearly variations in the timing and amount of precipitation. Although the Partial-season cover crop was able to reduce leaching in Year 2, the value of this winter-kill strategy to reduce nitrate leaching is limited by the need to kill the crop when relatively young, resulting in the release of nitrogen (N) from decaying residues back into the soil where it is subject to leaching.

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Richard Smith, Michael Cahn, Timothy Hartz, Patricia Love, and Barry Farrara

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 NO3-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 NO3-N leaching losses could substantially improve N efficiency in cole crop production.

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Renée L. Eriksen, Caleb Knepper, Michael D. Cahn, and Beiquan Mou

After a preliminary screening of over 3500 cultivars, we selected 200 butterhead, cos, crisphead, leaf, and stem lettuce (Lactuca sativa L.) and wild prickly lettuce (Lactuca serriola L.) varieties to test under high water (150% evapotranspiration [ET]) and low water (50% ET) conditions in the field, and tracked commercially relevant traits related to growth and marketability, maturity, and physiology. Plants typically reduced growth and appeared to reallocate developmental resources to achieve maturity quickly, as indicated by traits such as increased core length. This strategy may allow them to complete their life cycle before severe drought stress proves lethal. Although most cultivars experienced a reduction in growth under low water conditions relative to high water conditions, some cultivars had a significantly reduced yield penalty under stress conditions. Among the different types of lettuce, the fresh weight (FW) of cos cultivars was most affected by drought stress, and the FW of leaf lettuce was least affected. Cos cultivars tended to bolt early. Crisphead cultivars Cal-West 80, Heatmaster, and Marion produced large heads and did not bolt under low water treatments, and butterhead cultivars Buttercrunch and Bibb also produced relatively large heads with very little bolting and no signs of tipburn. The four green leaf cultivars Slobolt, Grand Rapids, Western Green, and Australian showed no statistically significant difference in FW among high and low water treatments in multiple trials, and may be good choices for growers who wish to minimize losses under reduced irrigation. The identification of potentially drought-tolerant varieties and the information from this study may be helpful for cultivar selection by growers under drought conditions, but this study also serves as a step forward in the genetic improvement of lettuce to drought stress.

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Lee F. Johnson, Michael Cahn, Frank Martin, Forrest Melton, Sharon Benzen, Barry Farrara, and Kirk Post

Estimation of crop evapotranspiration supports efficient irrigation water management, which in turn supports water conservation, mitigation of groundwater depletion/degradation, energy savings, and crop quality maintenance. Past research in California has revealed strong relationships between fraction of the ground covered by photosynthetically active vegetation (Fc), crop coefficients (Kc), and evapotranspiration (ET) of cool-season vegetables and other specialty crops. Replicated irrigation trials for iceberg lettuce and broccoli were performed during 2012 and 2013 at the USDA Agricultural Research Station in Salinas, CA. The main objective was to compare crop yield and quality from ET-based irrigation scheduling with industry standard practice. Sprinkler irrigation was used to germinate and establish the crops, followed by surface drip irrigation during the treatment period. Each experiment compared three irrigation treatment schedules replicated five times in a randomized block design. Two decision-support models were evaluated as follows: 1) an FAO-56-based algorithm embedded in NASA’s prototype Satellite Information Management System (SIMS) based on observed Fc, and 2) CropManage (CM), an online database-driven irrigation scheduling tool based on modeled Fc. Both methods used daily reference ETo data from the California Irrigation Management Irrigation System (CIMIS) to translate Kc to crop ET, with a target of 100% replacement of water use during the drip irrigation phase. A third treatment followed an irrigation schedule representing grower standard practice (SP) at 150% to 175% ET replacement during the drip irrigation phase. No significant treatment differences were seen in lettuce head weight or total biomass. Marketable yields of lettuce (near 45.4 Mg·ha−1) and broccoli (near 17.4 Mg·ha−1) were in-line with industry averages during both years and all treatments. During 2012, CM yield was below lettuce SP, and above broccoli SP, while in 2013 no treatment differences were detected for either crop. No significant differences were detected between SIMS and SP yields during any trial.

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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, 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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M. Murshidul Hoque, Husein Ajwa, Mona Othman, Richard Smith, and Michael Cahn

Commercial lettuce production requires adequate levels of nitrogen (N), phosphorus (P), and potassium (K) to provide high-quality postharvest attributes needed for longer shelf life. Factorial experiments were conducted in Salinas, CA, to evaluate yield and postharvest quality of both romaine and iceberg lettuce using fertilizers containing various levels of N, P, and K. Lettuce was evaluated for yield and postharvest quality parameters, including color, wilt, turgidity, glossiness, decay, brittleness, fringe burn, and salt burn. Uptake of N, P, K, calcium, and silicon by plants was also determined. Regardless of fertilizer treatment, shelf life and visual quality were better in the iceberg lettuce than romaine lettuce when cold-stored at 1 °C for 14 d. Yield increased with increased N application rate, but post-harvest quality fell at high levels of N (337 kg·ha−1) and P (225 kg·ha−1). The most economical treatment providing the highest yield and best post-harvest quality was the combination of 225 kg·ha−1 N and 112 kg·ha−1 P.

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Timothy K. Hartz, Paul R. Johnstone, Richard F. Smith, and Michael D. Cahn

Application of calcium (Ca) fertilizers is a common practice of California lettuce growers to minimize the occurrence and severity of tipburn, particularly in romaine lettuce (Lactuca sativa L. var. longifolia Lam.). An evaluation of the effect of soil Ca availability on the severity of tipburn in romaine lettuce was conducted in the Salinas Valley of central California in 2005 to 2006. Twenty representative soils from this region were evaluated for Ca availability by ammonium acetate extraction, saturated paste extraction, and extraction of soil solution through centrifugation of soil at field-capacity moisture content. Soil solution Ca in these soils was generally high, ranging from 5 to 80 mmolc·L−1, representing 44% to 71% of cations on a charge basis. Soil solution Ca was highly correlated with saturated paste Ca (r 2 = 0.70) but not with exchangeable Ca (r 2 = 0.01). However, saturated paste extraction significantly underestimated soil solution Ca concentration (regression slope = 0.19). A survey of 15 commercial romaine lettuce fields showed tipburn severity to be unrelated to either leaf Ca concentration or soil Ca availability. The most severe tipburn was observed in fields in which transpiration was reduced by foggy weather during the final 2 weeks of growth. Ca fertilizers (calcium nitrate, calcium thiosulfate, and calcium chloride) applied through drip irrigation during the final weeks of lettuce growth were ineffective in increasing romaine leaf Ca concentration in three field trials; tipburn was present in only one trial, and Ca fertigation had no effect on tipburn severity. We conclude that under typical field conditions in this region, tipburn severity is primarily a function of environmental conditions. Soil Ca availability plays no substantive role in tipburn severity, and Ca fertigation does not improve lettuce Ca uptake or reduce tipburn.