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Decades of heavy phosphorus (P) fertilization of vegetable crops in the Salinas Valley of California has increased soil test P (STP) levels, with bicarbonate-extractable P (Pbc) values >50 mg·kg–1 now common. To evaluate the response of lettuce (Lactuca sativa L.) to P fertilization in fields with elevated STP levels, 12 trials were conducted in commercial fields during 2002–03. Initial Pbc at the trial sites varied from 53 to 171 mg·kg–1. In each trial, four replicate plots receiving the growers' P application were compared with paired plots in which no P was applied. Leaf P was monitored at midseason and at harvest. At harvest, mean whole and marketable plant mass and percent of marketable plants were recorded. A significant increase in lettuce yield with P fertilization was achieved at only one trial site, a spring planting with 54 mg·kg–1 Pbc; at all other sites, including three with Pbc <60 mg·kg–1, P application resulted in no significant yield increase. Phosphorus application resulted in only a marginal increase in plant P uptake; in the nonresponsive fields leaf P concentration of nonfertilized plots was in excess of established sufficiency levels. In a laboratory study, the correlation of Pbc to bioavailable P (Pba) was evaluated using 30 representative Salinas Valley soils; Pbc varied among these soils from 15 to 177 mg·kg–1. Pba was estimated by P adsorption on an anion resin membrane during a 16 hour incubation. The effect of temperature on P bioavailability in six of these soils was estimated by conducting the Pba incubation at 5, 15, and 25 °C. Pba was highly correlated with Pbc (r = 0.89), and increased about 40% across soils with each 10 °C increase in soil temperature. Therefore, Pbc was determined to be an accurate reflection of bioavailable P in these soils, although the addition of a temperature correction factor in setting threshold values is desirable.
The effect of K fertigation through buried drip irrigation on processing tomato (Lycopersicon esculentum Mill.) was evaluated in two California field trials in 2004, and soil K dynamics was investigated in greenhouse trials. Fertigation trials were conducted in fields with exchangeable soil K of 190 (site 1) and 270 mg·kg-1 (site 2), above the yield response threshold by traditional preplant or sidedress K application established by prior research. Two fertigation strategies were compared to an unfertilized control: continuous fertigation at 100 mg·L-1 K from early fruit set through early fruit color development, and weekly application of 40 kg·ha-1 K over the same period. In both treatments, a total of 200 kg·ha-1 K (from KCl) was applied. K fertigation significantly increased fruit yield at site 2, and improved fruit color at both sites. In the greenhouse experiments, fescue (Festuca arundinacea) was grown for 2 weeks atop columns of eight soils ranging from 120–380 mg·kg-1 exchangeable K; the columns were wetted from the bottom, by capillarity. The fescue roots were separated from the soil by a nylon fabric that prevented root penetration while allowing the penetration of root hairs, creating a two-dimensional root/soil interface. In all soils, fescue K uptake reduced soil exchangeable K only in the top 2 mm of the columns, suggesting that effective K diffusion was very limited. In columns of 200-mm height, applying 100 mg·kg-1 K in the water used to wet the soil had minimal impact on fescue K uptake. In columns of 15-mm height, this method of K application more than doubled fescue K uptake in all soils, suggesting that the effective limit of K movement was between 15-200 mm.
California melon (Cucumis melo) growers commonly apply calcium (Ca) fertilizers during fruit development to increase fruit firmness and improve storage life. Drip-irrigated field trials were conducted in central California in 2005 and 2006 to evaluate the efficacy of this practice on honeydew (C. melo Inodorus group) and muskmelon (C. melo Reticulatus group). In the 2005 honeydew trial, three weekly applications of 10 lb/acre Ca from calcium nitrate (CN), calcium thiosulfate (CTS), or calcium chloride (CC) were injected into the irrigation system during early melon development. In the 2006 muskmelon trial, two applications of 15 lb/acre Ca from CTS or CC were made early, or two applications of CC late, in melon development. The effect of these Ca fertigation treatments on fruit yield, soluble solids concentration, flesh firmness, and Ca concentration were compared with an untreated control receiving no Ca fertigation. Calcium fertigation had no effect on marketable yield, quality, or Ca concentration of honeydew or muskmelon fruit regardless of application timing or Ca source applied. Loss of firmness during either 2 weeks (honeydew) or 1 week (muskmelon) of postharvest storage was unrelated to Ca fertigation treatment and was not correlated with Ca concentration in fruit tissue. We conclude that under conditions representative of the California melon industry, Ca fertigation at typical application rates is ineffective in improving honeydew or muskmelon yield or fruit quality.
Mustard (Brassica spp.) cover crop residue has been reported to have significant `biofumigant' action when incorporated into soil, potentially providing disease suppression and yield improvement for the succeeding crop. The effects of growing over-winter mustard cover crops preceding processing tomato (Lycopersicon escultentum Mill.) production were investigated in six field trials in the Sacramento Valley of California from 2002–04. A selection of mustard cover crops were compared to a legume cover crop mix, a fallow-bed treatment (the current grower practice in the region), and in two of the six trials, fumigation treatments using metam sodium. Mustard cover crops removed 115 to 350 kg·ha–1 N from the soil profile, reducing NO3-N leaching potential. Soil populations of Verticillium dahliae Kleb. and Fusarium spp. were unaffected by the cover crops, and there was no evidence of soilborne disease suppression on subsequent tomato crops. Mustard cover crops increased tomato yield in one field, and reduced yield in two fields. In one of two fields, metam sodium fumigation significantly increased tomato yield. We conclude that, while environmental benefits may be achieved, mustard cover cropping offers no immediate agronomic benefit for processing tomato production.
Mustard cover crop residue has been reported to have a “biofumigant” action when incorporated into the soil, potentially providing significant disease suppression and yield improvement for the succeeding crop. Such activity could be particularly useful in processing tomato rotations, where consecutive cropping invariably results in yield decline. Agronomic and environmental effects of growing over-winter mustard cover crops preceding tomato production were investigated in three field trials between 2002 and 2004. Two mustard cover crops [`Pacific Gold', a brown mustard (Brassica juncea), and `Caliente', a blend of brown and white mustard (Sinapis alba)] were compared to a legume cover crop mix, a fallow bed treatment (the standard grower practice in this region), and, in two of the three trials, a fumigation treatment using metam sodium. No suppression of soil populations of Verticillium dahliae or Fusarium spp. was observed with the mustard cover crops, nor was there any visual evidence of disease suppression on subsequent tomato crops. In these fields, the mustard either had no effect, or reduced tomato yield, when compared to the fallow treatment. At one of two sites, metam sodium fumigation significantly increased tomato yield. The presence of a cover crop, whether mustard or legume, reduced winter runoff by an average of 50% over two years of trials. No benefit of mustard cover cropping beyond this reduction in winter runoff was observed.
Fruit soluble solids concentration (SSC) is an important quality factor for tomatoes (Lycopersicon esculentum Mill.) grown for processing. The use of drip irrigation often results in undesirably low SSC. The effects of late-season irrigation management on fruit yield and SSC was investigated in a series of drip-irrigated field trials in California from 2000–04. The effects of irrigation cutoff or deficit irrigation implemented 40 to 50 days preharvest (the period corresponding to the initiation of fruit ripening) were compared to a standard grower practice of irrigation cutoff 20 days preharvest. Irrigation cutoff 40 to 50 days preharvest increased SSC but resulted in substantial yield loss, with significantly reduced brix yield (Mg fruit solids ha-1). By contrast, deficit irrigation significantly increased SSC compared to the standard practice, with no significant loss of brix yield. In three commercial fields the effect of deficit irrigation on fruit SSC was investigated. Fruits were sampled on three dates: 1) 4 to 5 weeks preharvest, early-ripening, pink-stage fruit only, 2) about 1 week preharvest, both late-ripening, pink-stage fruit and early-ripening fruit now fully ripe, and 3) commercial harvest, composite of early- and late-maturing fruit. SSC increased in response to soil moisture stress induced by deficit irrigation, with late-maturing fruit as much as 1.6 °brix higher than fruit maturing before significant soil moisture stress. However, once a fruit reached the pink stage of maturity, its SSC was not affected by subsequent soil moisture stress. An additional five commercial field trials were conducted to compare growers' irrigation practices with greater degrees of deficit irrigation. In each field the grower's deficit irrigation regime was compared to a reduced treatment receiving 25% to 50% less water over the final 4 to 7 weeks before harvest. Across fields, applying 20% to 60% of reference evapotranspiration (ETo) over the fruit ripening period resulted in acceptable SSC without significant brix yield reduction. We conclude that deficit irrigation initiated during early fruit ripening provides a flexible tool for SSC management. Brix monitoring of earliest ripening fruit can help classify fields as to the severity of irrigation deficit required to reach desirable SSC at harvest.
The effect of K fertigation through subsurface irrigation lines on processing tomato (Lycopersicon esculentum Mill.) fruit yield and quality was evaluated in four field trials in California from 2002–04. Fields had exchangeable soil K between 0.48 to 0.85 cmol·kg–1, with high exchangeable Mg (10.6 to 13.7 cmol·kg–1) and a history of yellow shoulder (YS, a fruit color disorder) occurrence. K treatments evaluated included seasonal amount applied (0 to 800 kg·ha–1), fertigation method (continuous versus weekly), and timing (early, mid or late season); foliar K treatments were also included in the 2002 trial. In two fields total and marketable fruit yield were significantly increased by K fertigation, and fruit color improvements were observed in all trials. Among color parameters improved by K fertigation were YS incidence, blended color, and L*, chroma, and hue of the shoulder region of fruit. K fertigation did not affect fruit soluble solids concentration. Yield increased only with fertigation treatments initiated during early fruit set. The effects of fertigation method and rate were inconsistent. Foliar K application was ineffective in increasing either fruit yield or quality.
Soluble solids concentration (SSC) is a major quality factor for tomatoes (Lycopersicon esculentum Mill.) grown for processing. The effects of early irrigation cutback were investigated in a series of drip-irrigated field trials in California from 2000-03. Irrigation cutback was initiated from 4-7 weeks preharvest, with irrigation volume reduced to 30% to 70% of reference evapotranspiration. Early irrigation cutback was compared to full irrigation until cutoff 2-3 weeks preharvest. SSC was monitored from the initiation of deficit irrigation until harvest, with breaker-stage fruit sampled at approximately 10-day intervals; additionally, early-maturing fruits were tagged on the plant at breaker stage and retrieved at harvest for SSC analysis. Fruit yield, overall SSC, and brix yield (Mg·ha-1 fruit solids) were evaluated at commercial maturity. Fruit SSC increased in response to soil moisture stress, with late-maturing fruit as much as 2.0 °Brix higher than fruit maturing before significant moisture stress. However, once a fruit reached the breaker stage of maturity, its SSC did not increase regardless of subsequent soil moisture stress. Across field trials, yield decline resulting from early irrigation cutback was matched by a corresponding increase in overall SSC, resulting in equivalent brix yield in all test fields. We conclude that the early irrigation cutback provides a flexible tool for SSC management and that °Brix monitoring of breaker-stage fruit can augment soil moisture monitoring to tune irrigation management to field-specific conditions.