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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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Charlie G. Summers, Jeffrey P. Mitchell* and James J. Stapleton

Trials were conducted in 2002 and 2003 in California's San Joaquin Valley to determine the efficiency of reflective plastic and wheat straw in managing silverleaf whitefly and aphid-borne virus diseases in late planted cantaloupes. In 2002, the incidence of aphid-borne viruses was lowest in plants growing over reflective plastic followed by those growing over wheat straw and then those growing over bare soil. Wheat straw mulch was as effective as reflective plastic during the early part of the growing season in reducing the incidence of virus disease, but by mid-season, the reflective plastic was superior. The incidence of virus diseases in plants growing over wheat straw was significantly (P < 0.05) lower than that in plants growing over bare soil throughout the season. Whitefly numbers (nymphs per cm2) and aphid numbers were significantly reduced on plants growing over both reflective mulch and wheat straw mulch compared to those growing over bare soil. Yields of all sizes of melons were significantly higher in the reflective mulch plots and yield for the straw mulched and bare soil plots were not significantly different. Results in 2003 were similar to those of 2002. Both whitefly numbers and aphid numbers were significantly lower in plants growing over both mulches than in those growing over bare soil. Virus incidence was initially low but following an aphid flight in late August, the number of infected plants increased rapidly. Both the reflective plastic and straw provided equal protection form aphid-borne viruses throughout the growing season. Yields were highest in the reflective plastic plots, followed by the straw mulch and finally the bare soil. Differences were significant (P < 0.05) among all three treatments.

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J.P. Mitchell, D.M. May and C. Shennan

Field studies were conducted in 1992 and 1993 to assess the effects of irrigation with saline drainage water on processing-tomato fruit yields and quality constituents. Saline water (ECiw = 7 dS/m) was used for 66% of the seasonal irrigation requirements in 1992 and 82% in 1993. Yields of tomatoes irrigated with saline water were maintained relative to nonsaline irrigation in 1992, but were decreased by 33% in 1993. Juice Brix and Bostwick consistency were generally improved by irrigation with saline water. pH was unaffected by irrigation treatment, and titratable acidity, an estimate of citric acid content, was increased only in 1993. Calculated quantities for various marketable processed product yields reflect the dominant influence of fresh fruit yield that masked, to a large extent, whatever quality enhancements that may have derived from saline irrigation. The substantial tomato yield reduction that occurred in the second year of this study in plots irrigated with saline drainage water, the gradual surface accumulation of boron, as well as the significant salt buildup in lower portions of the crop root zone following drainage water irrigations demonstrate definitive limitations to the reuse approach and restrict options for the crops that can be grown in this system and the frequency of saline drainage reuse.

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J.P. Mitchell, C. Shennan, D. Peters and R.O. Miller

Sustainable alternatives for saline drainage water management in areas such as California's San Joaquin Valley are needed. Previous work has demonstrated the short-term potential for reuse of saline drainage water for irrigation in this area. Results from our 6-year cyclic drainage reuse study, however, indicate that soil structural problems may occur which can greatly reduce stand establishment and crop yields in periodically salinized soils. To prevent these problems, we are evaluating the effectiveness of winter cover crop incorporation and gypsum applications relative to conventional fallows, for improving/maintaining soil physical properties and crop productivity in cyclically salinized soils. Six winter cover crop/fallow treatments have been imposed upon a rotation of tomatoes, tomatoes and cotton as summer crops. By monitoring water use, relevant soil physical and chemical properties as well as crop performance during the course of this 3-year rotation study, we are assessing the potential benefits and constraints of using winter cover crops in drainage water reuse systems.

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T.K. Hartz, J.P. Mitchell and C. Giannini

Nitrogen and carbon mineralization rates of 19 manure and compost samples were determined in 1996, with an additional 12 samples evaluated in 1997. These organic amendments were mixed with a soil: sand blend at 2% by dry weight and the amended blends were incubated at constant moisture for 12 (1996) or 24 weeks (1997) at 25 °C. Net N mineralization was measured at 4- (1996) or 8-week (1997) intervals, C mineralization at 4-week intervals in 1997. Pots of the amended blends were also seeded with fescue (Festuca arundinacea Shreb.) and watered, but not fertilized, for 17 (1996) or 18 weeks (1997); N phytoavailability was estimated from fescue biomass N and mineral N in pot leachate. An average of 16%, 7%, and 1% of organic N was mineralized in 12 weeks of incubation in 1996, and an average of 15%, 6%, and 2% in 24 weeks of incubation in 1997, in manure, manure compost, and plant residue compost, respectively. Overall, N recovery in the fescue assay averaged 11%, 6%, and 2% of total amendment N for manure, manure compost, and plant residue compost, respectively. Mineralization of manure C averaged 35% of initial C content in 24 weeks, while compost C mineralization averaged only 14%. Within 4 (compost) or 16 weeks (manure), the rate of mineralization of amendment C had declined to a level similar to that of the soil organic C.

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S.R. Grattan, R.O. Miller and J.P. Mitchell

Since the discovery of elevated concentrations of selenium in the water, sediments, and biota at the Kesterson Wildlife Refuge, several studies regarding trace element distribution in the San Joaquin Valley and their potential environmental impacts have been initialed. We conducted a reconnaissance investigation to assess the concentration of boron, selenium, arsenic, molybdenum, uranium and vanadium using inductively coupled mass spectroscopy in prominent vegetation in the San Joaquin Valley. Five regions representing a range of geochemical environments with known differences in trace element concentrations in their soils or shallow ground water were selected for plant and soil sampling. Concentrations of boron, selenium arsenic, molybdenum, uranium, and vanadium in soil and tissue will be presented for these geographic areas for alfalfa, almonds, cotton, garlic, grapes, onions, tomatoes, and wheat. Interpretations of the wide range of trace element tissue concentrations that have been found will be discussed.

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A.M. Boland, P.H. Jerie, P.D. Mitchell, I. Goodwin and D.J. Connor

Individual and interactive effects of restricted root volume (RRV) and regulated deficit irrigation (RDI) on vegetative growth and mineral nutrition of peach trees [Prunus persica (L.) Batsch (Peach Group) `Golden Queen'] were studied over 3 years (1992-95). Trees were grown in lysimeters of five different volumes (0.025, 0.06, 0.15, 0.4, and 1.0 m3) with either full or deficit (RDI) irrigation. Increasing soil volume increased vegetative growth as measured by trunk cross-sectional area (TCA) (linear and quadratic, P < 0.011) and tree weight (linear, P < 0.001) with the final TCA ranging from 29.0 to 51.0 cm2 and tree weight ranging from 7.2 to 12.1 kg for the smallest to largest volumes. Root density measured at the completion of the experiment decreased with increasing soil volume (linear and quadratic, P < 0.001) with root length density declining from 24.0 to 2.0 cm·cm-3. RDI reduced vegetative growth by up to 70% as measured by weight of summer prunings. Root restriction was effective in controlling vegetative vigor and is a viable alternative for control of vegetative growth. Mineral nutrition did not limit tree growth.

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A.M. Boland, P.H. Jerie, P.D. Mitchell, I. Goodwin and D.J. Connor

Individual and interactive effects of restricted root volume (RRV) and regulated deficit irrigation (RDI) on productivity and water use of peach trees [Prunus persica (L.) Batsch `Golden Queen'] were studied over 3 years (1992-95). Trees were grown in lysimeters of five different soil volumes (0.025, 0.06, 0.15, 0.4, and 1.0 m3) with either full or deficit (RDI) irrigation. In Years 3 and 4, fruit size was reduced by up to 30% on trees in the two smallest volumes. Tree water use was positively related to increasing soil volume (linear, P < 0.001; quadratic, P < 0.011) in all years ranging from 1.8 to 4.4 L·mm-1 Epan in the post-RDI period of Year 2. Water use of deficit-irrigated trees was less than fully irrigated trees and there was an interaction between soil volume and irrigation treatment during RDI. Water relations did not limit growth or productivity. Tree water use was reduced under root restriction as a consequence of canopy demand rather than leaf function. Results suggest that a combination of restricted root volume and development of water stress achieve the RDI response in the Goulburn Valley, Australia.

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J.P. Mitchell, C. Summers, T.S. Prather, J. Stapleton and L.M. Roche

Observations that tomato transplants died or were severely stunted when set into unincorporated sorghum-sudan hybrid surface mulch led us to further investigate the potential allelopathic impacts of this warm-season cover crop in a series of field experiments. Survival and dry weights of tomato, lettuce, and broccoli transplants were determined in fallow, incorporated sorghum-sudan-, and unincorporated sorghum-sudan-mulched soils. All three species transplanted into plots in which the sorghum-sudan had been cut and left on the soil surface had a significantly lower dry weight than plants transplanted into fallow soil or into soil where the sorghum-sudan had been incorporated. Additionally, fewer transplants survived in the mulch treatment. The surface mulch plots also significantly reduced weed biomass nearly 10-fold. We believe that a water-soluble compound that is leached out of the sorghum-sudan hybrid is toxic to all three of the plants tested. Further laboratory and greenhouse tests are under way to determine the exact nature of the toxic substance.

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J. C. Correll, J. K. Mitchell, R. T. Holland and P. E. Cooper

Zonate leaf spot (ZLS) caused by Cristulariella moricola, apparently a rare disease of tomato, was identified in a commercial tomato field in southeastern Arkansas in June, 1991. Although lesions of ZLS were similar to early blight (Alternaria solani) lesions, which were also present, there were several distinctions between the lesions: the concentric rings in lesions of ZLS were more symmetrical than early blight lesions, no distinct chlorosis was associated with the lesions, and ZLS lesions were not localized near the bottom of the tomato canopy. Koch's postulates were completed on several greenhouse grown tomato cultivars. Inoculum was produced on autoclaved tomato leaves incubated at 20 C and consisted of large (300-400 um long) “Christmas tree” shaped propagules. When free moisture was maintained at 20 C, large (> 1 cm), rapidly expanding, water-soaked lesions were observed on leaves of inoculated plants 1-3 days after inoculation. Lower humidities caused lesions to rapidly dry out. Epidemiological factors, such as temperature, leaf wetness, and cultivar susceptibility also have been examined.