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Thomas A. Obreza and Arnold Schumann

Florida citrus (Citrus spp.) producers must improve water and nutrient use efficiency to remain sustainable as they face increasing urbanization, plant disease, and environmental awareness. Producers have traditionally used water-soluble nitrogen (N) and phosphorus fertilizer sources with calendar-based irrigation, but they are slowly integrating enhanced efficiency fertilizers into nutrient management plans and converting to sensor or evapotranspiration-based irrigation scheduling. Recent research has improved the understanding of the citrus grove N budget, which has led to development of appropriate best management practices (BMPs) that will maintain production while protecting the environment. BMPs that have been shown to decrease N loss to groundwater include applying the appropriate fertilizer rate, splitting fertilizer applications, converting to fertigation, and improving irrigation scheduling.

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Thomas A. Obreza and Robert E. Rouse

The growth response of newly-planted 'Hamlin' orange (“Citrus sinensis L. Osbeck) on Carrizo citrange (C. sinensis × Poncirus trifoliata L. Raf.) trees to N-P-K fertilizer rates was studied to determine the minimum fertilizer required to bring trees into maximum early production. The highest fertilizer rate applied was 2.72, 5.45, and 8.17 kg·tree-1 of an 8-1.8-6.6 N-P-K fertilizer in 1989, 1990, and 1991, respectively. Additional fertilizer treatments equalled 50, 25, or 13% of the maximum rate. The response of trunk cross-sectional area, tree canopy volume, and fruit yield to fertilizer rate was described by a linear plateau model. The model predicted a fruit yield of 22.6 kg·tree-1 at the estimated critical rate of 48% of maximum. Fruit yield at 50% of maximum rate averaged 21.2 kg·tree. As fertilizer rate increased, total soluble solids (TSS) in juice and ratio (TSS:acid) decreased, but weight per fruit and TSS per tree increased. A fruit yield above 21 kg·tree-1 from 31-month-old trees was indicative of vigorous growth.

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Monica Ozores-Hampton and Thomas A. Obreza

In 1997, 24.7 million t of solid waste were produced in Florida (about 4.3 kg per person per day). If all biodegradable material was composted, 12.4 million t of compost would be produced annually. If this compost was used as a soil amendment in fruit and vegetable production, knowledge of its N mineralization rate would be important to determine the application rate. We measured the field N mineralization of four commercial Florida composts mixed with sandy soil (dry weight rate): Jacksonville (yard trimming compost, 127 t•ha-1), Sumter (municipal solid waste compost, 67 t•ha-1), and Nocatee and Palm Beach (yard trimming and biosolids composts, 63 and 56 t•ha-1). The control treatment was unamended soil. Open-top, 20-cm long PVC columns were filled with soil/compost mixtures and fitted at the bottom with a trap containing cation and anion exchange resin to capture leaching NO3 and NH4-N. The columns were buried in the soil at ground level and incubated in situ for 45 and 90 days in the spring. The resin was extracted with 1 N KCl and the mass of NO3-N and NH4-N adsorbed was determined. A similar procedure measured the NO3-N and NH4-N left in the soil/compost mixture. After 90 days in the field, net N immobilization was observed with Nocatee (-4.3%), Sumter (-3.0%), and Jacksonville (-1.3%) composts, while N mineralized (6.4%) from Palm Beach compost. Where N immobilization occurred, composts had initial C: N greater than 20: 1 and N concentration <1.6%. Mineralization occurred where compost had C: N ratio lower than 20: 1 and N concentration greater than 1.6%.

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Thomas A. Obreza and Robert E. Rouse

The growth response of young `Hamlin' orange (Citrus sinensis L. Osbeck) on Carrizo citrange (C. sinensis × Poncirus trifoliatu L. Raf.) trees to N-P-K fertilizer rates under field conditions in southwestern Florida was studied to determine the minimum fertilizer required to bring trees into maximum early production. The highest 8N-1.8P-6.6K fertilizer rate was 2.72,5.45, and 8.17 kg/tree in 1989,1990, and 1991, respectively. Additional fertilizer treatments equaled 50%, 25%, or 13% of the maximum rate. Fertilizer sources contained either all water-soluble N (applied more frequently) or 40% to 50% controlled-release N (applied less frequently), and they did not affect fruit yield or quality. The response of trunk cross-sectional area, tree canopy volume, and fruit yield to fertilizer rate was described by a linear plateau model. The model predicted a fruit yield of 22.6 kg/tree at the estimated critical fertilizer rate of 48% of maximum. Fruit yield at the 50% maximum rate averaged 21.2 kg/tree. As fertilizer rate increased, total soluble solids concentration (TSS) in juice and the TSS: acid ratio decreased, but weight per fruit and TSS per tree increased. A fruit yield >21 kg/31-month-old tree indicated vigorous growth.

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Thomas A. Obreza and Robert E. Rouse

Controlled-release N (CRN) fertilizer is receiving interest as a possible nutrient best management practice (BMP) for Florida citrus production, but grower acceptance will be limited until cost decreases and familiarity with CRN materials increases. The objective of this study was to compare long-term citrus production resulting from N fertilizer programs containing isobutylidene diurea (IBDU) or methylene urea (MU) with a conventional water-soluble N fertilizer program to determine the magnitude of horticultural utility provided by CRN. We applied N to a newly planted `Hamlin' orange (Citrus sinensis L. Osbeck) orchard using three sources (100% ammonium nitrate (AN); a 50/50 mixture of AN/IBDU; a 60/40 mixture of AN/MU) at four rates (0.25, 0.5, 1.0, and 2.0 or 1.5 times the recommended annual rate) in factorial combination, and continued for 7 years. During this period, AN was applied 31 times vs. about 15 times for CRN-containing fertilizers. We measured fruit yield, juice quality, and total soluble solids (TSS) yield in years 4 through 7 and found that they generally were not affected by N source, especially when year-to-year variation was taken into account. In year 7, fruit and TSS yields of well-fertilized trees reached 153 and 9.2 kg/tree, respectively. Maximum 4-year cumulative fruit and TSS yields (486 and 27.6 kg/tree, respectively) occurred at an N rate of 200 kg/ha. Maximum juice quality occurred at 180 kg N/ha. We feel the CRN materials tested could be used successfully in a nutritional BMP program that would maintain high yields while potentially decreasing N loss to the environment.

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Thomas A. Obreza and Jerry B. Sartain

Florida's citrus (Citrus spp.), vegetable, and turfgrass industries must improve nitrogen (N) and phosphorus (P) fertilizer use efficiency to remain sustainable in an era of emerging environmental policies designed to protect water quality. Producers have traditionally used water-soluble N and P fertilizers because they are plentiful and economical. Improving nutrient use efficiency (NUE) is being addressed through implementation of best management practices (BMPs) such as nutrient management planning, proper fertilizer material selection, better application timing and placement, and improved irrigation scheduling. Emerging technology that will aid in this effort includes increased use of enhanced efficiency fertilizers (EEFs), organic soil amendments, fertigation, and foliar fertilization. However, any new technology shown to improve NUE must be economically feasible before it can be considered a BMP. Future research in this area will aim to improve the economics of EEFs and precision fertilizer application.

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Monica Ozores-Hampton, Phillip A. Stansly and Thomas A. Obreza

Methyl bromide will be unavailable to conventional vegetable growers in the year 2005, and it cannot be used by organic growers. Chemical alternatives are more expensive and may also be subject to future restrictions. Non-chemical alternatives like solarization and organic amendments are as yet largely unproven but do offer promise of sustainable solutions free of government regulation. The objective of this study was to evaluate the effects of soil-incorporated biosolids and soil solarization on plant growth, yield, and soil fertility. Main plots were a biosolids soil amendment (37 Mg·ha-1 and a non-amended control. Treated main plots had received some type of organic amendment for the previous 6 years. Sub-plots were fumigated with methyl bromide as they had been for 6 years, or non-fumigated. Non-fumigated plots were further split into solarized and non-solarized plots. Bell pepper (Capsicum annuum `X 3R Aladdin') was grown for 8 months. Nitrogen fertilization was reduced to 50% of the recommended rate in the biosolids plots due to expected N mineralization from the biosolids amendment. Plant biomass was higher in the biosolids plots compared with the non-amended plots but there were no differences in marketable pepper yields between biosolids and non-biosolids plots. Plants grown in solarized soil produced lower plant biomass and yields than the methyl bromide and non-fumigated treatments. Soil pH and Mehlich 1-extractable P, K, Ca, Mg, Zn, Mn, Fe, and Cu were higher in biosolids plots than in non-amended control plots. Soil organic matter concentration was 3-fold higher where biosolids were applied compared with non-amended soil. The results suggest that regular organic amendment applications to a sandy Florida soil can increase plant growth and produce similar yields with less inorganic nutrients than are applied in a standard fertilization program. However, methyl bromide and non-fumigated treatments produced higher yields than soil solarization.

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Monica Ozores-Hampton, Thomas A. Obreza and George Hochmuth

Large volumes of compost produced from waste materials like yard trimmings, household trash (municipal solid waste), or biosolids (wastewater sludge) will likely become available for use by the Florida vegetable industry in the future. Using compost to produce vegetables has the potential to increase water and fertilizer conservation and reduce leaching from inorganic fertilizers in Florida's sandy soils. Compost quality for vegetable production systems should be based on soluble salts, phytotoxic compounds, C:N ratio, plant nutrients, trace metals, weed seeds, odor, moisture, pH, water-holding capacity, bulk density, cation exchange capacity, and particle size. In Florida, immature compost contained phytotoxic compounds that were harmful to crop germination and growth. Amending soil with mature composted waste materials has been reported to increase the growth and yields of vegetable crops grown in Florida. However, a beneficial response does not always occur, and the magnitude of the response is often not predictable.

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L. Carolina Medina, Jerry B. Sartain and Thomas A. Obreza

Slow-release fertilizers marketed to the public usually include a claim that nutrient release will last for a specific time period (e.g., 6, 9, or 12 months). However, no official laboratory method exists that can verify these claims. A long-term (180 days) incubation method has been developed that produces constants for an exponential model that characterizes nutrient release as a function of time. In addition, a relatively short-term (74 h) extraction method has been developed to assess nutrient release under accelerated laboratory conditions. Through regression techniques, release constants established for individual slow-release nutrient sources by the incubation method are used in conjunction with the laboratory extraction data to verify the release claims of slow-release fertilizers. Nutrient release for selected single materials has been predicted with greater than 90% accuracy in previous studies. Nutrient release from mixtures of slow-release products has been more variable. It is typical for water-soluble and slow-release fertilizers to be mixed in commercial products. Ultimately, it is intended that these methodologies will be accepted as an official method to verify nutrient release claims placed on slow-release fertilizers.

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Thomas A. Obreza, Robert E. Rouse and Kelly T. Morgan

No calibrated phosphorus (P) soil test exists to guide Florida citrus fertilization. Applying P fertilizer to citrus when it is not needed is wasteful and may cause undesirable P enrichment of adjacent surface water. The objective of this study was to establish guidelines for P management in developing Florida grapefruit (Citrus paradisi Macf.) and orange (Citrus sinensis L. Osb.) orchards by determining the effect of P fertilizer rate on soil test P and subsequently calibrating a P soil test for citrus yield and fresh fruit quality. Two orchards were planted on sandy soil with 3 mg·kg−1 (very low) Mehlich 1 soil test P. In Years 1 through 3, P fertilization increased soil test P up to 102 mg·kg−1 (very high). In Years 4 through 7, canopy volume, yield, and fruit quality did not respond to available soil P as indexed by soil testing. As tree size and fruit production increased, leaf P was below optimum where soil test P was below 13 mg·kg−1 (grapefruit) or 31 mg·kg−1 (oranges). Total P in the native soil at planting was ≈42 mg·kg−1, which was apparently available enough to support maximum tree growth, fruit yield, and fruit quality for the first 7 years after planting. Trees were highly efficient in taking up P from a soil considered very low in available P. Citrus producers can likely refrain from applying P fertilizer to young trees on Florida sandy soils if soil test P is very high or high and probably medium as well.