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  • Author or Editor: T.A. Obreza x
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Citrus trees planted in alkaline soils typically show iron (Fe) deficiency chlorosis. Currently, Fe-EDDHA (ethylenediiminobis-2-hydroxyphenyl acetic acid) chelate is the most effective source of Fe for high pH soils. Iron humate (FeH), a by-product of the drinking water decolorization process, was compared with Fe-EDDHA for Fe deficiency correction on nonbearing `Ambersweet' orange and `Ruby Red' grapefruit Citrus paradisi Macf., and bearing `Hamlin' orange Citrus sinensis and `Flame' grapefruit trees, all on Swingle citrumelo rootstock, planted on high pH (>7.6) soils. Iron humate was applied under the tree canopy in spring at rates from 2 to 200 g Fe (nonbearing trees), or 22 to 352 g Fe (bearing trees) per tree per year. Application of FeH to nonbearing trees decreased twig dieback rating and increased flush growth, flush color rating, tree size, and leaf Fe concentration. Addition of urea or ammonium nitrate to FeH did not increase Fe availability. Iron amendments (22 g Fe per tree per year) increased fruit yield after the 1st year of application. Further increases in the rate of Fe, from 22 to 352 g Fe per tree per year as FeH, did not significantly increase tree growth, fruit yield, or fruit quality. This study demonstrated that FeH was an effective Fe source for citrus trees planted on alkaline soils.

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Deep aquifer water, which contains high levels of bicarbonate and Ca, is used predominantly for citrus irrigation. Changes in soil pH and Mehlich 1 extractable Ca concentrations were examined inside and outside the microsprinkler-wetted zone in 3- to 5-year-old citrus groves on three soils. Soil pH at 0 to 15 cm inside the wetted zone was 0.4, 0.9, and 1.3 pH units higher than that outside the wetted zone in Immokalee, Myakka, and Holopaw sands, respectively. This pH increase was due to the addition of bicarbonate in the irrigation water. Extractable Ca concentrations were also about two-fold higher inside compared to those outside the wetted zone at depths of O to 15 and 15 to 30 cm. With young trees, a majority of the roots are within the microsprinkler-wetted zone; therefore, soil samples should be taken inside the wetted zone for measuring soil pH and status of plant nutrients.

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Understanding the growth pattern of fibrous, orange tree [Citrus sinensis (L.) Osbeck] roots enables proper fertilizer placement to improve nutrient uptake efficiency and to reduce nutrient leaching below the root zone. The objective of this study was to develop relationships defining citrus fibrous root length density (FRLD) as a function of soil depth, distance from the tree trunk, and tree size. Root systems of 18 trees with tree canopy volumes (TCV) ranging from 2.4 to 34.3 m3 on two different rootstocks and growing in well-drained sandy soils were sampled in a systematic pattern extending 2 m away from the trunk and 0.9 m deep. Trees grown on Swingle citrumelo [Citrus paradisi Macf. × Poncirus trjfoliata (L.) Raf.] rootstock had significantly greater FRLD in the top 0.15 m than trees on Carrizo citrange (C. sinensis × P. trifoliata). Conversely, Carrizo citrange had greater FRLD from 0.15 to 0.75 m below the soil surface. FRLD was significantly greater for ‘Hamlin’ orange trees grown on Swingle citrumelo rootstock at distances less than 0.75 m from the tree trunk compared with those on Carrizo citrange. Fibrous roots of young citrus trees developed a dense root mat above soil depths of 0.3 m that expanded both radially and with depth with time as trees grow and TCV increased. Functional relationships developed in this study accounted for changes in FRLD with increase in tree size.

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Growth and nitrogen (N) accumulation relationships based on tree size, rather than age, may provide more generic information that could be used to improve sweet orange [Citrus sinensis (L.) Osbeck] N management. The objectives of this study were to determine how orange trees accumulate and distribute biomass and N as they grow, investigate yearly biomass and N changes in mature orange trees, determine rootstock effect on biomass and N distribution, and to develop simple mathematical models describing these relationships. Eighteen orange trees with canopy volumes ranging between 2 and 43 m3 were dissected into leaf, twig, branch, and root components, and the dry weight and N concentration of each were measured. The N content of each tree part was calculated, and biomass and N distribution throughout each tree were determined. The total dry biomass of large (mature) trees averaged 94 kg and contained 0.79 kg N. Biomass allocation was 13% in leaves, 7% in twigs, 50% in branches/trunk, and 30% in roots. N allocation was 38% in leaves, 8% in twigs, 27% in branches/trunk, and 27% in roots. For the smallest tree, above-/below-ground distribution ratios for biomass and N were 60/40 and 75/25, respectively. All tree components accumulated biomass and N linearly as tree size increased, with the above-ground portion accumulating biomass about 2.5 times faster than the below-ground portion due mostly to branch growth. The growth models developed are currently being integrated in a decision support system for improving fertilizer use efficiency for orange trees, which will provide growers with a management tool to improve long-term N use efficiency in orange orchards.

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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.

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Horticulture is an important industry in Florida despite formidable soil limitations. Favorable climate often makes the expense of overcoming these limitations economically feasible. Challenges arise from high water tables and/or sandy textures, both of which limit plant-available water and nutrient retention. High water tables of flatwoods (Spodosols) and marshes (Everglades Histosols) restrict root proliferation and commonly require artificial drainage. Upper zones of these soils are dominated by uncoated sand (Spodosols) or organic matter (Histosols) that has minimal sorption capacity for phosphorus (P) such that its transport poses an environmental risk without careful management. Nitrogen can be lost via denitrification under prolonged near-surface water saturation. At the other extreme but also prevalent in Florida are excessively well-drained sandy “sandhills” soils with limited water and nutrient retention. Nitrogen leaching from the latter soils can result in nitrate contamination in groundwater. Soil morphology is an important consideration in gauging nutrient and moisture retention. For example, each is enhanced by the presence of sand-grain coatings. Some amendments show promise in reducing P and moisture loss from sandy soils. Precarious balance between horticultural production and environmental risks for Florida soils has spurred development of approaches providing for a more accurate determination of the safe soil P storage capacity. Testing and refinement of these approaches are needed.

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