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John D. Lea-Cox and James P. Syvertsen

Eighteen, 4-year-old Grapefruit (Citrus paradisi) cv. `Redblush' trees on either Volkamer lemon (C. volkameriana = VL) or Sour orange (C. aurantium = SO) rootstocks were grown in 7.6 kiloliter drainage lysimeters in a Candler fine sand (Typic Quartzipsamments), and fertilized with nitrogen (N) in 40 split applications at 76, 140 and 336 g N year-1 (= 0.2, 0.4 and 0.9 x the recommended annual rate). Labelled 15N was substituted for the N in a single fertigation at each rate at the time of fruit set the following year, to determine N uptake, allocation and leaching losses. “Nitrogen-uptake and allocation were primarily determined by the sink demand of fruit and vegetative growth, which in turn were strongly influenced by rootstock species. Larger trees on VL required at least 336 g N yr-1 to maintain high growth rates whereas smaller trees on SO of the same age only required 140 g N year-1. Of the 15N applied at the 336 g N rate to the SO trees, 39% still remained in the soil profile after 29 days. With optimally scheduled irrigations, 15N leached below the root zone was less than 3% of that applied after 29 days, regardless of rate. However, 17% of the applied 15N was recovered from a blank (no tree) lysimeter tank. Total 15N recovery ranged from 55-84% of that applied, indicating that a sizeable fraction of the 15N applied may have been lost through denitrification.

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C.A. Sanchez

The low desert region of Arizona is the major area of lettuce (Lactuca sativa L.) production during the winter. Most lettuce is grown on alluvial valley loam and clay loam soils. There is interest in moving some vegetable production onto sandy soils on the upper terraces (mesa) to partially relieve the intensive production pressure currently being placed on land in the valleys. Water and N management is a major concern in coarse-textured soils. Studies were conducted to evaluate the response of crisphead lettuce to sprinkler-applied water and N fertilizer on a coarse-textured soil (>95% sand). The experiments were irrigated using a modified lateral irrigation system that applied five levels of water and five levels of N in specified combinations. Nitrate-N concentrations were determined in samples collected in ceramic suction cups placed below the crop rooting zone. Leaching fraction was estimated by frequent neutron probe soil moisture measurements. Lettuce yield increased with water and N but rates required for maximum economic yield exceeded rates typically required on finer-textured valley soils. These data show the potential for large N leaching losses on this coarse-textured soil.

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K.M Whitley and J.R Davenport

Potato (Solanum tuberosum) production in Washington State's Central Columbia Plateau faces nitrogen (N) management challenges due to the combination of coarse textured soils (sandy loam to loam) and hilly topography in this region as well as the high N requirement of potato. Potato growth and development can vary with the N availability across the field. In this 2-year study, two adjacent potato fields were selected each year (1999 and 2000). Each field was soil sampled on a 200 × 200 ft (61.0 m) grid to establish existing soil N content. One field was preplant fertilized with variable N rate while the other was conventionally preplant fertilized, applying a uniform rate across the field based on the field average. During the growing season, each field was monitored for nitrate leaching potential using ion exchange membrane technology. Soil and plant nutrient status were also monitored by collecting in-season petiole and soil samples at two key phenological stages, tuber initiation and tuber bulking. Overall this research showed that variable rate preplant N fertilizer management reduced N leaching potential during the early part of the growing season, but did not persist the entire season. Since preplant N accounted for only 40% of the total seasonal N applied, it is possible that further gains could be made with variable rate in-season N application or with variable rate water application.

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J.E. Ells, A.E. McSay, P.N. Soltanpour, F.C. Schweissing, M.E. Bartolo, and E.G. Kruse

Water and nitrogen (N) are major inputs in the production of onions in the Arkansas Valley of Colorado. Because nitrates move with irrigation water, the effect of different rates of application of both N fertilizer and water on nitrate leaching were studied simultaneously. After a 2-year field study (1990-1991), it was concluded that >50 t·ha-1 of onions could be obtained without any N fertilizer when >42 ppm of nitrate nitrogen (NO3-N) were initially present in the top 33 cm of soil and up to 112 cm of irrigation water was applied. Total onion yield was not improved by applying more than the calculated irrigation requirement. The 2-m profile of soil under these experiments was found to contain >1400 kg·ha-1 of residual NO3-N prior to fertilizer treatments. When twice the estimated irrigation requirement was applied, >1000 kg·ha-1 of NO3-N was unaccounted for and presumed to have been mostly leached below the 2-m profile and partly denitrified. In both years, the onions were planted on land that had been fallowed the previous season, which does not help explain the presence of the high levels of nitrates found in the soil profile. It was concluded that sound water and N management practices in onion fields are crucial for preservation of water quality.

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Zhenli He, David. V. Calvert, Peter. J. Stoffella, and Mingkui Zhang

To evaluate effects of canopy and micro-irrigation under trees on accumulation and leaching of phosphorus (P) and heavy metals in agricultural sand soils, the horizontal and vertical variations of soil P and metals in a 408-m2 plot within a grove under grapefruit (Citrus paradisi Macf.) production near Fort Pierce, Fla., was examined. A high horizontal variation of labile soil P and metal concentrations was observed. Across the row, the highest values of pH, EC, water-soluble P, and all metals occurred in the soils under the canopies, and the lowest values occurred in the soils near the water furrow or the midway of the inter-row. Along the grapefruit row, the highest values of many measured variables occurred along the northern side of the citrus tree and close to the emitter. The downward movement of P, Cu, and Zn in the soils was more significant in the soils in open areas (near the water furrow and midway of inter-neighboring trees) than those under the canopies. The differences in labile P and metal spatial distributions in the soils were related to the location of emitter fertigation and differences in rainfall-induced leaching in the field. The results suggest that applying fertilizers to sites under the canopy rather than the spaces between the trees can minimize leaching losses of nutrients.

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Yuan-ling P. Lin, E. Jay Holcomb, and Jonathan P. Lynch

Alumina granules charged with P were used as an amendment to improve the ability of a soilless medium to retain P and provide it to plants. Commercially available alumina was acidified, saturated with P, and evenly distributed in a medium of peat, vermiculite, and sand to grow potted marigolds (Tagetes spp.) to a commercially salable stage. Marigolds grown in medium amended with P-charged alumina had adequate nutrition and similar or superior shoot growth (as measured by height, number of branches, and flower production) and fresh and dry weights compared to marigolds grown using commercial fertilizer. Phosphorus-charged alumina at 1% or 2% of total medium volume was sufficient to grow marigolds for at least 8 weeks and substantially reduced P leaching compared to conventionally fertilized controls. Alumina amendments in this range did not cause Al toxicity, as evidenced in root growth and leaf Al content.

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Catherine S.M. Ku and David R. Hershey

Poinsettias (Euphorbia pulcherrima Willd. ex Klotzsch `V-14 Glory') grown as single-pinched plants and received constant fertigation of Hoagland solution with N at 210 mg·L-1 of 100% NO3-N or 60% NO3-N : 40% NH4-N; P at 7.8 and 23 mg·L-1; and leaching fractions (LFs) of 0, 0.2, or 0.4. The P at 23 mg·L-1 used in this study was about half the P concentration typically provided from a 20N-4.4P-16.6K fertilizer at 200 mg·L-1 N fertigation. The total P applied via fertigation ranged from 51 mg at the 0 LF to 360 mg at the 0.4 LF. The leachate P concentration ranged from 0.2 to 46 mg·L-1. With P at 7.8 mg·L-1, the percentage of total P recovered in the leachate was 6% to 7%. At 23 mg·L-1 P fertigation, however, the total P recovered in the leachate with 60% NO3-N treatment was 2-times greater than with 100% NO3-N treatment. This result is attributed to a lower substrate pH, which resulted from NH4-N uptake and nitrification processes with 60% NO3-N fertigation. The P concentration in the recently matured leaves with 7.8 mg·L-1 P fertigation was in the normal range of 0.3% to 0.6%. Fertigation P can be reduced by up to 80% and still be sufficient for producing quality poinsettias. Reducing the fertigation P concentration is beneficial because it reduces P leaching, reduces fertilizer costs, and reduces luxury consumption.

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Kimberly A. Williams and Paul V. Nelson

Soilless container root media have little capacity to retain P, and preplant amendments of triple superphosphate (TSP) and water-soluble fertilizer (WSF) P applications are readily leached from them. A soilless medium modified with Al2(SO4)3 was tested to reduce such P losses. Aluminum sulfate solutions were applied to a 70 sphagnum peat: 30 perlite (v/v) medium to result in 0.32, 0.96, and 1.92 kg Al/m3 and dried at 70C. Adsorption isotherms (25C, 0 to 500 mg P/liter) showed that P retention increased as the rate of Al addition increased. In a greenhouse study, plants of Dendranthema ×grandiflorum (Ramat.) Kitamura `Sunny Mandalay' were grown in Al-modified media and an unmodified medium in factorial combination with P from preplant amendment of 0.1 kg TSP-P/m3, or P applied at each watering as WSF at rates of 5.5 or 21.8 mg P/liter. The two highest rates of Al were excessive and resulted in low pH and excessive soluble Al levels in the root medium solution early in the cropping cycle, which were detrimental to plant growth. When the root medium was modified with 0.32 kg Al/m3, soluble Al levels in medium solution were not significantly different than in the unmodified control. TSP-P that leached was substantially reduced by the addition of Al, yet sufficient P was released throughout the cropping cycle for adequate plant growth. Plants grown in Al-modified medium with 0.1 kg TSP-P/m3 did not differ from control plants in unmodified medium + 0.27 kg TSP-P/m3 and were larger than plants grown in unmodified medium + 0.1 kg TSP-P/m3. Aluminum modification of the root medium substantially reduced P leaching when used with WSF containing P. In addition, growth of plants in unmodified medium fertilized with 5.5 vs. 21.8 mg P/liter was similar.

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J.P. Syvertsen and M.L. Smith

Four-year-old `Redblush' grapefruit (Citrus paradisi Macf.) trees on either the relatively fast-growing rootstock `Volkamer' lemon (VL) (C. volkameriana Ten. & Pasq.) or on the slower-growing rootstock sour orange (SO) (C. aurantium L.) were transplanted into 7.9-m3 drainage lysimeter tanks filled with native Candler sand, irrigated similarly, and fertilized at three N rates during 2.5 years. After 6 months, effects of N application rate and rootstock on tree growth, evapotranspiration, fruit yield, N uptake, and leaching were measured during the following 2 years. When trees were 5 years old, low, medium, and high N application rates averaged about 79,180, or 543 g N/tree per year and about 126,455, or 868 g N/tree during the following year. Recommended rates average about 558 g N/tree per year. A lysimeter tank with no tree and additional trees growing outside lysimeters received the medium N treatment. Nitrogen concentration in the drainage water increased with N rate and exceeded 10 mg·liter-1 for trees receiving the high rates and also for the no tree tank. Leachate N concentration and total N recovered was greater from trees on SO than from those on VL. Average N uptake efficiency of medium N rate trees on VL was 6870 of the applied N and 61 % for trees on SO. Nitrogen uptake efficiency decreased with increased N application rates. Trees outside lysimeters had lower leaf N and fruit yield than lysimeter trees. Overall, canopy volume and leaf N concentration increased with N rate, but there was no effect of N rate on fibrous root dry weight. Fruit yield of trees on SO was not affected by N rate but higher N resulted in greater yield for trees on VL. Rootstock had no effect on leaf N concentration, but trees on VI. developed larger canopies, had greater fibrous root dry weight, used more water, and yielded more fruit than trees on SO. Based on growth, fruit yield and N leaching losses, currently recommended N rates were appropriate for trees on the more vigorous VL rootstock but were 22% to 69 % too high for trees on SO.

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T.E. Bilderback

Research reports documenting phosphorus leaching from soilless container media has changed commercial nursery phosphorus fertilizing practices. However, rhododendron growers are concerned that phosphorus levels are adequate as plants begin setting flower buds in July and August. Medium solution of 10 to 15 ppm P are recommended. Five replicated leachate samples were collected from 6 phosphate sources for 11 weeks following surface application to 2 container grown rhododendron cultivars. Each fertilizer source wax blended to an analysis of 14.0N-11.2P-5.0K except a 14.0N-0P-5.0K control. Phosphate sources included Diammonium Phosphate, Triple superphosphate, Sulfur coated Diammonium Phosphate, Sulfur coated triple superphosphate, and a commercial rhododendron sulfur coated fertilizer. With the exception of control, all treatment leachate phosphorus levels ranged from 180 to 145 ppm two days and 85 to 75 ppm one week after application. All sources ranged from 45 to 10 ppm weeks 2-5 and were lower than 10 ppm weeks 7-11. Leachate levels of the control were below 10 ppm at all sample times. Bud set and foliar P levels were different among phosphate treatments, but growth index measurements were not significant.