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Julie Fulenwider* and David Creech

General guidelines for the fertilization of azaleas in Southern landscapes often suggest applications be made after bloom. Early fertilizations are thought to encourage earlier blooms which are more likely to be damaged by spring freezes. Three years of data will be presented. Treatments include four fertilization rates, and various times of application from early (December-January) to late (March-April). Four to six varieties were evaluated (depending on the year of the study) in the SFA Ruby M. Mize Azalea Garden and in plantings on the Stephen F. Austin State Univ. campus. A randomized complete block design was utilized with three plants per replication. Influence of timing and rate of fertilizer application on bloom date and persistence. Influence of fertilizer treatments on leaf nutrient concentrations will be presented.

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Steven A. Weinbaum, Wesley P. Asai, David A. Goldhamer, Franz J.A. Niederholzer, and Tom T. Muraoka

There is legitimate concern that excessive fertilizer nitrogen (N) application rates adversely affect groundwater quality in the San Joaquin Valley of California. A 5-year study was conducted to assess the interrelationships between N fertilization rates, tree productivity, leaf [N], soil [NO 3], tree recovery of isotopically labeled fertilizer N, and NO 3 leaching. High N trees recovered <50% as much labeled fertilizer N in the crop as did trees previously receiving low to moderate fertilizer application rates. Our data suggest that the dilution of labeled N in the soil by high residual levels of NO 3 in the soil had a greater effect than tree N status (as expressed by leaf N concentration) on the relative recovery of fertilizer N.

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Jose Alvarez and C.A. Sanchez

The economics of producing sweet corn (Zea mays L.) and head lettuce (Lactuca sativa L.) on Florida Histosols were analyzed with and without P application constraints based upon quadratic response functions derived from two experiments with each crop. At the lower end of the historical price range, production of both crops is unprofitable, especially when P is constrained. At higher prices, net returns for sweet corn under P constraints are relatively small compared with the capital invested; net returns are higher for lettuce. However, an analysis of historical monthly prices showed that those high prices rarely occur. Values for the marginal contribution of the last unit of P show that constraints greatly limit net returns. Many sweet corn and head lettuce producers may be forced out of business if P fertilization rates are arbitrarily lowered below the economic optimal rate.

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Kalala Mwamba, E.G. Rhoden, R.O. Ankumah, and V. Khan

Amaranth (Amaranthus sp.) is a vegetable crop with grains and leaves high in protein, especially, lysine and the sulfur-containing amino acids which are limiting in many vegetables and grains. These nutritional qualities and the ease of growth make it a suitable alternate crop for limited resource farmers. A study was conducted to determine the effect of nitrogen sources and fertilization rates on amaranth production in Alabama and other Southeastern States. The experiment was set up as a complete randomized block design in Norfolk sandy loam (Fine silicoeus, thermic, Typic Paleudult). Four nitrogen sources (urea, sodium nitrate, ammonium sulfate, and ammonium nitrate) were used at three different levels (0, 40, and 80 Kg/ha) one week after transplanting. Sources of nitrogen did not have any significant effect on both fresh and dry vegetable yield (p 0.05). However, fertilization brought significant increases in both yield and total nitrogen content of vegetable amaranth (p 0.05).

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Ricardo Campos and David Wm. Reed

Salinity is a limiting factor in plant growth. The combination of water high in soluble salts and water-soluble fertilizers can induce salt damage in plants. The objective of this work was to investigate the effects of salinity in irrigation water on optimal fertilization rates in Spathiphyllum `Petite'. The combination of 5 levels of fertilizers and 5 salinity levels were tested. Maximum growth was observed at 250 mg l-1 N and no salts, and with 2000 mg l-1 salts at 125 mg l-1 N. As salt levels increased, height and leaf area decreased. Tissue calcium, sodium, and chloride increased with increasing levels of salinity. Tissue nitrogen, phosphorus, and potassium generally increased with increasing levels of fertilizers, and were not affected by salinity level. It is possible that high sodium and chloride concentrations in leaves, petioles, and roots produced an ion toxicity.

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Thomas H. Yeager, Rebecca H. Harrison, and Dewayne L. Ingram

Ilex crenata Thunb. `Rotundifolia' grown in sand culture with the root zone at 40C for 6 hours daily had smaller root and shoot dry weights after 6 weeks than plants grown with root zones at 28 or 34C. Root and shoot N accumulation (milligrams N per gram of dry weight) decreased when root-zone temperatures were increased from 28 to 40C and plants were fertilized twice dally with either 75, 150, or 225 mg N/liter. Nitrogen application rates of 150 or 225 mg·liter-1 resulted in increased root and shoot N accumulation for plants grown with root zones at either 28, 34, or 40C compared with the 75 mg N/liter treatment. Increased N fertilization rates did not alleviate reduced plant growth due to the high root-zone temperature.

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Bielinski M. Santos

Selecting the “right” nutrient rate for fertilization programs is one of the most important decisions growers face. On one hand, increasing fertilizer prices and environmental concerns have increased the awareness of accurately managing fertilization programs, thus reducing fertilizer amounts during cropping seasons. By contrast, many growers fear not obtaining the desired crop performance and economic returns, especially when fertilization is assumed as “inexpensive insurance” to improve yields, thus leading to overfertilization. The objective of this paper was to provide general principles for selecting and monitoring the right nutrient rate within the framework of the “4R” nutrient management concept (right rate, right source, right placement, and right timing) to protect environmental quality while maintaining productivity. Some methodologies to determine, apply, and adjust fertilization rates during the growing season were discussed, including in-season monitoring procedures, such as petiole sap testing, plant diagnostic analysis, leaf color evaluation, and plant growth index.

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James M. Spiers

A sand culture study arranged in a 3 × 3 factorial was used to determine the influence of Al and Mn levels on leaf nutrient content and plant growth of `Tifblue' rabbiteye blueberry (Vaccinium ashei Reade). Aluminum fertilization increased leaf Al content but did not affect plant vigor, leaf dry weight, or chlorosis. Manganese fertilization resulted in increased Mn in the leaves and a decrease in all growth parameters measured. The Al × Mn interactions were significant for Mn concentration in the leaves and vigor ratings. At the highest Mn fertilization rate, increasing Al fertilization had a synergistic influence on leaf Mn. Plant vigor at the highest Mn rate was lowest when no Al fertilizer was added. Increasing Al fertilization resulted in better plant vigor in plants grown with a high rate of Mn fertilization.

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James M. Spiers

The effects of varying rates of a complete fertilizer and irrigation on the growth and fruit yields of `Tifblue' rabbiteye blueberries (Vaccinium ashei Reade) established 3 years before treatment initiation were determined in a field study. Increased rates of irrigation resulted in increased plant growth and fruit yields. Five- and 6-year-old plants were more responsive to increasing irrigation rates than older plants. Irrigation water efficacy was greatest at lower rates and progressively less at higher rates of irrigation. Five- and 6-year-old plants fertilized with the lowest rate (14N–4P–7K; 150 g/plant) grew and yielded less than those fertilized with higher rates. Older plants did not respond to fertilization rates higher than 150 g/plant. Yearly rates of fertilization above 300 g/plant did not influence plant growth or fruit yields. Fertilization and irrigation responses were independent.

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C. Alt, H. Kage, and H. Stützel

Concepts of above-ground dry matter partitioning in cauliflower [Brassica oleracea L. (Botrytis Group)] as dependent on nitrogen (N) supply and light environment are presented. Leaf and stem partitioning depends on a functional relationship between stem dry weight and leaf area, independent of N status. Dry matter partitioning into the inflorescence is sink-limited (potential capacity) at the beginning, and source limited (daily available assimilates) later. The intrinsic specific growth rate of the inflorescence is dependent on leaf N content. The model is parameterized and evaluated with data from field experiments. Applied to an independent data set, the model predictions of proportions of inflorescence, leaf, and stem on total dry matter corresponded with measurements (r = 0.84, 0.92 and 0.22, respectively) for different N fertilization rates and light treatments.