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

Nutrient use efficiency, particularly for nitrogen (N) fertilizer, is still low despite significant improvements in crop production over the last few decades. Development of slow-release fertilizer (SRF) evolved as a potential way to enhance

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E.A. Guertal

A brief review of slow-release fertilizer technology Slow-release nitrogen fertilizers can be separated into three broad categories. The first is “natural” organic fertilizer, with the N contained as a part of crop residue, animal waste, or other

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Luz M. Reyes, Douglas C. Sanders, and Wayne G. Buhler

). Various types of slow-release fertilizers may extend the availability of nutrients, especially N, to the plant ( Maynard and Lorenz, 1979 ) and reduce N leaching losses from soil ( Wang and Alva, 1996 ). Research has been conducted with sulfur-coated urea

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Kelly T. Morgan, Kent E. Cushman, and Shinjiro Sato

with limited nutrient retention capacities, it is desirable to increase the number of split applications of N fertilizers to reduce leaching potential or to use products designed to release N over time. Nutrients from slow-release fertilizer (SRF) and

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Tim C. Knowles, Billy W. Hipp, and Mary Ann Hegemann

the slow-release N, P, and K formulations. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the Texas Agricultural Experiment Station. The cost of publishing this paper was defrayed in part by

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William Terry Kelley

Despite some advantages, adoption of slow-release fertilizers in vegetables has been slow primarily due to cost. In crops fertilized with ground equipment, growers can make fewer trips through the field and assure fertilizer is present when conditions prevent application. With drip irrigation, some materials are difficult to inject, however, Nitamin is a new injectable liquid produced by Georgia Pacific. Thus, with plasticulture, growers can inject less frequently and potentially use lower rates. Granular and liquid formulations of slow-release fertilizer were tested on onions (Winter 2003–04), cabbage (Winter 2003–04) and pepper (Spring 2004) in Georgia. Combinations of traditional fertilizer with slow-release formulations and various rates of slow-release fertilizer alone were compared to a standard fertilizer program on these crops in separate experiments. The slow-release contains only N. So, other nutrients were held constant. Otherwise normal cultural practices were employed. Crops were harvested at maturity and data collected on yield and quality. In cabbage, with at least 50% of the standard N rate using the slow-release fertilizer, yields were comparable to the standard. Results on onions were similar with N rates of at least 75% of the standard for the liquid material; the granular formulation did not perform well. Split applications of slow-release fertilizer and combinations with standard fertilizer worked well for cabbage, but not for onions. Results on pepper, although inconclusive, indicated it was possible to get comparable yields at lower N rates with the slow-release material. Based on these results, lower N rates are possible on cabbage and onions with slow-release fertilizers which may make them economically feasible while providing application advantages to growers.

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Bert T. Swanson, James B. Calkins, Daniel G. Krueger, and Theresa L. Stockdale

Media fertility is a critical factor in the successful production of container grown plants. Fertility treatments including fertigation and slow-release fertilizers (topdressed and incorporated) were compared. Fertility treatments were studied over a two-year period on a variety of deciduous and evergreen plant materials. Plant growth was quantified based on height, volume, branching, and quality. Soil fertility levels based on leachates were followed during the study. Nutrient release for incorporated fertilizers tested was variable although less so than when the same fertilizers were topdressed. Fertility treatment effects were species-dependent. Several incorporated, slow-release fertilizers, especially those high in nitrogen (Sierra 17-6-10, Sierra High N 24-4-6, Woodace Briquettes 23-2-0, Woodace 21-4-10), show promise for use in two-year container production systems.

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Edward Bush, Jeff Kuehny, and Patricia Branch

Three slow-release fertilizer formulations (Osmocote 14–14–14, 18–6–12, and Nutricote 17–6–10) at three rates (1, 2, and 3 lb/yd3) were incorporated into 4 pine bark: 1 sand (by volume) media filling 1-gal nursery containers. Additional treatments included slow-release fertilizer formulations at 1 lb/yd3 fertigated with 100 ppm N 20–10–20 fertilizer. As fertilizer rates increased, vegetative height, width, and dry-weight accumulation generally increased for both pinched and no-pinch mum crops. Fertigated pinch and no-pinch mums were the largest plants with the greatest dry-weight accumulation for each fertilizer formulation. The high rate for all slow-release fertilizers produced the greatest vegetative growth for nonfertigated treatments. This research suggest that higher rates for incorporated slow-release fertilizers and/or fertigation are required to produce maximum vegetative growth.

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Jong-Myung Choi and Paul V. Nelson

Mineralization of N from nonviable cells of Brevibacterium lactofermentum (Okumura et al.) mixed into soilless substrate in elution columns occurred largely during the first 5 weeks with a peak between 2 and 3 weeks. Over a 12-week period, 73% of the total N was recovered in the eluent. To prolong the period of N release to meet the requirements of a slow-release fertilizer, the bacterium was bonded to kraft lignin, a polyphenolic substance highly resistant to degradation. To retard mineralization further, the bacterium-lignin mixture was reacted with formaldehyde to form amino cross-links within and between protein chains. Bonding to lignin was undesirable because N release occurred during the same period as from the bacteria unbound to lignin and the total amount of N recovered was reduced to only 42%. Cross-linking with formaldehyde was less desirable since N was released mainly during the first 4 weeks with a peak during the first elution (0 time) and the total amount of N released was even lower than for the bacterium-lignin mixture. Additions of urea to the latter reaction did not satisfactorily improve subsequent N mineralization. In a second set of treatments lignin was withheld and the bacterium was reacted with weights of formaldehyde (a.i.) equivalent to 0.1%, 0.5%, 1.0%, 5.0%, and 10.0% of the dry weight of bacterium. Formaldehyde quantities ≤1.0% either had no effect or lowered the mineralization of N without altering time of release. Five percent and 10% formaldehyde successfully reduced release of N during the first 4 weeks and increased it thereafter. The best rate was 5%. In this treatment N was released from week 2 through the end of the test (12 weeks). Peak release occurred at 6 weeks. This resulting N source, while not a stand alone product, does have a slow-release property that could lend itself to use in combination with other slow-release N sources.

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Jong-Myung Choi and Paul V. Nelson

The structure of feather keratin protein was modified in attempts to develop a slow-release N fertilizer of 12 weeks duration or longer by steam hydrolysis to break disulfide bonds, enzymatic hydrolysis with Bacillus licheniformis (Weigmann) to break polypeptide bonds, and steam hydrolysis (autoclaving) to hasten mineralization followed by cross-linking of the protein by a formaldehyde reaction to control the increased rate of mineralization. Release of N in potting substrate within elution columns from ground, but otherwise untreated, raw feathers occurred mainly during the first 5 weeks with a much smaller release occurring from weeks 8 to 12. Steam hydrolysis resulted in an increase of N during the first 5 weeks and a decrease during weeks 8 to 11. Cumulative N release over 11 weeks increased from 12% in raw feathers to 52% for feathers steam hydrolyzed for 90 minutes. This favored an immediately available fertilizer but not a slow-release fertilizer. Microbial hydrolysis with B. licheniformis resulted in a modest reduction of N release during the first 5 weeks and a small increase during weeks 8 to 11. Both shifts, while not desirable for an immediately available fertilizer, enhanced the slow-release fertilizer potential of feathers but not sufficiently to result in a useful product. Steam hydrolyzed feathers cross-linked with quantities of formaldehyde equal to 5% and 10% of the feather weight released less N during the first 5 weeks, more during weeks 6 and 7, and less during weeks 9 to 12 compared to raw feathers. The first two shifts were favorable for a slow-release fertilizer while the third was not.