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  • Author or Editor: M. Gaskell x
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The effects of repeated application of two composts differing in carbon: nitrogen (C: N) ratio on soil NO3-N, soil NH4-N, and leaf lettuce yield was studied over three sequential crop cycles from 1995 to 1996. One compost type (HiCN) was prepared primarily from yard wastes and had a C: N ratio of 29 to 32:1 The other compost (LoCN) was a compost composed of a mixture of crude materials including yard wastes, feedlot manures, and vegetable trimmings and had a C: N ratio of 10 to 12:1. Before transplanting leaf lettuce, both composts were applied and incorporated in the same plots repeatedly over three crop cycles at rates of 9, 18, 36, and 54 Mg·ha–1 (dry mass) in each application. In the first crop cycle, no differences were observed for weekly soil NO3-N, NH4-N, or leaf lettuce yield among compost types or rates. In the second and third crop cycles, weekly soil NO3-N and soil NH4-N were directly related to LoCN compost application rates. First harvest lettuce yield was also directly related to LoCN rate in the second and third cycles, but total yield was not related to LoCN rate. In the second and third cycles, soil NO3-N and early and total lettuce yield were inversely related to rate of application of the HiCN material. Weekly soil NH4–N was not consistently related to application rates of HiCN or LoCN material.

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Limited soil nitrogen (N) availability is a common problem in organic vegetable production that often necessitates additional N fertilization. The increasing use of drip irrigation has created a demand for liquid organic fertilizers that can be applied with irrigation. The N availability of three liquid organic fertilizers was evaluated in an incubation study and a greenhouse bioassay. Phytamin 801 contained fishery wastes and seabird guano, while Phytamin 421 and Biolyzer were formulated from plant materials. The fertilizers ranged from 26 to 60 g·kg−1 N, 8% to 21% of which was associated with particulate matter large enough to potentially be removed by drip irrigation system filtration. The fertilizers were incubated aerobically in two organically managed soils at constant moisture at 15 and 25 °C, and sampled for mineral N concentration after 1, 2, and 4 weeks. In the greenhouse study, these fertilizers and an inorganic fertilizer (ammonium sulfate) were applied to pots of the two organically managed soils with established fescue (Festuca arundinacea) turf; the N content of clippings was compared with that from unfertilized pots after 2 and 4 weeks of growth. Across soils and incubation temperatures, the N availability from Phytamin 801 ranged from 79% to 93% of the initial N content after 1 week, and 83% to 99% after 4 weeks. The plant-based fertilizers had significantly lower N availability, but after 4 weeks, had 48% to 92% of initial N in mineral form. Soil and incubation temperature had modest but significant effects on fertilizer N availability. Nitrification was rapid, with >90% of mineral N in nitrate form after 1 week of incubation at 25 °C, or 2 weeks at 15 °C. N recovery in fescue clippings 4 weeks after application averaged 60%, 38%, and 36% of initial N content for Phytamin 801, Phytamin 421, and Biolyzer, respectively, equivalent to or better than the N recovery from ammonium sulfate.

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