Reduction of Toxicity of Immature Composts

in HortScience
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  • 1 Department of Plant and Soil Sciences, University of Massachusetts, Amherst, MA 01003

Uses of immature composts are difficult due to wide C:N ratio, high NH4 content, and phytotoxins, such as phenols and low molecular weight organic acids. This research focused on toxicity from high NH4 content. A compost of biosolids and wood chips was used. The compost was treated with (NH4)2SO4 to 2000 mg N·kg-1 (dry weight) to simulate an immature compost. The same compost without any external NH4 was used as a mature compost. Different proportions (regimes) of compost and soil provided 1/3, 1/6, and 1/12 compost (by volume). Each regime received potassium treatment at 0 or 0.6 g K·kg-1 as KC1. A nitrate treatment, at the same N rate as NH4 in immature compost, was factored into both mature and immature composts. For the mature compost, adding K generally decreased tomato (Lycopersicon esculentum Mill.) growth (measured by shoot mass) regardless of regimes. Adding Ca(NO3)2 to mature compost greatly increased plant growth for the regimes of 1/6 and 1/12. When the regime was 1/3, this increase diminished. For the immature compost, adding nitrate restricted plant growth due to excessive amount of N, including already high amounts of NH4. This response was especially true for the 1/3 regime. Adding K to immature compost greatly increased plant growth for the regimes of 1/3 and 1/6; K suppressed plant growth at the regime of 1/12. The results indicated that using K properly can effectively reduce immature compost toxicity due to high amount of ammonium. E-mail barker@pssci.umass.edu