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Fishwaste Compost Effects on Rhododendron Growth and Nitrogen Leaching and Transformation

Washington State Univ. Res. and Ext. Center, 7612 Pioneer Way East, Puyallup, WA 98371-4998.

^* Corresponding author (skuo{at}wsu.edu).

ABSTRACT

Availability and leachability of N in fishwaste compost should be considered before it is used as growing media for nursery stock. This study examined N availability and leaching, and transformation of organic N constituents in fishwaste compost (HGF) as well as the effect of the compost on growth of rhododendron (Rhododendron fortunei ‘Scintillation’). With an initially high inorganic N concentration, the compost enhanced growth of rhododendron during its first flush of growth. Because of extensive N leaching, this beneficial effect was short-lived. Periodic additions of N fertilizer at 150 mg N kg^–1 were required to improve growth of rhododendron. Approximately 45% of inorganic N present initially in the HGF was leached regardless of the proportions of HGF in the mixtures. With N uptake by the plants representing a small fraction of inorganic N added, the inorganic N lost not accounted for probably was lost by denitrification and/or ammonia volatilization. Most N fertilizer applied as NH₄NO₃ to 100% bark was also leached. Concentrations of labile amino acids changed significantly in 4 mo, from 53.6% to <5% of total organic N in the HGF. Similar reduction also occurred for amino sugars. Even with such a magnitude of change in the two labile N fractions, there was no marked increase in inorganic N in the leachate 2 wk after transplanting and in growth of rhododendron. A large proportion of amino acids and amino sugars present in the compost initially were probably transformed to a more stabilized organic N form.

Scientific Paper 9604-10, Dep. of Crop and Soil Science, College of Agriculture and Home Economics Research Center, Washington State Univ., Pullman, WA 99164.

Received for publication May 17, 1996.