A review of factors influencing organic matter decomposition and nitrogen immobilisation in container media

Abstract

The organic fraction of a potting mix is subject to decomposition and, therefore, is important in relation to nitrogen (N) immobilisation. Immobilisation of N is the reduction in plant available N (i.e., nitrate or ammonium) as a result ofmicroorganisms using this N as they decompose organic materials with a high carbon (C) content. The organic portion of potting mixes usually constitutes 50% or more of their volume and in New Zealand Pinus radiata bark and sphagnum peat are the most commonly used materials. Pinus radiata sawdust, tree fern fibre, composted mixed vegetation, and other sources of bark or sawdust are also used on a limited scale. Spent mushroom compost has also been successfully used in bark and peat container media overseas although there have been reports ofproblems with its use in New Zealand. It has good physical properties and is a useful source of nutrients except for N. Spent mushroom compost may cause temporary N immobilisation, after which N is slowly mineralised from it. Spent mushroom compost also has a highsoluble salt content that can be ameliorated by leaching, and a pH of 6.5-8.1. This paper seeks to review the factors causing decomposition of these organic materials and the implications of these processes on N immobilisation.