Phosphorus immobilization in wood waste-based potting media

Microbially induced nitrogen (N) immobilisation in potting media is accompanied by immobilisation of soluble phosphorus (P), with the P/N ratio of the immobilised elements being about 0.15. Fertiliser N applied to counter N immobilisation should be accompanied by this amount of soluble P if plants are not to suffer from P deficiency. Essentially none of the immobilised P in potting media that contained aged pine bark or eucalypt sawdust was available for short-term growth of 'Giant Butterfly' pansies or Hakea francisiana subsequently grown in them. One implication of these results is that the N drawdown test for potting media will underestimate N requirements if P is not included in the charging solution.