ESTIMATION OF PHOSPHORUS AVAILABILITY IN COMPOSTS AND COMPOST/PEAT MIXTURES BY DIFFERENT EXTRACTION METHODS

A trial was carried out with compost and compost/peat mixtures to test several extraction methods for the estimation of availability of phosphorus (P). The test plant was Dendranthema grandiflorum All composts had a high pH and salt content. Amounts of P extracted by different extraction methods decreased in the order: Formate < CAL < NH4-acetate < CaCl2/DTPA < CaCl2. Dilution of compost with peat decreased pH and increased availability of P. The better availability of P caused by dilution with peat was not reflected by the Formate-, CAL-, and NH4-acetate method. These acid and well-buffered extraction solutions overestimate P, and are therefore not suited to estimate availability of P in composts and compost/peat mixtures. Weak extraction solutions, like CaCl2 and CaCl2/DTPA, gave results which showed a good correlation with P content of plants and P uptake. The advantage of the latter method compared with CaCl2 is the extraction of amounts of P comparable to amounts taken up by the plants. Therefore of all the extraction methods tested, the CaCl2/DTPA method showed the best suitability to estimate the availability of P in composts and compost/peat mixtures.