A comparison of methods for the analysis of compost-based growing media

The objective of this study was to evaluate 3 analytical methods used for the characterization of compost-based substrates and their ability to predict nutritional disorders during plant cultivation. Five composts (CO) from different origins (green waste and a mixture of green waste and organic fraction of household waste) were mixed with light peat (LP) and perlite (PE) in the following rates: 33% CO, 50% LP, 17% PE; 50% CO, 33% LP, 17% PE and 66% CO, 17% LP, 17% PE. The growing media obtained were analysed in two laboratories with the following water extraction methods: Sonneveld 1:1.5 (V/V); EN 13652 1:5 (V/V); Regione Piemonte 1:5 (W/V). Electrical conductivity (EC), pH and plant available nutrients (NO3-N, NH4-N, P and K) were determined. A field trial with the 15 substrates analysed was carried out in order to correlate results of the chemical analysis with plant growth responses. Tested species were Impatiens hawkerii 'New Guinea', Prunus laurocerasus 'Rotundifolia' and Euphorbia pulcherrima. Results showed good correlation between the methods; for EC, pH, NO3-N and K it was possible to define conversion factors. Plant growth was adversely influenced by the amount of compost in the growing medium. High values of EC reduced plant development, especially in the stages following the transplanting of rooted cuttings into pots. Relations between analytical methods and plant growth show that both the methods according to EN 13652 and Sonneveld seem to be the most suitable to predict nutritional disorders in the cultivation of ornamental plants, while Regione Piemonte method is less accurate to characterize compost-based substrates.