DETERMINATION OF NITROGEN IMMOBILIZATION INDEX IN GROWING MEDIA: EFFECT OF INCUBATION TIME AND INOCULATION WITH COMPOST

Microorganisms can immobilize mineral nitrogen in some growing media with high C/N ratio. The nitrogen Draw Down Index-NDI (Handreck, 1992) is used to determine the nitrogen immobilization in growing media. The mineral N immobiliza-tion of seven organic growing media and growing media constituents was measured by NDI methodology and by two modified versions concerning the incubation time and the addition of microbes to the substrates. The materials tested were peat, two kinds of pine wood fibre, oak wood fibre, coconut fibre, pine sawdust and oak sawdust. Results showed nitrogen immobilization in all materials tested, but the results were different depending on the methodology employed. The Nitrogen Immobilization Index changed along the incubation time in some substrates. Moreover, the addition of microorganisms to the substrates increased the immobilization of nitrogen in the substrates with pine wood fibre. Know the NDI evolution in a longer incubation time could be interesting for fertilization management in container crops. In light of the results we conclude that NDI original methodology should be adapted to the specific characteristic of some growing media.