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Critical soil test values for desired yield targets of 400, 450 and 500q ha−1 were computed from the fertilizer prescription equations of potato in Ultic Hapludalf of Nilgiris. The critical soil test values for all the three major nutrients were low under IPNS treatments (NPK plus FYM, NPK plus Azospirillum) as compared to NPK alone, which in turn resulted in reduction of NPK fertilizer application for the same levels of yield targets.