Soil testing of horticultural substrates (i) Evaluation of 1: 1.5 water extract for nitrogen

Abstract The 1: 1.5 water extraction of horticultural substrates for N was evaluated using N uptake as the criterion. Three crops, tomato (1 trial), chrysanthemum (2 trials) and verbena (2 trials) were grown in a number of substrates including peat, peat + pumice, bark, peat + sawdust + sand and JIP. The number of N application rates varied from 7 in the tomato trial to 20, 22 or 25 in the chrysanthemum and verbena trials. The relationship between soil test value and N uptake was quadratic rather than linear, and was very good, especially for the chrysanthemum and verbena trials (R2 nearly always greater than 0.9). The relationship varied among the substrates depending on whether N was retained (e.g. bark) resulting in overestimation of N uptake, or on whether N was released (e.g. JIP) resulting in underestimation.