Soil testing of horticultural substrates (ii) Desirable nitrogen values for the 1: 1.5 water extract

Abstract

Abstract The initial nitrogen desirable value (DV) to give maximum dry weight and maximum growth rate for various growing periods has been estimated for the 1: 1.5 water extraction procedure by regressing dry weight and net growth rate against soil test values using a quadratic function. The DV for dry weight and growth rate has been estimated for chrysanthemum and verbena (2 trials each) and for dry weight for tomato (1 trial) in a number of substrates. The DV for maximum dry weight of chrysanthemum, grown for 48 days under winter conditions, were: Peat 151–211 ppm and greater, Bark 140‐168 ppm and Peat and Sawdust and Sand (PSS) 135–162 ppm. Under summer conditions and grown for 35 days, the DV were as follows: Peat 134 — 172 ppm, Peat and Pumice (PP) 142 ‐170 ppm, Bark 168–202 ppm and PSS 152–172 ppm. The DV for maximum dry weight of verbena grown for 42 days under spring conditions were as follows: Peat 57 ppm, Bark 41 ppm and PSS 48–52 ppm and greater. Under summer conditions and grown for 38 days, the DV for verbena were: Peat 77–90 ppm, PP 70–85 ppm, Bark 67–73 PPm and greater, and PSS 72 ppm. For the chrysanthemum and verbena trials, the DV for maximum growth rate over various periods were generally in the range of DV found for maximum dry weight. However, the values did tend to increase later in the life of the crop. The DV for maximum dry weight of tomato were as follows: Peat 260–290 ppm, PP 211–240 ppm, Bark 220–254 ppm, PSS 186–236 ppm and Sawdust and Sand 190–220 ppm. This trial, in contrast to the others, was not conducted in conditions free from leaching and this may have given a higher DV.