| Abstract: | Abstract The initial phosphorus 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 and Olsen's procedure by regressing dry weight and net growth rate against soil test values using a quadratic function. 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 water extractable P to give maximum dry weight and growth rate in the tomato, chrysanthemum and one verbena trial tended to be higher for peat than for other substrates. For tomatoes the DV for maximum dry weight ranged from about 2 ppm in JIP to about 40 ppm in peat but the range was smaller for other substrates (14.3 ‐ 24.7 ppm). The DV for Olsen's extract was much less affected by substrate; it varied from 27.6 ‐ 32.5 μg/ml to 53.8 ‐ 62.3 μg/ml. The DV to give maximum dry weight for chrysanthemum varied from 2.3 ‐ 2.4 ppm in peat + soil to 19.0 ‐ 23.6 ppm in peat for the water extract, and 14.8 ‐ 18 μg/ml in peat + sawdust + sand (PSS) to 7 4 ‐ 78 μg/ml in peat for Olsen's method. For the verbena trials the DV to give maximum dry weight for the water extract varied from 3.0 ‐ 3.2 ppm in PSS to 17.7 ‐ 20.1 ppm in peat, and for Olsen's method 12.0 ‐ 14.2 μg/ml in peat to 27.8 ‐ 33.1 μg/ml in bark. The DV estimated for maximum growth rate were similar to those for maximum dry weight for both extracts. |
