Soil testing of horticultural substrates (v) Evaluation of 1: 1.5 water extract and ammonium acetate extract for potassium and desirable potassium values

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Abstract
The 1: 1.5 water extraction of horticultural substrate for K was evaluated using K uptake as the criterion. Two crops, chrysanthemum (2 trials) and verbena (2 trials) were grown in peat, peat + pumice (PP), pine bark and peat + sawdust + sand (PSS). The ammonium acetate (NH4OAc) extraction for K was evaluated in one trial. The number of K application rates varied from 9 to 23. The relationship between both soil tests and plant uptake was very good for all substrates, although it was generally poorer in bark, particularly in the verbena trials. This was probably due to the relatively high level of native K in bark and the lower K requirement of verbena. The relationship between water extractable K and NH4OAc extractable K was linear and very good. Percentage maximum dry weight and net growth rate were regressed against soil test values using a quadratic function and desirable values (DV) were estimated from the response curve. The initial DV for water extractable K For maximum dry weight in the chrysanthemum trials varied from about 45 ‐ 139 ppm and for NH4OAc extractable K in one chrysanthemum trial varied from 284 ‐ 469 ppm. The DV obtained from growth rate measurements were similar to those obtained using percent maximum dry weight. It was not possible to determine DV for plants growing in bark and in PSS in one trial. The DV for verbena, which could be determined in peat in only one trial due to lack of response in the others was 10–11 ppm in the water extract.

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