Abstract
In order to ascertain whether the growth response to K application could be accurately predicted or not with the exchangeable K content of the soil regardless of conditions, pot experiments with factorial design were carried out on three soils containing different exchangeable K. The original volcanic ash soil containing 91ppm exchangeable K (shown in Table 1) was mixed with a proper amount of K2SO4 and was left for several weeks. These soils contained 240ppm and 442ppm exchangeable K. Radishes (var. Ooakamaru) were grown in 1/2000-are Wagner pots filled with 11kg field moist soils (equivalent to about 6.5kg oven dried soils) containing 91ppm, 240ppm and 442ppm exchangeable K, respectively. Factors combined with K were the light intensity, the soil water regimes, the amount of N, P, lime, Mg and Na. (The levels of these factors were shown in Table 2. ) K treatment was consisted of 2 levels, 0g and 5g K2SO4 per pot. All pots received 2.1g N as ammonium sulfate and 8g P2O5 as superphosphate as the basal dressing except in the N or P experiment.
On the soil containing 91ppm exchangeable K, significant increases in the weight of the fleshy root due to K application were obtained in all experiments, regardless of variables of factors combined with K.
On the soil containing 442ppm exchangeable K, K application did not increase the weight of the fleshy root in all experiments.
On the soil containing 240ppm exchangeable K, significant increases in the weight of the fleshy root due to K application were obtained only in the experiments where the levels of Mg and the soil water regimes were altered, but not in the experiments where the levels of N (calcium nitrate), P, lime, Na and the light intensity were altered. In the N (ammonium sulfate) experiment, K application did not increase the weight of the fleshy root when 2.1g N was supplied, but it increased significantly the weight of the fleshy root when 8.4g N was supplied.
In the experiments where significant increases in the weight of the fleshy root due to K application were obtained on the soil containing 240ppm exchangeable K, K concentration of the youngest expanding leaves, the largest leaves and the fleshy roots in radishes grown without K fertilizer was below ca. 3.4%, 3.0% and 6.0%, respectively.
Significant and non-significant increases in the top weight due to K application were obtained on the soil containing 91ppm exchangeable K, but this variability in growth responses to K application could not be explained by the levels of K concentration in radishes grown without K fertilizer.
