Potassium uptake by Chinese cabbage (Brassica pekinensis Rupy.) from fused potassium silicate, a slow-releasing fertilizer

Fused potassium silicate (FPS), which contains K2Ca2Si2O7, has been prepared as a slow-releasing potassium fertilizer. Moreover, it is difficult to estimate the proportion of nutrients utilized by plants that come from the soil versus the slow-releasing fertilizer applied. To trace the uptake of potassium (K) by plants from FPS supplied to the soil, the fertilizer K was partially replaced with rubidium (Rb). The growth and K+Rb uptake (moles) of Chinese cabbage (Brassica pekinensis Rupy. cv. Kekkyu) plants in sand culture experiments were not affected by the replacement of K with Rb. In pot experiments using a volcanic ash Ando soil, Chinese cabbage was grown with no application of K, with K and Rb salts, or with Rb-containing FPS for three cycles of 40 days each. The amounts of fertilizer-derived K in the shoots estimated by the Rb-tracer method were smaller than those estimated by the difference of K accumulation between K fertilized and unfertilized plants. Such result suggests the involvement of 'K priming', a process by which the addition of K fertilizers enhances plant K uptake from the soil. The amount of K absorbed from FPS, calculated by the Rb-tracer method, indicated that the sparingly soluble K, that is soluble in 0.2 g L-1 citric acid solution but not in water, was absorbed at least partly through direct contact with the roots without prior exchange with soil K. Moreover, the plant absorption of sparingly soluble K from FPS was also confirmed by the difference method in K uptake.