ABSTRACT

To study the effect of nutrient conditions on the weathering of rock, hydroponic cultivation experiments of two crop plants (soybean and maize) were conducted with two different levels of nutrient supply and the release of elements from rock (basalt and andesite) particles were compared. The rock was encased with two different mesh-sized nets to allow or restrict the free action of fine roots. In the nutrient sufficient experiments approximately 4 times as much amount of the nutrient solution as supplied in the nutrient deficient condition was supplied in total. The release of elements (Ca, Mg, Al, Si, Mn and Fe) was calculated by subtracting the depletion of media from the amount absorbed by the plants, after the appropriate analyses of the media and plants. The experimental results showed that the release of elements in rock depended on nutrient condition. Generally the release of Ca, Mg, Si and Al by the plants was significantly smaller in the nutrient sufficient condition than that in the nutrient deficient condition, whereas Fe showed an opposite change. The difference was the most prominent for both the plants in the experiments with basalt and less clear only in the combination of soybean and andesite. The difference in the release amounts was explainable in terms of physiological requirement of the elements. The plants enhanced the release of elements from rock by secreting H⁺ and/or extending their fine roots to the surface of rock. The effect of the latter action was more important when nutrient was more deficient. When released Si is taken as a measure of rock weathering, basalt and andesite were weathered more rapidly by plants in nutrient deficient conditions.