Abstract
According to a plot of pAlt' (-log of total inorganic Al concentration) as a function of pH in soil solution, aluminum species and their concentrations are summarized as follows. (1) Al^<3+> became the dominant Al species when the pH and pAlt' values were smaller than 4.5, respectively. (2) Al_6(OH)_<15>^<3+> became the dominant Al species when the pH values were around 4.5 to 6 and Alt' concentration in these pH values was relatively high. (3) Al(OH)_2^+ appears to become the dominant Al species when the pH value is higher than 5 and the pAlt' value is higher than 5.5. (4) The Al species which were present between (2) and (3) described above didn't exceed 50% of the Alt' independently. It was difficult to observe root systems in soil culture. Therefore, the degree of wheat response to soil solution aluminum may be represented by retention capacity of soil by the root system. The retention capacity of soil by a root system showed the lowest value in the Al^<3+> dominant area, such as (1) mentioned above, followed by the Al_6(OH)_<15>^<3+> dominant area, such as (2). However, even if the pH and pAlt' values of soil solution were in the Al_6(OH)_<15>^<3+> dominant area, the influence of Al_6(OH)_<15>^<3+> on the retention capacity of soil by the root system was negligible when the pH value was higher than 5. This is because the Alt' value is smaller than 10^<-5> mol L^<-1>. As the acidic aluminum in soil solution is neutralized, root elongation including root hair formation is promoted, resulting in the increase of the soil retention capacity of roots. Hence, a rhizosphere with an enlarged area of soil adhering to roots is created so that diffusion and mobility of water and nutrients from soil to plant can be facilitated favorably.
