Weak acids, uptake from soil

Figures 1 and 2 show relationships among concentrations of U and selected major and trace elements in spinach leaves and petioles, respectively. It is noteworthy that concentrations of U in spinach were significantly positively correlated (p<0.01) with concentrations of Fe and A1 in both leaves and petioles. These relationships suggested that the absorption and transport processes of U in spinach could be related to those of Fe and Al, as was also suggested by Kametani et al. who showed that plants with higher Fe concentrations tended to absorb more U. Less U was extracted by 1 mol L ammonium acetate solution from soil (Table 2), meaning that U in soil was less available to plants. Spinach favours neutral-to-weak alkaline conditions and has the ability to acquire insoluble mineral nutrients such as Fe under neutral-to-alkaline conditions. Helal et al. compared spinach and beans with respect to the ability of the root to uptake Fe and found that spinach root absorbed Fe more efficiently. The differences in Cu, Zn, and Cd uptake by two spinach cultivars were attributed to different abilities to exude oxalate, citrate, and malate from root l The application of organic acids to soil facilitated the phytoextraction of U by hyperaccumulator plants thus, those root exudates could induce U dissolution from soil. Since part of U is associated with Fe and Al minerals in the soil it was likely that the absorption of U was accompanied by Fe and Al absorption, possibly triggered by the secretion of protons or organic acids to solubilise Fe and Al from soil.

Sorption in soil of weak-acid herbicides increases as the pH decreases (see, e.g.. Ref. 35), but this effect is much less than the effect of pH on uptake by roots. Accordingly, uptake of weak acids by plant roots is substantially greater from soils of low pH than of high pH, and this may explain why such herbicides are often found to be more effective in more acidic soils. 

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