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Phosphate, Fe and Mn uptake of N 2 fixing red clover and ryegrass from an Oxisol as affected by P and model humic substances application. 2. Phosphate and aluminum species distribution as modified by plant roots
Author(s) -
Gerke Jorg
Publication year - 1995
Publication title -
zeitschrift für pflanzenernährung und bodenkunde
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.644
H-Index - 87
eISSN - 1522-2624
pISSN - 0044-3263
DOI - 10.1002/jpln.19951580506
Subject(s) - chemistry , red clover , humic acid , phosphate , environmental chemistry , botany , fertilizer , biology , biochemistry , organic chemistry
In a previous paper we reported that model humic substances (0–50 mg kg −1 soil) increased P‐, Fe‐, and AI‐solubility as well as P uptake of red clover and ryegrass on a strongly P‐fixing soil. In this paper P‐ and Al‐species calculations under equilibrium assumptions were conducted to determine the species which were mobilized by the humics and red clover root exudates. Phosphate distribution between free ortho‐P and humic‐Al‐P complexes was calculated by an iterative procedure. In the soil without plants more than 50% of P in solution was present as humic associated P even at the lowest humic level. In soil under ryegrass the proportion of humic‐P complexes was much lower due to the relatively low concentrations of humic‐Al complexes in solution. In soil under red clover, between 55 and more than 84% of P in solution were present as humic‐P. This increase compared to soil without plants or with ryegrass is caused by the high concentrations of humic‐Al complexes in solution. The higher P‐influx of red clover compared to ryegrass and P species distribution indicate that red clover can take up P from solubilized humic‐P complexes. Aluminum species calculations showed that, even at the lowest level of humics addition, more than 60% of Al was complexed by the humics in soil without plants. The proportion of Al complexed by humics increased to about 90% at the highest level of humics addition. In soil under ryegrass cover, the proportion of humic‐Al complexes was lower, not exceeding 73% due to the reduced solubility of humic substances. In contrast, higher humics concentrations in soil solution under red clover increased the proportion of humic‐Al complexes to more than 80% of total Al even at the lowest humic level. It was concluded that the strong complexation of Al by the humics in the soil solution under red clover reduces Al toxicity.