SOLUBILITY PRODUCTS OF 6 METAL-GLYPHOSATE COMPLEXES IN WATER AND FORESTRY SOILS, AND THEIR INFLUENCE ON GLYPHOSATE TOXICITY TO PLANTS

The solubility products (K-sp) of 1:1 complexes of glyphosate, [N-(pho sphonomethyl)glycine], with Mg2+, Ca2+, Mn2+, Zn2+, Cu2+ and Fe3+, wer e determined in buffered (pH 7.0) distilled water, moist Ottawa sand, sandy loam and clay loam soils, each adjusted to 0.02 M with respect t o KNO3. The K-sp values decreased in the order of Mg approximate to Ca > Mn > Zn > Cu > Fe, regardless of the medium in which they were dete rmined. The constants measured in Ottawa sand were similar to those in water, but those in the forestry soils depended upon the type of meta l ion involved. The values for the Mg, Ca, Mn and Zn complexes were ab out 2 to 3 times lower in sandy loam soil than those in water, but tho se in clay loam were about 3 to 10 times lower. The K-sp of the Cu and Fe complexes were similar to those in water regardless of the soil ty pe used. In a bioassay experiment using tomato plants, immersed in the saturated solutions of the complexes or planted in the sand and soils containing saturated solutions of the complexes, no mortality occurre d although slight inhibition in growth was observed in all cases. The most soluble complexes of Mg and Ca caused the most reduction in plant height, while the least soluble complex of Fe caused little growth in hibition. The Zn, Cu and Mn complexes caused some growth inhibition de pending on their K-sp values. The larger the solubility product, the g reater the concentration of glyphosate ion in solution, and the greate r the growth inhibition. In a similar experiment using white spruce se edlings, growth inhibition was insignificant over the 12-d bioassay pe riod.