THE EFFECTS OF USING COPPER FOR MITIGATING HISTOSOL SUBSIDENCE ON: 2. THE DISTRIBUTION OF COPPER, MANGANESE, ZINC, AND IRON IN AN ORGANIC SOIL, MINERAL SUBLAYERS, AND THEIR MIXTURES IN THE CONTEXT OF SETTING A THRESHOLD OF PHYTOTOXIC SOIL‐COPPER1

Our earlier report on the yield and mineral composition of oats and lettuce grown on an organic soil (Histosol) containing three times the copper required for mitigating subsidence and on various mixtures of the organic soil with mineral sublayers formed the context of this study. This report deals with the comparative distributions of Cu, Mn, Zn, and Fe, in (1) organic soils A and B containing 135.7 and 1060 ppm Cu, respectively; (2) three mineral sublayers—a shell-rich deposit (S), a gyttja (G), and a clay sediment (Cl); (3) 1:1 vol/vol mixtures of the soil B with inert quartz sand, S, G, and Cl, resulting in soils B1, S1, G1, and Cl1, respectively; and (4) soils S2, G2, and Cl2, which were S, G, and Cl enriched with organic-bound Cu to possibly phytotoxic levels. The distributions were determined by seven sequential and three nonsequential extractions of the 12 soils. The main aim was to determine whether some enhancements in uptakes of Fe, Mn, and Zn, which accompanied increased plant absorptions of Cu, in apparent conflict with knowledge of antagonisms between the metals, occurred because the added Cu increased the availability of Fe, Mn, and Zn in the soils, rather than because of physiological stress on the plants containing elevated levels of Cu. In both organic soils A and B, the latter obtained by applying mostly bound Cu to the former, about 97 percent of the Cu was tightly held, not extractable by reagents milder than DTPA-TEA. Apparently the added Cu displaced some of the strongly held Mn and Zn in soil A to weaker sites of chelation or adsorption (in soil B), thus increasing their mobility and plant-availability. The Cu in soil B1 similarly caused the presence of large proportions of soil Mn and Zn on the exchange complex. The percentages of total Fe present in NaOH-extractable mobile form in soils A, B, and B1 were 1.32, 1.55, and 2.09 of the total, respectively, and thus availability of Fe was not decreased by the Cu. About 88 to 90 percent of the Cu in soils S1, G1 and Cl1 was also in the tightly held forms, while the availability and mobility of Mn, Fe, and Zn were seemingly adequate for balanced plant nutrition. Soils S2, G2, and Cl2, which supported crops showing some signs of incipient Cu-phytotoxicity, contained 52.7, 80.46, and 280.4 ppm of acetate-extractable Cu, the three highest concentrations among the 12 soils. In general, the results suggested that some of the changes caused by Cu in Fe, Mn, and Zn uptakes by the crops grown on soils B, B1, G1, S1, and Cl1 reflected alterations in the forms of these elements in the soils, rather than physiological stress. The data also allowed us to test various criteria proposed for setting a safe limit for soil-Cu. Our results were most consistent with the proposal that soil Cu becomes phytotoxic when its level exceeds 5 percent of the soil's cation exchange capacity (CEC), that is 16 ppm Cu for every milliequivalent of CEC per 100 g soil as determined by the ammonium acetate method.