The effects of using copper for mitigating histosol subsidence on: 1. the yield and nutrition of oats and lettuce grown on histosols, mineral sublayers, and their mixtures

As a part of extensive investigations into the suitability of applying moderate amounts of copper for slowing down the undesirably rapid biodegradation and subsidence of some Histosols (organic soils), this greenhouse study revealed that the yields of oats and lettuce grown on an organic soil (B) containing 1060 ppm Cu (wt/wt) were similar to or greater than those grown on an organic soil (A) with 135.7 ppm Cu. Also, the mineral nutrient contents of crops on soil B showed no signs of physiological stress due to their higher Cu contents.This study simulated a situation where organic soil B, containing about threefold the Cu thought to be necessary for mitigating subsidence by 50 percent, is mixed with a mineral sublayer in 1:1 vol/vol proportion for agronomic convenience, Organic soil B so mixed individually with sand, shell-rich deposit (soil S), gyttja (soil G), or a clay (soil Cl) sublayer thus gave soils B1, S1, G1, and C1, respectively. Similarly, soils S2, G2, and Cl2 represented a situation where about 98 percent of the carbon, but none of the Cu added by soil B to the S1, G1, and Cl1 mixtures, was dissipated, and further Cu was added to increase Cu concentrations by 350, 400, and 862 ppm, respectively.Like soils A and B, soil B1 supported healthy crops of lettuce and oats, both devoid of any adverse effects of Cu on the uptake of other elements, such as Fe and Mn. Although soils of the S group, like other calcareous soils, were probably poor in plant-available iron and manganese, crops grown on soil S1 neither contained phytotoxic levels of Cu nor suffered Fe deficiency. In contrast, soils of the G group probably had excessive amounts of Mn, which may have created some stress on crops grown on soil G1. However, lack of Fe, a diagnostic feature of Cu phytotoxicity, was not observed in the crops on soils G, G1 and G2, even when the Cu content in lettuce tops was >30 ppm—commonly considered to be a level where Cu phytotoxicity begins to be exerted. Similarly, neither the yields nor the mineral contents of the two crops grown on soil Cl1 showed any evidence of Cu phytotoxicity or even physiological stress. The yields of the two crops from soils S1, G1, and C1 were higher than those from corresponding soils S, G, and Cl, respectively, with one exception (lettuce tops grown on G1), thus indicating that even 1060 ppm of Cu in an organic soil would not adversely affect the growth and nutrition of crops grown on this soil or on its mixtures with the mineral sublayers found under cultivated organic soils in Canada. As for the mineral soils with extremely high Cu contents, an Fe deficiency induced or aggravated by the excess Cu appeared to have hampered the oat crop grown on soil S2 but not the succeeding lettuce crop. Apparently the absorption of Cu by the crops grown on soil G2 was not accompanied by a commensurate uptake of Fe, although this possible physiological stress was not reflected in crop yields. Similarly, excess Cu in the lettuce, but not the oat crop, grown on soil Cl2 may have interfered with the absorption, translocation, or both of Zn and Fe.Of all the accepted or proposed diagnostic features of incipient Cu phytotoxicity, decrease in Fe concentration and Fe:Cu ratio and Zn:Cu ratio were indicated by the data presented here to be more reliable and consistent than changes in total concentrations or uptakes of Cu, Mn, P, K, N, Ca or Mg.