SOIL TESTS FOR COPPER, IRON, MANGANESE, AND ZINC IN HISTOSOLS

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Abstract
We selected 55 cultivated organic soils from nine areas in eastern Canada that represented important properties of the investigated population, in order to evolve useful tests for plant-available Cu, Fe, Mn, and Zn in the soils. As a first step, the level and distribution of the soil Cu into H2O-sol, Ca-exchangeable, free-oxide-associated, weakly complexed, strongly complexed, carbonate, and sulfide forms were determined by sequential extractions, and the influence of soil properties on the level and distribution of Cu was ascertained. The data revealed that the level of total Cu in the soils, ranging from 8.3 to 537.5 .mu.g .cntdot. g-1 was independent of soil pH; percentage of ash; pyrophosphate index; bulk density; cation exchange capacity; and levels of Fe, Mn, and Zn. The Fe (0.15 to 3.57%) and Mn (32.5 to 916.8 .mu.g .cntdot. g-1) levels reflected the degree of decomposition and mineralization (percentage of ash: 6.0 to 53.7) of the organic soils, but Cu was mainly of external origin and thus increased with length of cultivation, due to the general practice of periodic applications of Cu. On an average, 57% of the soil Cu was unextractable, 28% strongly complexed, 9% sulfide-bound 2% in carbonates, 1.4% in free oxides, 1% weakly complexed 0.85% H2O-sol, and 0.67% on the exchange complex. The distribution was generally not influenced by soil properties or by levels of total soil Fe, Mn, and Zn. Total Cu was positively correlated with absolute concentrations of all forms of Cu on both wt/wt and wt/vol bases. The various forms of soil Cu also correlated with each other to varying extents. Water-soluble Cu had the best and most frequent correlations with other forms, and the free-oxide form was indicated to be the least interrelated. On the basis of percentages, all seven of the extractable forms that correlated with total Cu did so negatively, thus indicating that, in cultivated organic soils fertilized with Cu at regular intervals, the residual fertilizer value of Cu decreases as its residence time in soil increases.