Effects of soil properties and amendments on the availability of zinc in soils

In a soil incubation experiment with different rates of Zn, the amounts of Zn extracted with 0.005 M DTPA, 1 M MgCl 2 , and 0.01 M CaCl 2 increased with an increase in the organic matter content of a neutral sandy loam soil and with alfalfa added as an organic amendment. Addition of muck and peat increased the amount of Zn exchanged with 1 M MgCl 2 but decreased the amount soluble in 0.01 M CaCl 2 , whereas addition of clay increased the amount of exchangeable Zn but decreased the amounts in the DTPA and 0.01 M CaCl 2 extracts. Liming of an acid sandy loam soil (pH 4.9) to about the neutral point reduced the amounts of extractable Zn markedly. A pretreatment of the soils with phosphate almost invariably increased the amounts of extractable Zn. In a corresponding pot experiment, the highest rate of Zn (250 ppm) reduced the yield of corn slightly, prevented the growth of lettuce, and reduced the yield of alfalfa markedly when these crops were grown successively in the acid soil. The concentration of Zn reached levels of 792 ppm in the corn and 702 ppm in the alfalfa. Addition of 50 ppm Zn to the acid soil restricted the growth of lettuce and increased the concentration of Zn to 523 ppm. Despite discrepancies, the concentrations of Zn in the plants as influenced by soil organic matter, organic amendments and liming were usually in accord with the amounts of Zn extracted from the soils. But the P pretreatment tended to decrease the concentration of Zn in corn and lettuce. The mean weight concentrations of Zn in the three species were correlated significantly with the amounts of Zn extracted with 0.005 M DTPA (r = +0.73), 1 M MgCl 2 (r = +0.93) and 0.01 M CaCl 2 (r = +0.90).