oa Agrochemophysica - Studies on zinc in selected Orange Free State soils : IV : factors affecting the availability of zinc*

Volume 2, Issue 2
  • ISSN : 0302-7112



The effect of NH 4 NO 3, Ca(H2 PO 4 ) 2 , CaCO3 , MgCO3 , ZnSO4, and organic material applications to soils, and of changes in soil pH, on the availability of zinc in soils was studied in three short-term pot experiments. Effects were measured in terms of Zn content of Japanese millet seedlings which accurately reflect the Zn status of the soil. ZnSO 4 applications significantly increased the Zn content of millet seedlings at all pH values. The magnitude of the effect decreased with increasing soil pH. NH 4 NO 3 applications significantly increased the availability of Zn in soils. This was a secondary effect, however, attributable to shifts in soil pH. When divorcedfrompHchanges ??.,??, did not affect Zn availability. Ca CO 3 and MgCOs applications effected marked reductions in Zn availability primarily through soil pH changes. There was no evidence that the pH effect was altered by the nature of the liming material. Increasing soil pH values between 5.0 and 7.0 sharply reduced Zn availability. Increasing pH values above 7.0 did not, however, further reduce Zn availability. Ca(H2P04 ) 2 applications significantly reduced the availability of Zn. The effect increased progressively between soil pH values 4.7 to 6.5, but diminished progressively as the pH rose above 6.5. The application of finely-ground maize leaves to soils resulted in an initial increase in Zn availability. With longer decomposition periods, the availability decreased, in certain soils to a level less than that in the untreated soil. It is suggested that the Zn contained in the maize leaves dissolves rapidly in the soil solution, and thereafter becomes complexed by organic decomposition products in an unavailable form.

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