oa South African Journal of Agricultural Science - Studies on zinc in selected Orange Free State soils. I. An assessment of the zinc status of selected surface soils.

Volume 9, Issue 3
  • ISSN : 0585-8860



Various extractiori procedures for the assessment of the plant available zinc status of surface soils were investigated. In order to obtain a suitable procedure which may be related to the plant avail able zinc, several factors affecting the results were examined. These included the ratio of soil to extractant, time of extraction an4 pH of extractant and for each the optimum conditions were selected. The modified procedures were then applied ,to 88 surface soil samples from the agronomically important areas of the Orange Free State. Neutral normal solutions of NH.OAc, KCl, MgCI. and CaCl. failed to extract detectable quantities of zinc from any of the soils, but slightly acidified solutions of these salts extracted extremely small amounts. Dithizone-ammonium acetate"" 01 N HCI and di-sodium ethylenediamine- tetra-acetic acid extractable zinc and total soil zinc vary in order of magnitude. There are, however, highly significant relationships (P = 0'01) between the total zinc and the extractable zinc values, and also between each pair of the three extractable zinc values. The total and extractable zinc contents of these soils are in most cases extremely low compared with accepted standards. Taking only the total zinc for all the soils investigated, it is evident that the cpntents vary between 55 and 256 ppm with a mean value of 112 ppm. Only eight soils have total zinc contents of more than 15 ppm. These results indicate conclusively that many Orange Free State soils are zinc deficient. Comparing the total and extractable zinc contents of the cultivated and the corresponding virgin soils it is evident that the values for the former are significantly lower (p = 0'05) than those of the latter.

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