Vanadium availibilty to maize (ZEA MAYS L.) grown on two contrasting soils

Abstract

Soils on the Eastern Highveld acidify naturally due to the high rainfall and the use of certain nitrogen fertilizers. Liming materials are used to ameliorate soil acidity. A large fraction of this liming material used is Basic oxygen furnace slag (BOF slag), a secondary by-product from the Iron and Steel industry, commercially known as Aglime. These slag contain various concentration of heavy metals, like vanadium. No vanadium guidelines exist in South Africa. This study was done to determine the V threshold values where maize experienced reduced growth and to determine the V loading rate through slag application. A pot trial was used to determine the concentrations where vanadium reduces plant growth, and to establish toxicity levels in maize. The pot trail was used to establish V threshold values with various indicators, like total V concentration in the soil, total V concentration in the plant, Bray 1 extractable V and ammonium acetate extractable V, where maize experienced reduced growth. The threshold value where maize showed reduced growth in sandstone derived soils was at a total V concentration in the soil of 73.3 mg kg-1. The Bray 1 extractable V at this threshold was 23.5 mg kg-1 and there was no V in the above ground plant material in the maize. The ammonium acetate extractability at this level was 1.68 mg kg-1. V toxicity occurred at a total V concentration of 150 mg kg-1, with Bray 1 extractable V at 77.6 mg kg-1 and total V in the maize plant 14.8 mg kg-1 For the dolerite derived soil the threshold value was determined to be 235 mg kg-1 for the total V concentration in the soil. The Total V concentration in the plant was 0.5 mg kg-1 and the Bray 1 extractable V was 30.3 mg kg-1. The ammonium acetate extractable V was 1.69 mg kg-1. A pot trail and field evaluation site was used to determine the V loading through slag application. Three slag where used containing different V concentrations, slag A containing the highest V (918 mg kg-1) and B (153 mg kg-1) and C (88.6 mg kg-1) had a lower V concentrations. Theoretical V loading values was determined for three different slags containing different V concentrations and by using the threshold V concentration generated in chapter 3, the period to reach the critical V threshold value for liming with slag A was determined. If all factors (V concentration and incorporation depth), were to be kept constant, it will take an estimated 186 years of liming with slag A for the sandstone derived soil to reach the threshold value of 100 mg kg-1 where V negatively affect the growth of maize plants. This period was calculated to be 472 years for the dolerite derived soil, due to the higher Fe content and finer textured soil, which increase the V sorption capacity of the soil. The safe period for the slag B and C in respect with V is much longer than slag A, but other heavy metal concentrations must be kept in mind for they too can accumulate in the soil and can influence the growth of maize negatively if certain threshold values are reached.The V concentration of all the parameters generated in the V toxicity pot trail was far below the threshold values of the slag pot trail and field evaluation site. This indicated that the slag use with high V concentration on the short term (10 years) is unlikely to negatively influence crop production.