Growth of Plants on Soils from Two Metalliferous Sites in Rhodesia

Abstract

The vascular plants of Rhodesia are fairly well known taxonomically but, although there are detailed geological maps of areas interesting to the mining industry, and the soils have been studied in most parts of actual or potential agricultural value, little has been written about edaphic influences on species distribution. The Flora Zambesiaca (Exell & Wild 1960-) mentions the apparent soil 'preferences' of certain species and Rattray (1960) reported that some grasses occur more frequently on certain soil types. There are relatively large areas of highly metalliferous soils. Wild (1965, 1968, 1970) and Jacobsen (1967, 1968) have drawn attention to the over-riding effects on their distinctive vegetation of parent rocks containing copper or nickel. However, in a detailed study of Becium homblei, a species previously regarded as characteristic of metalliferous soils, HowardWilliams (1970) showed that it survives competition with the similar B. obovatum not only on metalliferous but also on base-poor soils. The present work has been concerned to discover whether species and ecotypes from the metalliferous areas generally have greater tolerance of metalliferous soils than species and ecotypes from non-metalliferous sites. It is known (for review see Antonovics, Bradshaw & Turner 1971) that some endemic species and ecotypes from metalliferous soils in Europe have evolved specific tolerance of the metals. This is not necessarily the case where, as in Rhodesia, the soils adjacent to metalliferous sites are also infertile, but for other reasons including ready availability of aluminium, manganese and iron. Comparisons of tolerance within and between species were based on the interaction soil x population on yield of plants raised from seed and grown in soil from one or both of two metalliferous areas and in a control soil. Similar methods were used by Kruckeberg (1951), Bradshaw (1952), Walker (1954) and Proctor (1971a, b) but all these workers employed unfertilized soils. In order to eliminate as far as possible effects of differing major nutrient content, all three soils used were fertilized equally with a balanced nutrient solution. Also, the soils were diluted with two parts by weight of sand to reduce effects of differing texture; nutrient applications were made in numerous small portions to avoid exposing the plants to high salt concentrations and differential losses by fixation or denitrification. It was found (Wiltshire 1972a) that fertilization with ammonium and nitrate had different effects on accumulation and distribution of nickel in certain plants on one of the metalliferous soils. Ammonium fertilization acidified and thus increased availability of nickel from this soil. On the other hand, some species grew better with ammonium than with nitrate whether on metalliferous or non-metalliferous soil (Wiltshire 1972b, 1973). * Present address: Tobacco Research Institute, Private Bag, Rustenburg, Transvaal, South Africa.