Fertilising Commercial Forest Species in Southern Africa: Research Progress and Problems (Part 1)

SYNOPSIS Since the inception of forest fertiliser research in Southern Africa in 1926, eight eucalypt, six pine and four acacia species have been tested, encompassing a wide range of sites in summer, winter and uniform rainfall areas. Marked deficiencies of P, Mn and sometimes N, usually associated with hydromorphic and elluviated soils derived from sandstone, are easily diagnosed and corrected at planting or after canopy closure. On the other hand, ferralitic soils showing adequate growth rates respond mainly to applications of P and Ca but balanced for N and K at planting. However, requirements after canopy closure are difficult to determine, varying with site, species and stand development. Fertilising at planting promotes the development of a vigorous root system, which allows for continually improved growth. Thus responses increase with the effective rooting depth and soil water availability of sites, as well as being larger for the faster growing species. Hence MAI has been increased up to 11 m3/ha for eucalypts, 9 m3/ha for pine and 5 m3/ha for wattle. Similarly responses to fertilising after canopy closure appear greatest on deep soils with an adequate water supply. An understanding of nutrient cycling is necessary for ameliorating site fertility, if effective and efficient use of fertilisers is to be made. Similarly, the development of diagnostic techniques such as ORIS and soil norms will enable site-specific fertilising to be carried out. Initial investigations indicate that intensive short-rotation forestry may lead to declining yields with successive rotations through factors such as depletion of soil water reserves, the loss of soil nutrients through harvesting, soil erosion, leaching and burning, and nutrient “lockup” in undecomposed litter. Although fertilising may correct this temporarily, research is also necessary into the correct management of organic matter, species rotation, and the use of cover crops and other practices which improve soil fertility.