Dark Island Heath (Ninety-Mile Plain, South Australia). VIII. The Effect of Fertilizers on Composition and Growth, 1950-1972

Abstract

In the early 1950s fertilizers, i.e. phosphorus, nitrogen, copper and zinc, were added in various factorial combinations to raise the fertility of an area of native heath vegetation to that of the more fertile soils found in southern Australia. The first 10 years of observations on the effects of increased soil fertility on the composition and growth of heath vegetation were reported in 1963. The field experiment was re-examined in 1972. No significant response to the application of nitrogen, copper, or zinc has been observed in this experiment. A response to nitrogen plus phosphorus was reported previously; no response to any other essential nutrient has been demonstrated in the field. Phosphorus fertilizer showed five effects which, collectively, have gradually changed the heath vegetation towards an 'herbaceous sward' over the last 22 years. (1) Much of the phosphorus fertilizer applied in 1950-1953 was retained in the ecosystem for at least two decades. (2) The fertilizer increased the growth of well-established 'adult' species of heath, and tended to speed up their life cycle and cause them to die many years earlier than usual. (3) Herbaceous species with a spring peak of growth which gave them a competitive advantage (for water and nutrients) over heath seedlings which produced new shoots towards the end of spring and over summer invaded the fertilized heath. (4) 'Phosphorus toxicity' and water stress in competition with invaders induced the death of almost 100% of the seedling heath species. (5) Bushfires, an integral part of this ecosystem, released phosphorus stored in above-ground vegetation to the surface soil; seedling regeneration of the heath was further inhibited by this accession of nutrient. The conservation of heath vegetation subjected to phosphate contamination from adjacent farms and forest plantations, and from litter (even fruit peelings) deposited along tracks, poses a major problem in the management of conservation areas.