Dark Island heath (Ninety-mile plain, South Australia). VII. The effect of fertilizers on composition and growth, 1950-60

Abstract

Field experiments described in this paper analyse the effect of increase in soil fertility level on heath vegetation growing on the Makin sand. Fertilizers, P, N, Cu, and Zn, were added to raise the fertility of the plots to that of more fertile soils which, in coastal eastern and southern Australia, support a herbaceous understorey. Increased fertility did not affect germination of heath plants, but it influenced adversely the establishment of seedlings. Application of even small quantities of fertilizers (2 cwt superphosphate per acre, 2 cwt sodium nitrate per acre) greatly reduced the survival rate of all seedlings. Phosphorus fertilizer produced greatly increased growth in both surviving seedlings and mature heath plants; nitrogen together with phosphorus improved the growth of some species still further. There was great variation in the responses of individual species to fertilizers: some species showed maximum growth at 2 cwt superphosphate per acre, others at 4, others at 8 cwt per acre. The first group is dominant in the climax vegetation. Growth of the heath community was almost trebled by additions of 16 cwt superphosphate per acre. "Phosphorus toxicity" symptoms were observed in some mature heath species. Phosphate fertilizer hastened maturation and flowering in some heath seedlings which, after the addition of superphosphate, flowered some two years before the controls. This treatment also induced annual flowering a week or two earlier than on the control plots. Annual grasses, herbs, and a moss appeared and survived on all phosphorustreated plots, rarely in the unfertilized heath. Seeds of these annual species were windborne either from neighbouring farms five miles away or from adjacent plant communities. The results of these field experiments suggest that: (1) herbaceous species fail to survive on impoverished soils because of the low level of phosphorus which precludes satisfactory growth; (2) heath species are excluded from the more fertile soils firstly by some harmful effect on seedlings and secondly by competition from the more vigorous herbaceous plants in this environment. Gradually over the last 10 years heath plots fertilized with phosphorus fertilizers have shown changes towards a herbaceous sward.