The ionic composition of rainwater at Katherine, NT. and its part in the cycling of plant nutrients

Abstract

Rainwater was collected from individual showers at Katherine, Northern Territory, Australia, during the 1958–59 wet season, and again in 3.4 mm portions of individual showers during the 1959–60 wet season. Strict precautions were taken during rainless periods to prevent particulate material from accumulating in the rain collector. The laboratory examination of the samples included determinations of nitrate, ammonium, sodium, potassium, magnesium, and chloride ions, and of insoluble silica. Soil and plant material samples collected in the surroundings of Katherine were also analysed for a number of elements. In spite of a total annual average rainfall of 925 mm the annual total amount of soluble material in the rainwater was less than 6–7 kg 'ha. The concentration of all ions decreased during the progress of a shower, and from shower to shower, during the progress of the season. Correlation coefficients between the different ions were all positive and generally high. No relation could be detected between the nitrate content in the rainwater and the incidence of lightning. The correlation between chloride content and amount of insoluble silica was high (+ 0.73). It is argued that most of the material in the rainwater is part of a terrestrial cycle and cannot be regarded as a true accession.