UQ eSpace

Abstract

A series of experiments were carried out in the glasshouse at the University of Queensland to investigate soil factors limiting seedling growth of leucaena (Leucaena leucocephala (Lam.) de Wit) cv. Cunningham. Preliminary experiments, using five soil types with all known deficiencies corrected, indicated that growth of Cunningham leucaena was limited in soils with low pH values. Liming of the Wallum sand improved both growth and nodulation of the plant. Hence, later experiments were designed to clarify the effects of soil acidity on growth and nodulation of Cunningham leucaena. As an aid to interpretation of factors limiting leucaena growth in acid soils, a series of experiments was conducted in Wallum sand to determine the critical concentrations in youngest fully expanded leaves for deficiency of nitrogen, phosphorus, potassium, calcium and sulphur and ' for manganese toxicity. These experiments involved both nitrogen-supplied plants, and in the case of phosphorus, calcium and manganese, inoculated plants dependent on biological nitrogen fixation. Critical nutrient concentrations in the youngest fully expanded leaves of Cunningham leucaena were: nitrogen - 4.1%, phosphorus - 0.21% for nitrogen supplied and 0.25% for inoculated plants, potassium - 2.0%, calcium - 0.38% for nitrogen-supplied and 0.49% for inoculated plants, and sulphur - 0.24%. The critical concentration for manganese toxicity was 325 mg kg-1 for both nitrogen-supplied and inoculated plants. Experiments revealed that a soil pH at ? 4.2 (1:5, soil:0.01M CaCl2) , with monomeric aluminium and manganese concentrations of 100-250 and 100-200 µM respectively, was marginal for growth of nitrogen-supplied plants. Further study in solution culture suggested that aluminium toxicity was likely to be the primary limiting factor at soil pH ? 4.2. Growth of leucaena plants was severely limited at monomeric aluminium concentrations in solution ? 18 µM and at solution pH ? 4.0. A manganese toxicity limitation to plant growth was only evident when high rates of manganese (? 192 kg Mn ha-1) were applied to Wallum sand.