Aluminum, Ca, and Mn Concentrations in Macadamia Seedlings as Affected by Soil Acidity and Liming

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Abstract
Macadamia (Macadamia integrifolia) is increasingly becoming an important tree crop in many parts of the world. However, knowledge about the plant's nutritional behavior, especially under adverse soil conditions, has been deficient. To address this deficiency, a pot experiment was conducted to study the effects of Al, Mn and Ca (soil acidity and liming) on macadamia seedlings. Three soils having different mineralogy and fertility were used; soil pH was adjusted based on lime requirement curves so that several pH levels ranging from 4.5 to 7.5 were obtained for each soil. Chemical composition of the soil solution and of recently fully mature leaves was monitored periodically to assess the growth response. Results suggested that Al was detrimental to physiological processes of macadamia seedlings when leaf Al was greater than 275 mg kg and soil‐solution Al exceeded 1.2 mg L‐1. Furthermore, Al seemed to have reduced Mn uptake by the plant, although macadamia could accumulate as much as 1200 mg Mn kg‐1 in leaves without apparent toxic symptoms. The internal Ca requirement of the plant was not clearly defined; however, maximum growth could be expected when soil solutions contained 160 mg Ca L‐1 , which corresponded to 0.9 cmol(+)kg‐1 of exchangeable Ca (or 10% of CEC) in a highly weathered Oxisol.