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Abstract
In a series of nitrogen-phosphorus-potassium (NPK) fertiliser trials on citrus (Citrus sinensis (L.) Osbeck) conducted on 2 soil types of Rarotonga, Cook Islands, the relationships between fertiliser applied and nutrient levels in soils and citrus leaves were studied. Topsoils of the 2 soil types were distinguishable by their chemical properties. The older, more weathered and leached Pouara soil had lower pH, exchangeable calcium (Exch Ca), percent base saturation (BS), and reserve K levels, but a higher level of phosphate-extract-able sulphate than the younger Tikioki soil. During 1976–79, soil P, K, and sulphur (S) levels reflected the balance between fertiliser inputs and nutrient uptake by citrus. High rates of NPK fertiliser (which also contained S) resulted in increases, and low rates in decreases in soil levels of these elements. The level of exchangeable magnesium (Exch Mg) decreased in all treatments, but the decrease was greater at higher rates of fertiliser, presumably because of greater tree uptake. Strong, positive correlations were observed between leaf and soil levels of Mg, and between levels of Ca in the leaves and levels of Exch Ca, BS, pH, and Bondorff P in the soil Leaf N levels increased significantly with increasing fertiliser rates, and tended to increase with time. Leaf P levels declined with increasing rate of fertiliser application in some orchards. Leaf K levels generally declined with time, the decrease being significantly greater at higher fertiliser rates. Leaf S levels also tended to decrease with time, but the decrease was significantly less at the high rate of fertiliser.