Reynolds (1985) Responses of citrus (Citrussinensis) to nitrogen-phosphorus-potassium (NPK) fertiliser on 2 soils of Rarotonga, Cook Islands 2. Effects of NPK fertiliser rate, soil properties, and leaf nutrient levels on yield and tree size, AbstractThe application of increasing rates of nitrogen-phosphorus-potassium (NPK) fertiliser to citrus (Citrus sinensis (L.) Osbeck) resulted in significantly increased overall fruit yields from 4 trial orchards on 2 soil types of Rarotonga, Cook Islands. In 2 of the trial orchards, higher fertiliser rates consistently resulted in significant increases in fruit yields. For these 2 orchards, yields were positively correlated with leaf N levels, negatively correlated with leaf P and K levels, and uncorrelated with leaf sulphur (S) levels. These correlations, which occurred even though higher fertiliser rates resulted in comparatively higher soil levels of P, K, and S, suggest that the increased yields resulted from the increased fertiliser N input. Decreased leaf P and K levels at higher fertiliser rates appeared to be caused by increased translocation of these nutrients from the leaves to the fruit, because of higher fruit yields. On one of the 2 soil types, fruit yields were also positively correlated with leaf magnesium (Mg) levels, which were closely related to soil Mg levels. There was no significant effect of fertiliser rate on tree size during the experiment.

Reynolds (1985) Responses of citrus (Citrussinensis) to nitrogen-phosphorus-potassium (NPK) fertiliser on 2 soils of Rarotonga, Cook Islands 1. Effects of NPK fertiliser rate on soil properties and leaf nutrient levels, New Zealand Journal of Agricultural Research, 28:4, 475-486, DOI: 10.1080/00288233.1985 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.

In a recent Soil Survey of the Cook Islands the fertility of some major soil types was assessed from chemical analysis and glasshouse pot trials using the grass green panic (Panicum maximum var. trichog/ume). The glasshouse study has been extended to field trials with maize on three major agricultural soils in Rarotonga: Matavera clay loam, Pouara clay loam, and Tikioki clay loam. A 'subtractive' design was used to assess the response to fertiliser N, P, K, and S, and the maize was harvested at the tasselling stage and again at grain maturity. Relationships between maize yields from the subtractive treatments, nutrient levels in soils before planting, and nutrient levels in leaves at silking were examined. Nitrogen was shown to be the nutrient most limiting to the growth of maize; yield responses to N were obtained on all three soils. Responses to P were obtained on the Pouara and Tikioki soils, but not on the Matavera soil, reflecting its high level of available phosphorus. The response patterns observed in the field experiments were consistent with those observed in a glasshouse study using the same soil types. Added N significantly increased leaf levels of N, P, K, S, Mn, and Zn and levels of crude protein in grain. The trials also showed that grain yields of up to 11 000 kg/ha may be obtained in a growing period of 4'/2 months, which compares favourably with yields for this maize variety grown in New Zealand in a 6 month growing period.

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