Responses of citrus (Citrus sinensis) 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

Abstract

The 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.

Keywords:

• citrus
• Citrus sinensis
• fertilisers
• nitrogen
• phosphorus
• potassium
• soil fertility
• Rarotonga
• Cook Islands