Plantation tree growth responses to P, N, K and minor and trace elements on low fertility savanna soils

Context Widespread soil nutrient limitations in savanna soils typically constrain plantation tree growth, and hence limit economic opportunities in tropical regions. Fertilisation offers an approach to overcome soil nutrient limitations, but requires research on nutrient contents and rates to maximise plant growth while avoiding nutrient imbalances that have stunted plant growth under some fertiliser regimes. Aims To test the hypothesis that multiple nutrient limitations exist in savanna soils, with nutrient deficiencies exposed in sequence with fertiliser addition. Methods Factorial field experiments tested the growth of the plantation timber species African mahogany (Khaya senegalensis) to applications of phosphorus, potassium, nitrogen and minor and trace elements (referred to as the T treatment) on a kandosol soil near Darwin, Australia. Key results Under high stocking rates to induce deficiencies sooner through utilisation of a high proportion of the available nutrient capital, positive responses and interactions to all four main treatments were recorded. There were step-wise responses to phosphorus, potassium, nitrogen and the T treatment. Treatments with greater mean tree growth were more uniform than lesser-growing treatments, even though the largest and smallest individuals were similar across treatments. Conclusions Consistent with our hypothesis, correcting one soil nutrient deficiency exposed another in sequence as nutrient reserves were depleted in a drying soil. Variation in tree performance across plots indicates that testing of soil nutrients and fertiliser responses need to be assessed in replicate and dispersed samples. Implications Khaya senegalensis demonstrated potential for plantation use in northern Australia with minimal mortality from termites and other causes, if supported with broad-spectrum fertilisation balanced to match plant growth and water availability.