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

Abstract

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.

Fertilisation with P, N and S requires additional Zn for healthy plantation tree growth on low fertility savanna soils

“…The N, P and S experiments described above were terminated after the 3.81-year measurements. At that time, late in the dry season, termites were killing trees at GP, and trees had become unhealthy at all three sites with symptoms ranging from short needles to fasciation and stem tip die-back, similar to symptoms on other plantation species nearby (Rance et al 2022). The incidence of affected trees was highest at TW and lowest at HD, although occasionally a tree or two appeared to be healthier than those surrounding.…”

“…Sulfur (S), either with or separate from T, increased growth in the presence of P + N in glasshouse trials (Rance et al 1983(Rance et al , 2020. However, trees in the field frequently develop unhealthy new growth near the end of their second or third dry season, more commonly or sooner on fertilised trees than unfertilised trees (Rance et al 2022). The availability of cations also appears to play an important role in plant growth, mediated through soil pH which in turn can be altered by fertiliser addition (Rance et al 2020).…”