Investigation into the optimal fertilizer regime of Swietenia macrophylla (Mahogany)

Abstract

With the increase of the population in Sri Lanka, and increase of the demand for timber, the requirement to produce wood in country in large scale as intensively managed plantations has become very important and urgent. As forest tree planting has to compete with agriculture for land, the land relegated to forestry is usually marginal and therefore fertilization and nutrient management are important requirements. The objective of the present study is to provide prescriptions for fertilization and on site nutrient management to obtain best growth and development in Mahogany plantations in wet zone of the country owned by the Touchwood Investments Ltd.

The Study comprised of three parts; a pot trial and two field trials, was conducted in mahogany plantations located in three districts of Sri Lanka namely Kaluthara, Mathale and Rathnapura. With regard to pot trial, 16 combinations of N and P fertilizer levels with a constant K level were used on 6 months old seedlings planted on soil taken from Panthiya, Gomaragala and Ambanganga Estates. The second trial dealt with the biomass and nutrient distribution in component parts of the tree in different ages and assessing the Nutrient Use Efficiency. The third trial tried to establish a relationship between the site index and foliar nutrient content. The latter two trials were set up to provide an insight into nutrient management on site.

A statistically significant (p<0.05) relationship was observed in the fertilizer mixtures and height growth (square root) in the pot trial using Panthiya and Ambanganga soils. The responses to fertilizer mixtures were not significantly different in diameter growth and number of leaves generated during the trial period of 6 months. Further, in general treatments having more phosphorus (double or triple) than the treatment already in use (N15.45, P5.15, K15.45 kg/ha/six month) give better results in growth improvement. This was more intense in soils having comparative low initial levels of Phosphorus. Treatments having higher nitrogen levels than currently in use (more than N15.45) tended to retard growth. Plantations which are poor in soil nutrients gave a higher response to the fertilization.

Nutrient content within tree components decreased in the order of stem > branches > leaves > roots. A higher Nutrient Use Efficiency was observed in stem while foliage had the lowest. Nutrient distribution was almost independent from the plant age within the age series. A relationship was observed between site index and leaf nitrogen and calcium levels of similar aged plantations and therefore foliar levels of N and Ca can be used in comparing the productivity of sites and also to determine optimum level of nutrients. As the growth response to fertilizer applications is a long term process it is recommended that long term field experiments should be established and maintained.