Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The economic mainstay of tropical Sri Lanka, is agriculture. Plantation monocultures earn 43% of export trade. Under natural conditions, tropical rainfall leads to a paradoxical combination of lush vegetation and infertile soils; regeneration is maintained through rapid recycling of litter. In monocultures nutrient removal is not similarly compensated for, as the increasing number of deficient nutrient element diagnoses indicate.;Distinct differences exist between Sri Lanka's intensely cultivated Wet Zone soils and Dry Zone soils.;While Sri Lankan soils are reassuringly fertile compared to Amazon soils, average crustal levels of Na, Mg and Ca obtain in only {dollar}\sim{dollar}10% of Wet Zone soils sampled and in 33% for potassium (a major plant nutrient). Corresponding values for Dry Zone soils are Na(83%), Mg(14%), Ca(39%) and K(100%).;Mass balance calculations indicate that while K is slightly more resistant, less than 5% of parent rock Mg, Ca and Na remain in a Wet Zone soil. Ti vs. Zr plots (successfully used for the first time to discriminate soil horizons derived from gneissic banding) indicate extensive physical removal of soil particulates due to intense rainfall.;Groundwater percolation rates govern soil mineral formation. In the Wet Zone, oxyhydroxides and kaolin can be the first and last minerals to form since groundwaters remain dilute and aggressive. Primary minerals are nonexistent even within the silt fraction. Low percolation rates result in abundant primary minerals and smectitic clays in Dry Zone soils. Kaolin-smectite interstratification previously not identified in Sri Lanka may be common.;Wet Zone soils have a maximum 2 year supply of exchangeable potassium (compared to 3-4 years from Dry Zone soils) and if utilisable, total potassium reserves for 40 years (compared to 1000-2000 years for Dry Zone soils).;An adequate 50 kg/ha supply of exchangeable potassium, is equivalent to 34 tonnes of rock powder and would lead to logistical problems if substituted for fertiliser. However, when the new insight into weathering conditions of Wet Zone soils is considered it becomes clear that applications of rock powder and smectite are essential for long term soil maintenance, if these soils are to continue to support an agricultural economy.



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