Nitrogen, phosphorus and potassium in Salvinia molesta Mitchell in the field: effects of weather, insect damage, fertilizers and age

Abstract

More than 1700 samples of Salvinia molesta Mitchell were collected from sites distributed from close to the equator in Papua New Guinea to 33°S in Australia. Nitrogen content ranged from 0.62 to 4.0% dry weight, phosphorus from 0.03 to 1.07% and potassium from 0.31 to 5.32%. Tops contained more concentrated N and P than roots, while K was more concentrated in roots. Concentrations of N and P were significantly correlated. Rainfall, followed by runoff, resulted in peaks of N, P and K in plants in one lake, but the sequence could not be demonstrated at other sites. At sites experiencing marked seasonal changes in temperature, N and P contents were inversely correlated with temperature, probably due to greater growth and utilisation of nutrients in summer. Damage caused by the biological control agent Cyrtobagous salviniae Calder and Sands decreased K content, while N and P contents were increased in some cases, but not in others. Thick mats of S. molesta in Papua New Guinea contained less concentrated N and P than thin mats, but this was not necessarily the case in the Australian sites. The reasons for these differences, and the likely effects on biological control agents, are discussed. Urea and superphosphate were found to increase N and P concentrations in the plant by amounts useful for experimental purposes and for aiding establishment of biological control agents. An equation related N concentration to the amount of urea applied and background N content of the plant. Concentrations of N, P and K decreased with age. Levels of N and P dropped most quickly as buds opened, then dropped slowly until the onset of senescence caused another rapid decline. The implications of these changes for insects feeding on S. molesta are discussed.