Abstract:
The effects of irrigating banana plants with secondary treated sewage effluent were examined using lysimeters at Woolgoolga, N.S.W. Lysimeters were sheltered from rain so that they received only effluent or tap water without substantial leaching occurring. Total application was c. 4700 mm over c. 30 months, equivalent to about 8 years field requirements for supplementary irrigation. At the end of the growth period, plants and soils were analysed for a wide range of chemical parameters. Soil profiles were then leached and the collected leachate was also analysed. Irrigation with effluent had no effect on plant size. Plants watered with effluent contained 225% more sodium, 81% more boron, 43% more copper, 26% more chloride, and about 16% more nitrogen, phosphorus, potassium and magnesium than plants watered with tap water. Soils watered with effluent accumulated more phosphorus, sulfate, chloride, calcium and sodium than soils watered with tap water. Phosphorus sorption results indicated that it would take over 200 years of supplementary watering with effluent to saturate the profile with P. Leachate collected after the growth period from lysimeters watered with effluent contained negligible phosphorus, but considerable sodium, chloride, sulfate, and magnesium. There was no evidence of sodium in effluent displacing calcium from the profile. The high concentration of magnesium in leachate from both treatments indicated that magnesium was displaced by the high levels of potassium fertilizer applied to ensure adequacy for growth of banana plants. Boron concentrations in the leachate indicated that boron would not accumulate in the profile under natural rainfall conditions. Other trace elements were also most unlikely to accumulate to problem levels. Pesticide residue testing of banana pulp, soils and leachate indicated minimal likelihood of problems associated with such residues in local effluent. It was concluded that supplementary irrigation of bananas with the effluent tested was unlikely to cause any problems with either soil or plant chemistry, given the considerable surplus of rainfall which occurs locally in most autumns.