Abstract:
The effects of irrigating banana plants in the field with secondary treated sewage effluent were examined, using plots watered with sprinklers and drippers at a site on the north coast of N.S.W. A total of 1030 mm of effluent was applied over 19 months during which rainfall totalled 3231 mm. Water budget calculations indicated that c. 57% of the irrigation applied would have been lost as profile overflow, and only c. 43% transpired. An 84 year water budget indicated that profile overflow could average 751 mm/year without irrigation, and 929 mm/year if irrigated according to a slightly drier schedule than used in this study. Compared to typical characteristics of secondary treated effluent, the material used in this study was towards the dilute extreme of the normal range, and failed to supply the plants' requirements for most nutrients. Soil sodium increased by up to 5 x on the effluent plots during the trial, and remained at double the control level at the end. Exchangeable sodium percentage (ESP) values reached 4% during the trial, and remained at 2.5% at the end. No other soil parameters were affected substantially. Size of mature banana plants and number of bunches produced were not measurably affected by irrigation with effluent. The only plant chemistry parameter affected was leaf lamina boron concentration, which increased by almost 100% at times during the trial, and remained significantly elevated at the end. A possible hazard from irrigating with the effluent used may be the displacement from the profile of other cations by sodium. A separate lysimeter trial is underway to check for such an effect. Heavy metal and trace element concentrations in the effluent were such that it could take between 270 and 5000 years to exceed recommended cumulative loading limits if effluent use was appropriate to crop water requirements.