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.
A series of physiological parameters was monitored under glasshouse conditions on micro-swards of four temperate herbage species. The micro-swards were grown on large soil cores so that the slow onset of water stress usually encountered in the field was simulated when water was withheld. Generally, water use per unit cover continued at a high rate even when water stress was causing considerable leaf death. Leaf diffusive conductance fell only gradually and did not reach minimum values until after much of herbage on the swards was dead. Stomatal closure in all species reduced water use per unit foliage cover by only 20-30%. Under increasing water stress, gross photosynthesis resembled stomatal conductances in remaining substantial (>50% of controls) even when relative water contents had fallen to below 80%. Both water use and gross photosynthesis were reduced more as a result of the reduced leaf area available for gaseous exchange than by the influence of stomatal action. Leaf moisture retention curves were similar for all species, there being a loss of c. 10% of relative water content per unit leaf water potential, down to drier than -5 MPa. It was concluded that the generally inferior herbage yields of white clover under dryland field conditions could be due in part to its relatively high leaf diffusive conductances when under water stress, while the better than average tall fescue yields under similar conditions may be attributed in part to its ability to roll its leaves tightly when water stress prevails.
Measurements were made over a 12-month period of the water use and leaf area index (LAI) of both dryland and irrigated monoculture swards of four temperate pasture species under two defoliation regimes. All four species used similar quantities of water on the dryland plots despite large differences in their ability to grow under such conditions. Even though very dry conditions prevailed during part of the study, the dryland swards generally failed to exploit reserves of soil moisture below a depth of c. 120 cm. The water use of the irrigated swards was sensitive to the manipulation of LAI by defoliation, while in contrast, dryland water use was not. On the irrigated swards, at an LAI of 1, a 1% decrease in LAI was associated with a 1% decrease in water use. This sensitivity of water use decreased as LAI increased until, at an LAI of 3 and above, water use appeared to be insensitive to charges in LAI. During the late spring to early autumn period both irrigated and dryland water use were significantly related to LAI. In this period, those irrigated and dryland swards which had common values of LAI generally used similar quantities of water. This finding indicated that stomatal control was ineffective in reducing water use per unit of leaf area. The quantity of dead herbage present in the swards suggests that pronounced leaf senescence (and hence reduction of leaf area) may have been a consequence of ineffective stomatal control of transpiration.
Bearing macadamia trees (cv. Keauhou 246), received varying rates of irrigation at weekly intervals over 8 years, and the effects on nut yield and tree size were measured. Annual rainfall ranged between 1232 and 2283 mm and was supplemented by 9-24 irrigations per year. Unsatisfied evaporative demand in the control treatment was estimated to vary from 1 to 380 mm per season. Canopy and trunk areas were not affected by irrigation. The average annual yield per unit of canopy area across all treatments was about 900 g/m2 of nut in shell. Irrigation reduced nut in shell yield per unit trunk area slightly, and depressed the individual mean weight of nut in shell by an average of 5%. There was a highly significant (P<0.01) inverse linear relationship between individual nut weight and irrigation amount, with an individual mean nut in shell reduction of 7% at the highest irrigation rate. Kernel weight, as a percentage of total nut in shell weight (kernel recovery), was not affected by irrigation, but the percentage of kernels that floated in tap water (grade 1 kernels) was 2.8% higher from control trees than from irrigated trees.
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