Water use efficiency, nutrient uptake and productivity of micro-irragated citrus

Abstract

The comparative patterns of water and nutrient uptake, soil salinity and water use efficiency resulting from 2 irrigation systems were studied in a 20-year-old Valencia orange orchard in Sunraysia on the Murray River. A conventional full ground cover system with the sprinklers in the middle of the rows operated at 14 day intervals (peak water demand), was compared with a partial (60-65%) ground cover system using under-tree micro-sprinklers operated at 7-day intervals. Irrigation applications were calculated to replace water depleted from the soil (including a leaching fraction) and were scheduled using tensiometers. During the 4 years of the experiment, approx. 10% less water was applied using micro-sprinklers. The average measured values for the crop coefficients were 0.62 (micro-irrigated) and 0.64 (full ground cover) during midsummer. Measurement of soil water extraction patterns by plant roots showed that only 5% of water use occurred below 1.0 m in under-tree microsprinkler irrigated trees, whereas 17% of water was taken up below this depth by trees irrigated with the mid-row system. Water uptake was limited by water availability rather than root density and declined sharply in mid-row irrigated trees during the second half of the 14-day irrigation cycle, when lower values of soil matric potential and shoot water potential were observed. Root distribution reflected these water uptake patterns with the highest density of fine roots in the 30-60 cm layer. Fertiliser injection with the micro-sprinkler system significantly increased the efficiency of N and P uptake compared with surface application, whereas leaf K levels were lower under micro-irrigation. Soil and plant levels of Na and C1 were low and unaffected by irrigation system during the experiment indicating adequate leaching of salt occurred with both systems. Fruit yield averaged 12% (5 t/ha) higher from micro-irrigated trees due to higher fruit numbers in light crop years. Tree growth was depressed in micro-irrigated trees suggesting a change in the partitioning of photosynthetic energy from vegetative to reproductive growth. The net effect of micro-irrigation was to increase water use efficiency by 22%, from 3.56 to 4.34 t fruit per ML applied water. No adverse effects on orchard productivity were observed following conversion from full to partial wetting of the soil surface. Under-tree micro-irrigation can therefore be recommended as a practical and efficient system for citrus irrigation in the irrigation areas of the Murray River.