Effect of irrigation management on nitrate movement under a lettuce crop

Abstract

Irrigation management practices have a significant impact on the leaching of nutrients and salts within a soil profile. Lettuce irrigation is often characterised by high frequency, small volume irrigations to maintain the shallow rootzone in a moist condition. The introduction of drip irrigation has provided the opportunity to apply soluble nitrogen fertiliser in the irrigation water to maintain high levels of soil nitrogen in the rootzone throughout the season. However, the combination of high soil moisture and nitrogen levels and well drained soils for extended periods of time raises concerns over the potential for nitrate leaching from the rootzone into local groundwater systems. This research involved a field trial to evaluate nitrate movement under a commercial fertigated lettuce crop. This trial was conducted on a commercial lettuce crop grown on the eastern Darling Downs. Irrigation and fertigation was scheduled and recorded by the grower, based on observation of weather, crop and soil conditions. Soil cores were obtained both pre- and post-season to measure soil moisture, bulk density, nitrate, ammonium and electrical conductivity (EC). Capacitance probes and ceramic soil suction cups were installed in each plot. Soil solution samples were extracted at two or three day intervals throughout the season and the nitrate concentration and EC measured. The results showed that deep drainage did occur during the season and that nitrate would have been moving out of the root zone. Substantial spatial and temporal variations in soil solution nitrate and EC were observed during the season. Solute movement appears to be related to the pattern of soil-water movement from the irrigation applications. This data suggests that the amount of deep drainage and nitrate leaching is influenced by the irrigation design and management practices. In-season rainfall and soil physical conditions may also play a role.The key to nitrogen management is minimising the amount of nitrogen and water in the soil, whilst ensuring adequate nitrate and water is available for plant growth. A large amount of water and fertiliser was applied after transplanting, during a period when the plant roots were shallow and plant water requirements were small. Substantial nitrogen was lost to leaching before the plants had reached 20% ground cover. Approximately one-fifth of the total nitrogen was applied during the last week before harvest. There is some debate over the requirement for nitrogen in the last week before harvest. If nitrate applied during the last week is unused then it would be highly susceptible to leaching during the following fallow period