Whilst the field data measured higher soil water nitrate-nitrogen concentrations than those calculated by the daily water and nitrogen balances, it did confirm the timing of leaching events and difference in average soil water nitrate concentration between the two trials. The late season trial had much lower nitrate-nitrogen concentrations than the early season trial, presumably due to leaching during the large rainfall events that occurred between the two trials.

From the daily water and nitrogen balances it was calculated that 1944 kL of water and 118 kg of nitrogen were leached per hectare during the early season trial. In the late season trial lower soil nitrogen concentrations and lower rainfall resulted in only 456 kL of water leaching 16 kg of nitrogen per hectare. The majority of the leaching occurred as a result of major rainfall events, however minor leaching was predicted to occur as a result of excess irrigation, especially at the beginning and end of the irrigation periods.

Whilst the field data measured higher soil water nitrate-nitrogen concentrations than those calculated by the daily balances, it did confirm the timing of leaching events and difference in average soil water nitrate concentration between the two trials. The late season trial had much lower nitrate-nitrogen concentrations than the early season trial, presumably due to leaching during the large rainfall events that occurred prior to the start of this trial.

Average electrical conductivity in the root zone was higher than the threshold levels for most of the early season trial, suggesting that there would have been some yield decline due to salt toxicity. During the late season trial root zone electrical conductivity was near the threshold, resulting in only a small (if any) yield loss.  To limit yield losses due to the salinity, pre-existing salts should be leaching from the root zone prior to nitrogen application. Keeping the application of fertiliser to a minimum will also limit the amount of salts that are added to the root zone during the season.

The key to nitrogen management is minimising the amount of nitrogen and water in the soil, whilst ensure that adequate nitrate and water is available to meet the plant needs. Currently a large amount of water and fertiliser is applied after transplanting, during a time when the plant roots are very shallow and the plant requires only a small amount of water and nitrate to meet its growth needs. In the early season trial, daily balances calculated that 31 kg of nitrogen were lost to leaching before the plants had reached 20 % ground cover.

The uptake of nitrogen by head lettuce during the week prior to harvest requires further investigation. Current research is conflicting, however Huett and Dettman (1992) found that plant uptake during the week prior to harvest was negligible. During these trials, approximately one-sixth of the total nitrogen was applied to each