Reducing the nitrate content of protected lettuce.

Abstract

A research project was carried out jointly between Teagasc, Kinsealy Research Centre and University College Dublin, Department of Crop Science, Horticulture and Forestry which studied the effects of cultivar, nitrogen fertilisation and light intensity on the nitrate content of protected butterhead lettuce. In a series of cultivar trials of winter and summer butterhead lettuce, significant differences in the nitrate content of the lettuce between cultivars were found only in one experiment. In this instance, the differences were not consistent between successive harvests. It was concluded that screening lettuce cultivars for tissue nitrate level is unlikely to contribute to an overall reduction of nitrate levels. The application of N in a liquid feed throughout the cropping period resulted in higher nitrate levels in lettuce plants grown in soil filled containers compared with a similar amount of N applied to the soil before planting. Withdrawing N for the final 10 days of the cropping period did not affect the nitrate content of the lettuce. In an experiment studying nitrogen source and rate on lettuce grown in containers, the use of calcium cyanamide as a N source resulted in lower nitrate levels in the lettuce and gave a reduced head weight compared with calcium ammonium nitrate (CAN) or ammonium sulphate. Increasing the rate of CAN or ammonium sulphate gave higher lettuce nitrate levels. A nitrification inhibitor reduced the soil nitrate levels especially with sulphate of ammonia as the N source but did not affect the plant nitrate levels significantly. The addition of chloride to the soil reduced nitrate levels in the lettuce. In a further fertilisation study using containers, calcium cyanamide again resulted in lower plant nitrate levels than CAN. Increasing the rate of CAN increased soil nitrate levels, lettuce head weight and plant nitrate levels. The relationship between soil nitrate levels, lettuce head weight and plant nitrate level indicates that the level of 100-150 mg·L-1 of nitrate N in the soil, advocated in the Code of Good Practice, is a compromise between maximising plant growth and minimising lettuce nitrate content. A comparison between CAN and calcium cyanamide in a border soil experiment again showed that the latter N source resulted in lower lettuce nitrate levels. In this experiment the addition of chloride to the soil did not affect plant nitrate levels. Lettuce was grown, in late summer, in small tunnels using a range of polyethylene cladding materials. Head weight correlated well with the overall light transmission of the materials. In one of the materials that had a low light transmission, lettuce nitrate content was doubled compared with those grown under the materials with high light transmission. Under both winter and summer conditions, the nitrate content of lettuce heads was not influenced by the time of day at which harvest took place. In experiments in which multiple harvests were carried out there was no consistent trend in nitrate content as the heads developed and matured. Within individual heads of lettuce there was a steep concentration gradient with the older outer leaves having much higher concentrations of nitrate than the younger inner leaves. Herbicides commonly used in protected lettuce production did not influence the nitrate content of the lettuce.