End of Project ReportA 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