NITROGEN REQUIREMENTS OF LETTUCE UNDER SPRINKLER IRRIGATION AND TRICKLE FERTIGATION ON A SPEARWOOD SAND

The nitrogen (N) requirements of lettuce (Lactuca sativa L.) on a Spea rwood sand were studied over two years, comparing N applied daily (6 t o 12 times per day) by trickle fertigation (TD) with N broadcast weekl y and sprinkler irrigated (SW). N was applied as ammonium nitrate (NH4 NO3) at 0 to 550 kg/ha and water was applied at 100% or 150% of the pr evious day's pan evaporation (Epan A) to the TD and SW (two times per day) plots, respectively. Lettuce yield showed a significant response to N (P < 0.001) in both years. The TD plots were 21-42% (P < 0.01) mo re profitable than the SW plots. In Year 1, yield was significantly (P < 0.05) higher on the TD than the SW site. In Year 2, the interaction between irrigation/fertilisation treatment and N was significant (P < 0.05). Under TD, maximum yield (and profit) of 93 (Year 1) and 100 t/ ha (Year 2) required 288 and 344 kg N/ha, respectively, while under SW , maximum yield of 86 and 93 t/ha required 230 and 321 kg N/ha, respec tively. These rates of N are between 30 and 60% of those recommended f or maximum yield of lettuce on coastal sands particularly on new land and would reduce the impact on groundwater pollution considerably. Agr onomic nitrogen use efficiency (t yield divided by kg applied N both p er ha) of lettuce under TD was 25% and 13% higher than under SW at 450 and 550 kg N/ha respectively. This was attributed to (i) less leachin g of nitrate-N, (ii) a more constant concentration of nitrate-N over t ime, (iii) better placement of N, (iv) higher nitrate-N:ammonium-N, an d (v) a lower than toxic concentration of ammonium-N in the soil, unde r TD than under the SW treatment. The concentration of N in the wrappe r leaves (WL) required for maximum yield at heading was 4.7% under bot h TD and SW treatments during the first Year and 4.7% for SW and 5.2% for the TD treatment in the second Year. Critical levels for maximum y ield of % N in the WL tended to be higher under the more frequent appl ication of N in the TD systems.