Nitrogen requirements of lettuce under sprinkler irrigation and trickle fertigation on a spearwood sand

Abstract

The nitrogen (N) requirements of lettuce (Lactuca sativa L.) on a Spearwood sand were studied over two years, comparing N applied daily (6 to 12 times per day) by trickle fertigation (TD) with N broadcast weekly and sprinkler irrigated (SW). N was applied as ammonium nitrate (NH₄NO₃) at 0 to 550 kg/ha and water was applied at 100% or 150% of the previous 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) more 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, respectively. These rates of N are between 30 and 60% of those recommended for maximum yield of lettuce on coastal sands particularly on new land and would reduce the impact on groundwater pollution considerably. Agronomic nitrogen use efficiency (t yield + kg applied N both per 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 leaching of nitrate‐N, (ii) a more constant concentration of nitrate‐N over time, (iii) better placement of N, (iv) higher nitrate‐N:ammonium‐N, and (v) a lower than toxic concentration of ammonium‐N in the soil, under TD than under the SW treatment. The concentration of N in the wrapper leaves (WL) required for maximum yield at heading was 4.7% under both 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 yield of % N in the WL tended to be higher under the more frequent application of N in the TD systems.