Effect of differentiated nitrogen fertilisation on the yield and quality of leaf lettuce

Two field experiments with leaf lettuce were conducted in 2006-2008. The first factor tested was the diversified contents of mineral nitrogen in the soil, amounting to 50, 100 and 150 mg N dm^-3 before lettuce cultivation. The second factor tested four nitrogen fertilisers differing in chemical composition and in nitrogen form introduced into the soil: ammonium nitrate 34% N - (NH⁴NO³), calcium nitrate 15.5% N - (Ca(NO³)² × H²O + NH⁴NO³), ammonium sulfate 20.0% N - ((NH⁴)²SO⁴), and ENTEC 26% N - (NH⁴NO³ + (NH⁴)²SO⁴ + 0.8% DNPP). Two cultivars of leaf lettuce were used: green leaf ‘Casabella’ (Lollo Bionda type) and red leaf ‘Klausia’ (Lollo Rosa type). The best source of nitrogen for ‘Casabella’ cultivar lettuce was ENTEC 26 fertiliser and calcium nitrate, which resulted in significantly higher yields as compared to ammonium sulfate, yet did not differ from the yields comprising plants fertilised with ammonium nitrate. In the case of the ‘Klausia’ cultivar, ENTEC 26 fertiliser and ammonium nitrate generated the best production results. Regardless the type of nitrogen fertiliser used, the increase in concentration of this element in soil from 50 mg N dm^-3 to 150 mg N dm^-3 resulted in a considerable increase in marketable yield of both cultivars with simultaneously higher nitrate accumulation and decreased dry matter in the ‘Klausia’ cultivar. Lettuce fertilised with ammonium sulfate as well as with ENTEC 26 presented significantly lower average nitrate content in leaves before harvesting than after application of ammonium and calcium nitrate as a source of nitrogen for the plants. Introduction of ammonium nitrate and calcium nitrate in the dose of 150 mg N dm^-3 contributed to nitrate accumulation in the amounts exceeding permissible by legal regulations regarding the content of this element in lettuce, while application of ENTEC 26 allowed us to obtain a high yield size of both lettuce cultivars without risk of exceeding permissible nitrate content.