Effect of Nitrogen Fertilization on the Yield and Quality of Field-grown Leaf Lettuce for Spring Harvest

Two field experiments with leaf lettuce were carried out in 2006-2008. Each of them was set up in a random sub-block design in 3 replications and a two-factorial system. The first factor represented different nitrogen doses that were used to obtain the required N level in the soil prior to planting the seedlings, the levels being: 50, 100 and 150 mg N·dm⁻³. The second factor incorporated 4 nitrogenous fertilizers which were different in terms of chemical composition, and, more importantly, contained different nitrogen forms for introduction to the soil: ammonium nitrate 34% N-[NH₄NO₃], calcium nitrate 15.5% N-[Ca(NO₃)₂xH₂O+NH₄NO₃], ammonium sulphate 20.0% N-[(NH4)₂SO₄], and ENTEC 26% N-[NH₄NO₃+(NH₄)₂SO₄+0.8% DMPP]. Two varieties of leaf lettuce were used: Casabella (of the type Lollo Bionda) with green leaves, and Klausia (of the type Lollo Rosa) with red leaves. The best sources of nitrogen for the lettuce plants were the fertilizers ENTEC 26 and ammonium nitrate, which in contrast to calcium nitrate and ammonium sulphate made it possible to obtain significantly higher yields of lettuce. Irrespective of the kind of the nitrogenous fertilizer used, increasing the concentration of nitrogen in the soil from 50 mg N·dm⁻³ to 100 mg N·dm⁻³ resulted in a significant increase in the marketable yield of lettuce. At 150 mg N·dm⁻³ this yield remained at a level similar to that at 100 mg N·dm⁻³. The lettuce fertilized with ammonium sulphate was marked by the lowest mean nitrate content in the leaves before harvest. The use of gradually higher doses of nitrogen resulted in a reduced dry weight content and lower levels of total and reducing sugars in lettuce leaves. With respect to yield size and quality, the recommended concentration of nitrogen in the soil for leaf lettuce should not exceed 100 mg N·dm⁻³.