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The effect of differentiated nitrogen-potassium fertilization on the substrate reaction and the yields of some vegetables grown under covers has been examined. A pot experiment was carried out in the glasshouse where high moor peat of pH 3.6 was used as the substrate. In the experiment with lettuce the substrate reaction was maintained on the level of pH 6.3 and the following N fertilization was applied, in mg N-NO₃/dm³: 150 and 300 as (NH₄)₂SO₄, NaNOr NH₄NO₃. In the experiment with red pepper the pH was 6.5 and the fertilization was as follows: nitrogen in mg N-NO₃/dm³ - 150, 200, and 250 as (NH₄)₂ SO₄, Ca(NO₃)₂ and potassium in KCl and K₂SO₄. The results obtained clearly indicate the acidifying effect of (NH₄)₂SO₄. It caused the decrease in substrate reaction on the average from pH 6.4 to 5.1. The NaNO₃ has alkalizing effect while Ca(NO₃)₂ and NH₄NO₃ very slightly affected the substrate reaction. In the experiment with red pepper unfavourable effect of the increasing nitrogen doses (200 and 250 mg N-NO₃/dm³) on the substrate reaction as well as on the pepper fruits yields and quality was found. Moreover, a high contribution of fruits attacked by dry rot was noted. The applied KCl and K₂SO₄ fertilization slightly decreased the substrate reaction.
A study on the influence of doses nitrogen on salt concentration, yield, biological value, and chemical composition of butterhead lettuce, kale, and leafy celery was carried out under greenhouse conditions in 2006-2008. The experiment was set up as a completely randomized design. A single experimental unit consisted of one pot with a plant. Increasing rates of nitrogen in the form of ammonium nitrate (34% N): 0.3; 0.6; 0.9; 1.2 g N·dm⁻³ of growing medium, were applied in all investigations, while the other macronutrients were the same for all plants. The obtained results revealed that the salt concentration in the growing medium depended both on the rate of nitrogen and the plant grown. Significantly lower crop yield was recorded when the highest nitrogen rate was applied (1.2 g N·dm⁻³), although this relationship was shown when there was high variation in EC in particular types of medium. It was found that excessive salt concentration in the growing medium due to increasing nitrogen rates significantly reduced vitamin C content in butterhead lettuce and leafy celery. An opposite relationship was observed for kale leaves. It was found that the protein levels in the tested plants increased with an increase in nitrogen rates. As compared to the lowest rate, the increase was 32.2% for butterhead lettuce, 30.6% for kale, and 41.2% for leafy celery.
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