Tomato yields in relation to the nitrogen, potassium and magnesium status of the plants and of the peat substrate

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The effects of 96 combinations of nitrogen, potassium and magnesium were studied in four tomato crops grown in peat. Yield responses were found to each of these nutrients. In a late-planted crop, nitrogen also increased the early yield. The nutrient contents of the leaves corresponding to maximum yield were 4.5–5.1% N, 4.4–5.6% K and 0.31–0.40% Mg. Yields increased with applied nitrogen until the peat contained 45–65 mg N/l and decreased at concentrations above 70 mg N/l. Maximum yields were achieved in two of the crops when the peat contained 330–430 mt K/l; in the other two crops 100 mg K/l sufficed, though this was insufficient for high quality fruit. A magnesium content of 250 mg Mg/l was fully adequate. A high proportion (>90%) of the extractable potassium in the peat was water-soluble, thus making a major contribution to the overall salinity.