Some Responses of Lettuce, Grown in Beds of Peat, to Nitrogen, Potassium, Magnesium and Molybdenum

Abstract

The effects of 96 combinations of N, K and Mg were studied in three crops of lettuce grown in peat; there were two sources of N, Nitrochalk (26% N) and sulphur-coated urea (32% N), and molybdenum was included as an extra factor in one crop. The yield and proportion of hearted lettuce increased with applied N until the peat contained 60 mg N I^-1. In two crops higher N concentrations were unfavourable, due to increased acidity in the peat which resulted in greater uptake of Mn by the plants. The yield and the proportions of both hearted and of marketable lettuce declined as the Mn content of the leaves increased. In the third crop, which was not affected by Mn toxicity, there was little response to N over the range 60–250 mg N I^-1. The yield and quality of the produce improved with increasing K until the peat contained 200 mg K I^-1, but concentrations of up to 600 mg K I^-1 had little further effect. The N and K contents of the leaves corresponding to maximum yield were 5% N and 8% K. There was no response to added Mg. The yield and proportion of hearted lettuce were increased by the addition of Mo in plots receiving the heaviest dressing of Nitrochalk. The increase in the proportion of hearted lettuce was greatest in plots also receiving sulphur-coated urea. The water-soluble phosphate content of the peat increased from 19 mg P I^-1 to >50 mg P I^-1 as the pH of the peat declined from 6–7 to 5.5 in response to increasing N application rates.