Some effects of micronutrients and liming on the yield, quality and micronutrient status of lettuce grown in beds of peat

Abstract

Summary The effects of a fritted micronutrient mixture on the growth, yield and quality of winter lettuce were studied in relation to the pH of the peat substrate. The plants were grown under glass and the atmosphere was enriched with CO₂ to 1000 vpm. The treatments consisted of three levels of the micronutrient mixture (0, 200 and 500 g m⁻³ F 253A) in factorial combination with four rates of liming (average pH values 5.cv 1, 5.6, 6.5 and 7.2). Responses to the omission of individual micronutrients (B, Cu, Fe, Mn, Mo or Zn) at pH 6.7 and the interaction between molybdenum level and pH (4.9 and 6.9) were also studied. The omission of the micronutrients caused a marked depression in the yield; few of the lettuce hearted and the leaves had low Cu contents (2 μg g⁻¹) irrespective of pH. At pH 5.0 the Mo content was very low (0.15 μg g⁻¹) and pH > 7.0 reduced the Mn content to 18–22 (μg g⁻¹. The yield and proportion of hearted lettuce were also reduced at pH 5.0–5.2 by the application of micronutrients at 200 and 500 gm⁻³ due to excessive accumulation of Mn (−1) in the leaves. High pH (7.1–7.4) always reduced the proportion of hearted lettuce. When individual micronutrients were omitted at pH 6.7, deficiencies of B, Mo and Cu reduced the yield by 23, 68 and 95% respectively. Omission of Mo reduced the yield by 99 and 41% at pH 4.9 and 6.9 respectively; at the lower pH the yield was reduced by 10% where Mo was supplied.