Damage to glasshouse crops by over‐manuring

Severe damage to lettuce involving tip‐burn of the leaves, wilting and death has occurred in several tomato glasshouses in west Scotland. Soils from such houses lose their capacity to cause damage if their soluble‐salt content is reduced by leaching or liming. The damage was produced experimentally when mixed artificial fertilizer was added to normal soil in amounts equivalent to four tons and more per acre. It is therefore concluded that the diseased condition results from over‐manuring, i.e. too high a content of soluble salts in the soil. Heavy manuring of the preceding tomato crop may contribute to this over‐manuring of the lettuce. Only one extreme case of actual damage to a tomato crop has been reported, but the yield of tomatoes begins to fall at higher levels of soluble mineral matter than for lettuce. It is suggested that the yield of tomatoes begins to fall at lower values of soluble‐salt content when added manures are grossly unbalanced than when they are attuned to the needs of the crop.