It the greenhouse border soil or frame is occupied year after year by the same crops, soil sterilization becomes essential. The ideal sterilant is steam, and steam sterilizers may be available for hire from the larger equipment-hiring shops. Removing the soil from the border, sterilizing it, and then replacing it is, however, a long and laborious process. An alternative method is thoroughly to soak the soil with a 2 per cent solution of formalin. This chemical has the drawback that its fumes kill plants, and consequently the greenhouse has to be completely cleared and left empty for several weeks. Before returning the plants to the greenhouse after this treatment, the soil and atmosphere should be submitted to the 'cress test'. Sprinkle cress seed 011 to the soil and lightly rake it 111. Cress is highly sensitive to the formalin fumes, and only if it growls satisfactorily should plants be returned to the greenhouse.
If the greenhouse is small, ir may be worthwhile to change the soil completely, bringing in fresh garden soil. All these methods mentioned are time consuming and the increasing popularity ot growing in containers of one type or another reflects their relative ease ot use. At all events, the labour and time expended in sterilizing border soils stresses the virtues of rotation.
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