HAZARDS OF VARIETY TRIALLING

Research workers who investigate biological material soon become aware of the various hazards which hinder their work and make the obtaining of reliable results particularly difficult. Many of the difficulties are related directly to the biology of the species under investigation, but others are products of the environment in which the material is grown, are due to management problems, or are inherent in the trials themselves. It is important to recognise these hazards, and the classifications outlined below may guide some investigators to sources of serious errors. It is based on experiences when trialling tomato, cucumber and lettuce varieties grown in glasshouses (Darby 1966). Recognition of a hazard to experimentation should obviously be followed by its elimination, but this is not always as readily achieved as might be imagined.

The term 'protected cropping' may conjure up thoughts of very uniform conditions for plant growth, but this is often far from true. The very act of protecting by growing plants under glass inevitably creates gradients in light intensity and temperature, and the more complex are the environmental controls the greater is the chance that something will go wrong.

Hazards relating directly to the protection of the crop may be grouped as follows:

• Site - natural or created
• Equipment - design or failure
• Crop management

As will be seen, further subdivision is possible within these groups.

Site - natural. All of the usual hazards due to variation in soil type and fertility occur in trials under glass, though they tend to be mitigated by some cultural operations. Steam sterilisation necessitates movement of the soil and inevitable mixing: flooding is intended to reduce soluble salt concentration to a uniform low level: heavy fertiliser application helps to smooth out small differences in nutrient status. A sloping site or excessive exposure to a prevailing wind are much more troublesome, especially in heated glasshouses. Temperature gradients may give considerable differences in plant growth which not only to statistical techniques but also lead to crop management problems, especially with species of indeterminate growth such as the tomato.

Site - created. All builders seem to take a malevolent delight in ruining the land on which they are working; and intense vigilance is necessary to ensure that the sub-soil from the footings of a new glasshouse is not deposited across the future experimental area. Only by living and sleeping on the site is it possible to prevent the digging of trenches for waterpipes intended to supply taps unwisely located in the middle of the glasshouse. Failure to prevent such action results in the local disturbance of deep soil which may have been unmoved for thousands of years. Subsequently there will be excessive compression of the soil around the taps, with local wet patches when they are allowed to drip or hoses are left to drain.