OBJECTIVES AND LIMITATIONS OF ENVIRONMENTAL CONTROL

Environmental control may be required for research purposes to find the effects of various factors upon plants, or for commercial horticulture to apply chosen levels of the factors in order to obtain maximum profit from the crop produced. Further objectives in commercial horticulture may be to keep diseases and pests at bay and to reduce labour requirements. Control engineers ask plant physiologists to specify the requirements of crops, but the answer to this question is complex, especially as the accuracy as well as the level of control must be specified if possible. The specification may depend not only on the species but on the stage of plant development, the spacing of the plants, and possibly on their recent environmental history.

Because of interactions between the crop and the environment, the relation between the measurement made by control engineers and the environment at the plant surface is sometimes very complex. Some environmental factors should probably be controlled in relation to the intensity of the solar radiation, but in a glasshouse this varies greatly in time and space, and it is not possible to make allowance for the fine structure of this variation.

Engineers use physical criteria to assess a control system but a horticulturist must decide whether the cost of improving control has been balanced by increased value of the crop. Present control systems cannot match skilled growers who use the environment as a tool in controlling the condition of the plants hour by hour throughout their lives.