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This term is used to express the amount of moisture in the atmosphere, as determined by a hygrometer, or by calculation from the temperatures of a wet and dry bulb thermometer. Obviously little can be done out-of-doors to deal with problems of humidity other than to choose plants to suit conditions in a particular area. But in the greenhouse humidity is of very great importance and modern commercial houses are fitted with automatic humidity controls, linked with temperature and ventilation.
Plants vary in their reactions to different degrees of humidity, some thriving in a dry atmosphere and others doing best in air almost at saturation point. Diseases and pests are equally varied in their response. Red spider mites and thrips, for example, thrive in a dry atmosphere and are checked by a damp one. Fungus diseases are encouraged by damp conditions. The main point to remember is that the total amount of water that can be held in the air varies according to its temperature. Thus 100 percent humidity at a low temperature means a lower water content than the same figure at a high temperature. This is one of the reasons why greenhouse plants, such as orchids, or a crop which requires a high level of humidity, for example the cucumber, must also have a high temperature.
For all greenhouse crops, it is useful to know their needs in this respect and the humidity level can then be regulated by increasing the temperature so as to allow the air to absorb more moisture, by opening or closing ventilators, by spraying the plants, walls and paths or by placing pans of water in the greenhouse to increase the water content of the air by evaporation.
A good many plants grown as house plants do better if they can be given a more humid atmosphere. Although this is not as easy to achieve in a living room as it is in a greenhouse, much can be done by placing the pots in which the plants are grown inside larger pots or other containers and filling the space between the two with moist peat, sphagnum moss or other moisture-retaining material.