DISEASES CAUSED BY THE NON-LIVING ENVIRONMENT.
From what has been said in previous para graphs, it is evident that unfavorable physical and chemical conditions of the soil, excessive dryness or moisture of the soil or atmosphere, unfavorable conditions of temperature, light, etc., may cause derangements resulting in temporary or chronic disease. Plants which have become adapted by natural or artificial selection to a certain range and sequence of soil, moisture, temperature and light conditions usually suffer some sort of derangement if exposed to a set of conditions materially different in any par ticular factor, group of factors or in the rela tion of these to each other. It is absolutely necessary, therefore, in studying the aetiology of diseases to know the natural or constitutional requirements of the plants and their powers of adaptation. This knowledge can be obtained only by long and intimate acquaintance with the kind of plants in question under varying con ditions.
Soil.— In natural conditions it is a matter of general observation that certain varieties of plants thrive best on particular kinds of soil, the same as in particular conditions of light, temperature and moisture. The storage and movement of water in soils and its availability to plants, the solution of soil foods, nitrifica tion, temperature and aeration depends largely upon the structure or physical composition of the soils. Water, food, air and temperature are among the fundamental requirements of vegetation. It is the varying amounts and rela tions of these factors maintained by a given soil that largely determine whether or not it is naturally suited in a given region to a par ticular class of plants. Where all these con ditions are under easy control the kind of soil is a matter of less importance. The work of furnishing and maintaining propef conditions is done by the gardener instead of by the soil in the latter case. It is not profitable, however, to employ soils that require close and constant attention, even where conditions are largely under control. It is the aim of economical production to select a soil for a given crop that, with a minimum of labor expended on the part of the cultivator, produces the desired results.
It is evident from these considerations that the conditions of the soil must be considered in connection with all of the other factors of environment, as no one of them ever acts inde pendently and alone in the production of dis ease. The plant, too, has to be considered. It may be adapted to withstand the variations that take place in a sandy soil in a locality charac terized by particular meteorological conditions, while it could not thrive in a clay soil in the same locality. On the other hand, with changed meteorological conditions the same plant might do best in the clay soil.
Root Suffocation.— This is one of the most usual diseased conditions caused by physical conditions of the soil preventing the ready access of atmospheric oxygen or the ventilation of the soil. A very fine sand closely packed, or a clay soil with a crust excludes the air, unless the sand is stirred or the crust broken. Carbon dioxide accumulates and the oxygen is reduced. The root hairs and feeding roots die for lack of sufficient oxygen for respiration. Growth is checked by starvation due to reduced root ac tion. The tissues become hard and woody. The plant may then die in dry, hot weather by desiccation, or it may simply starve to death. Plants injured in this way, even slightly, often become the prey of insects and fungi, especially of those that attack the roots. This form of suffocation is also often found where the roots of plants have to grow under close pavements or hard packed ground or heavy sod. Another form of root suffocation results from poor drainage or excess of moisture in a soil, thus excluding the oxygen and preventing soil venti lation. The feeding roots die and ferment, not only cutting off the proper absorption of soil food elements but resulting in poisoning the plant by products of fermentation. The leaves rapidly turn yellow and drop, and unless the soil is dried out and aerated the plant soon dies. This latter form of root suffocation is very common among plants grown in pots or tubs. When the natural drainage is stopped up or the plant is over-watered, the soil becomes sour.