Page:Popular Science Monthly Volume 25.djvu/405

Rh variety of ways; it may be through excessive humidity, as determined by the stagnation of water through imperfect drainage, or the natural condition and position of the water-table, or it may be through the mechanical condition. While these conditions may not actually cause disease, they will certainly promote it when once developed, and we therefore find a certain part of remedial measures to consist in thorough drainage and cultivation. But more than this, we find in special or general exhaustion of the soil a fruitful source of disease. Lands which have been cropped for a long period become at least specially exhausted, and in such case usually in the direction of that food-element most essential to the growth of the plant which has brought about the exhaustion. There is thus developed a debilitated condition of the entire system, by means of which the normal functions are impaired, and this in itself constitutes a disease. But the debilitated state permits the operation of other forms of plant-life which would otherwise be unable to develop readily, and also allows certain abnormal physiological and chemical changes to occur, all of which promote secondary features and thus bring about complication. This, it seems tolerably certain, is the case in peach-yellows, and may also prove to be the case in other diseases such as pear-blight. Diseases developed in this way, however, are most difficult to treat, because the entire system is involved. Remedial measures must therefore be directed toward—1. Removing the cause; 2. Building up the general system; 3. Restoring to a normal condition the disordered organic function. In the case of peach-yellows, the results of chemical analysis, as well as the changes produced by special treatment, show that in all probability the specific is chlorine as contained in muriate of potash, while a general toning of the system may be accomplished by the judicious application of a complete food as determined from the ash composition.

Atmospheric conditions are largely, if not wholly, beyond the control of man. They include, of course, the varying conditions of heat and moisture, and are thus either highly stimulating and favor the excessive growth of weak structure and parasites, on the one hand, or they are depressing and cause a stagnation of vital activity, and thus injure the plants, as through excessive drought; while this, in turn, leads to the development of parasites, which would not otherwise gain a firm hold. We can not expect to modify the conditions which produce these results; we can only hope to so prepare the plant, by judicious treatment, that it will suffer no material injury from the peculiar meteorological conditions in which it is placed. With this in view, we would doubtless find it wise to apply strong food, which will retard the vegetative process, and tend to the more solid maturity of the parts already formed. Nor must we neglect the importance of a judicious course of irrigation during drought. Doubtless the time will come when every man who depends upon the growth of plants for his living