Loss of Plant-food.--The soil is composed chiefly of material that never will enter into the structure of plants, but that serves us by affording a congenial place for plant-roots. It anchors the plants, holds moisture for them, and offers opportunity for all the processes necessary to the preparation of plant-food and to its use. In this material are the abundant supplies of such plant-food as silica, but, as has been previously stated, their very abundance leads us rightly to disregard them in our thinking. Our interest is only in the very small percentage of material that is composed of the four constituents which may be lacking in available form in the soil: nitrogen, phosphoric acid, potash, and lime. We believe that the only consideration that now need be given lime is as a soil-corrective and, when there is no acidity, we may assume that there is plenty of lime present. When yields of crops tend to decrease, the only plant-foods with which we are concerned are nitrogen, phosphoric acid, and potash.
The materials were stored in all agricultural land, and much of the supply is in inert forms. They help to make what we call the natural strength of the land. The rotting of organic matter, tillage, and many other agencies bring about some availability. The removal of crops, leaching, etc., reduce the supply. The right use of commercial fertilizers involves the addition of some plant-food when the available supply in a particular soil is inadequate.
Prejudice against Commercial Fertilizers.--The owner of land that was made very fertile by nature, and that has not been cropped long enough to reduce the supply of available fertility to the danger-point, rarely fails to entertain a prejudice against commercial fertilizers. It is the rule that he refuses to consider their use until the decrease in crop yields becomes so serious that necessity drives. If his land is not contributing its fair share of grain, vegetables, etc., to the markets, but has all its products converted into meat or milk, the supply of available plant-food may remain sufficient for so long a time that the matter cannot have any interest for him. If the land is producing some crops for market, there is reduction in its mineral store. It is the rule that the boundary of profitable use of commercial fertilizers pushes westward from the older and naturally poorer seaboard states about one generation after need shows in the crop yields. Lack of knowledge, the association of the use of commercial fertilizers with poor land, and some observation of the unwise use of fertilizers, combine to create a lively prejudice. They are viewed as stimulants only, and costly ones at that.