In the past we believed that fertilizers acted as plant foods and that the substances to be applied must contain one or more of four elements which are probably the most important to the plant, namely: Nitrogen, phosphorus, potassium and calcium. Several other elements go to make up the plants; but these are considered most difficult for the plant to secure from the soil and were therefore of the most importance for the farmer. Trials, by experimental work with different plants, soon showed that the forms of combination of these elements make a great difference. For example, nitrogen is now applied either in the form of nitrate of soda and of potassium, which is called saltpetre, or in the form of sulphate of ammonia; or, it may be applied as organic matter in dried blood, tankage, hoof- and horn-meal and even muck. Both the nitrate and sulphate forms are readily dissolved, but the nitrate form is the one in which plants take up their nitrogen, so that either nitrate of soda or saltpetre is specially valuable for forcing a quick growth. The effect may be seen upon plants in a very few days after it is first applied. But sulphate of ammonia, although it will dissolve, must be changed into a nitrate, in the soil, before the plants can use it, and this probably takes several days, hence, it is not quite so quick acting. The organic forms of nitrogen vary much in their usefulness. The nitrogen of dried blood or tankage is the most easily absorbed. These substances are sometimes sown at seeding-time, and become useful later in the season when the soil becomes warm. Other forms, however, as muck, leather parings, etc., are used considerably in making some low-grade fertilizers. Notwithstanding the tradition that you must bury an old shoe at the foot of the vine, the nitrogen in these may never be of very much value. The phosphorus is found in fertilizers both in forms that will dissolve and in forms that won’t. Acid phosphate and dissolved bone contain a large portion of phosphorus in the form of a soluble phosphate of lime; this is often marked on the bag as being equal to 12, 14, or 16 per cent. of phosphoric acid. Ground phosphatic rock, also called “floats,” and raw bone-meal contain phosphorus combined with lime in the form of an insoluble phosphate. Basic slag is still another form of phosphate of lime, which, although insoluble, readily becomes available as plant food in the soil.