generally considered the most valuable form of phosphoric acid for use as a fertilizer. At first sight it seems useless to go to the expense of making the phosphate soluble when it is again rendered insoluble by the soil before the plant can make use of it. The real object in making it soluble is to aid in its distribution in the soil. When an insoluble phosphate is applied it

remains where it falls except for the slight distribution it receives by cultivation. In the case of the soluble phosphate, on the other hand, the phosphate dissolves. in the soil water and is widely distributed before it becomes fixed by the soil. In the former case the roots must go to the phosphate while in the latter the phosphate is carried to the roots. It follows from what has been said that after the soluble phosphate is distributed throughout the soil the individual particles must be very much smaller than is the case with the

insoluble phosphate; the importance of fineness of division was clearly shown in the discussion of tillage.

There are some soils upon which the superphosphates cannot be used without injury, usually those that are deficient in lime, the superphosphate in such cases having a tendency to make them acid. Indeed, it is even asserted that soils containing an abundance of lime in

Relative value of phosphate fertilizers. All pots received the same amounts of plant food, but 7 received its phosphoric acid from acid phosphate, 5 from bone meal and 3 from ground phosphate rock or

floats "

the beginning may be made acid by the continued use. of superphosphate if no lime is added.

When the natural phosphates alone are considered there is no doubt that the preference should be given to those derived from bones. The organic matter present in the bones decays when it is incorporated with the soil, and this process doubtless causes the phosphate to become more readily available to the plant, while the rock phosphate on the contrary is very

slowly decomposed. The degree of fineness to which bone meal or mineral phosphate is ground is of prime importance. Very fine bone meal is much more available than that which is coarser and is always rated at a higher price a ton.

Using Floats With Manure.-The use of floats, or finely ground phosphate rock, has not met with general favor, and it probably does not give good results when used alone. Some of the earlier experiments indicate that it has practically no value as a source of phosphoric acid for the plant. Recent investigations at the Ohio and Illinois Experiment Stations show that when floats is added to farm manure it has a very high fertilizing value; in fact the increased crop production in Ohio due to adding the ground rock phosphate to the stall manure was nearly as large as that obtained from the addition of superphosphate. The acid substances produced during the decay of the manure apparently make the phosphoric acid in the rock more available, and it would seem from these experiments that the comparatively inexpensive floats might, partially at least, replace superphosphate if used in connection with the manure. Other experiments have demonstrated that good results can be obtained from the use of ground rock phosphate, when plowed under with a green manure crop like clover, but that it is of very little value if used on a soil low in organic matter. In a plot experiment at the Massachusetts Experiment Station two "equal money's worth" of ground Carolina rock and superphosphate were compared. In this case the superphosphate proved superior at first, but within a few years the plot to which rock phos

phate was added gave higher yields. It would seem, on the whole, that the use of floats with manure is worthy of a trial by anyone needing a phosphate fertilizer. Ohio Bulletin 134, recommends that the ground rock be used "as an absorbent in the stable, thus securing an intimate mixture with the manure in its fresh condition."

CHAPTER XIX

MIXED FERTILIZERS

Complete Fertilizers.-Mention was made of the fact that the basic materials described in the foregoing sections contain only one, or at most two, of the essential elements of fertility. By far the larger part of the commercial fertilizers used by the farmers in this country are purchased in the form known as complete fertilizers. A complete fertilizer, in the sense in which the word is used in trade, is one that contains nitrogen, phosphoric acid and potash, in proportions that are supposed to be suited to the requirements of farm practice. Practically all of these fertilizers are made by mixing two or more of the basic materials heretofore described, the different ingredients being so combined as to give the desired percentage of nitrogen, phosphoric acid and potash. In case the basic materials alone yield a product that is richer in the essential ingredients than is desired by the manufacturer, sufficient gypsum, dry earth, peat or other inert matter is added to bring the percentage of these ingredients down to the desired point. Materials added in this way are known as fillers.* These fertilizers are indiscrim

*There is a mistaken notion which is quite prevalent that anything contained in a fertilizer except nitrogen, phosphoric acid and potash is a filler. As a matter of fact it is impossible to make any rational combination of the basic materials which will contain more than one-third of its total weight of the three "essential ingredients," for even in the