named after the place where they are obtained, as, Carolina phosphates, Florida phosphates and Tennesee phosphates. These rocks contain from 18 to 32 per cent of phosphoric acid, and differ from the bone products in that they are purely mineral substances and contain no organic matter. Ground into a fine powder they are sometimes sold under the name of floats, but the rock phosphates are used only to a limited extent in the crude condition.

Superphosphates or Manufactured Phosphates.The phosphoric acid in all of the natural phosphates

described is combined with lime in a form that is extremely insoluble in water. In order to make the phosphate soluble it is sometimes treated with sulphuric acid which unites with part of the lime leaving a phosphate which contains only one-third as much lime as the natural phosphate and which is soluble in water. The lime and sulphuric acid make a compound which is the same as that found in gypsum or land-plaster. This combination of soluble phosphate and gypsum, made by treating the natural phosphates with acid, is

called by the various names of superphosphate, soluble phosphate, acid phosphate, acidulated rock, etc. For its manufacture the rock phosphates are generally employed both because they are cheaper and because the organic matter in the bones interferes with the use of sufficient acid to make all the phosphate soluble. A good sample of superphosphate or acidulated rock contains about 16 per cent of phosphoric acid in a form that is soluble in water.

Sometimes when insufficient acid has been used a part of the soluble phosphate will change into a form intermediate in solubility between the natural phosphate and the acid phosphate, and the phosphate is said to have undergone reversion, and the new compound is called reverted phosphate. The latter product is supposed to be more available to the plant than the insoluble or natural phosphate, hence, the soluble and reverted phosphoric acid taken together are known as the available phosphoric acid.

In some instances bone meal is treated with a limited amount of sulphuric acid and the product is called acidulated bone. This substance contains a much smaller proportion of its phosphoric acid in the soluble form than does the rock superphosphate. When soluble phosphates are added to the soil they soon combine with the mineral matter, and are converted first into the reverted phosphate, and finally into the insoluble form such as is found naturally in the soil. In this way the phosphoric acid is fixed and there is no danger of its being lost by leaching.

Relative Value of Phosphate Fertilizers.-The soluble phosphate present in the acidulated goods is

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