The differences in price between the materials should be given consideration when purchased—the superphosphates being much cheaper even when the nitrogen of bone is considered.

Ammonium phosphate is the most soluble form of phosphorus and, where both nitrogen and phosphorus are desired, makes the most satisfactory material of all the phosphates. Monocalcium phosphate . which contains  cent phosphoric acid and is being used in gravel culture solutions, is too expensive for average soil application. It is quite highly soluble in water.

Availability of Phosphorus. Fineness of grinding is important in such phosphorus fertilizers as bone to permit each individual particle being in contact on all sides with soil and later with root hairs Such fineness is not desirable in the soluble phosphates, even though they may change over to a somewhat insoluble form upon contact with soil moisture. This form becomes soluble through the action of various organic acids in the soil, so that at least a portion may be used by the plant.

Phosphorus is utilized by the bacteria and the fungi of the soil and may be tied up for a period but later is released for plant use. As in the case of nitrogen when straw mulch is applied, the loss is only temporary. The spread of roots through the soil causes phosphorus availability, as certain solvents are released by the roots in contact with particles of phosphorus. Hence it is very important that phosphorus in the form of bone or superphosphate be mixed thoroughly with the soil and particularly in the areas where roots abound. The solubility of ammonium phosphate will force its passage to a greater depth than other forms of phosphorus. The rate of phosphorus penetration through the soil is only aboutIA in. in depth per year; hence any surface applications are of little value. It should be mixed with soil whenever possible.

Granulated Phosphates. As pointed out previously, superphosphate should not be used in fine form, and this is also true of ammonium phosphate. Granular materials permit of less care in application. They have much less tendency to stick together. Likewise, the granules expose less surface to the surrounding soil, thereby reducing the amount of fixation (insolubility) of phosphoric acid by the soil.

Applications. Superphosphate  and bone meal may be used at the rate of preferably as mixtures in the soil. If applied to the top, they should be worked in. The beneficial effect from surface applications of bone meal is due to the nitrogen content which, however, is quickly dissipated. If an organic nitrogen fertilizer is desired, tankage is the preferable form Phosphorus deficiency is indicated by dwarfing of plants caused by a small root system. The color of the foliage is very dark purplish green at first with marginal yellowing developing later and followed by dropping of the leaves.

Potassium. Potassium tends to balance both nitrogen and phosphorus by encouraging longer root systems and delaying maturity. It is essential in starch formation and its translocation. It is needed in chlorophyll formation and is helpful in assimilation of carbon dioxide so that in the greenhouse in the winter when light intensity is low, additions of potassium tend to compensate for that lack. In general, potassium seems to add tone and vigor to plants and reduces susceptibility to disease. Dahlia and other root crops benefit by its presence, and the coloration of flowers is sometimes intensified through its application.

Many of our cultivated soils, particularly sands and peats, are lacking in sufficient quantities of potassium for the needs. Even in clay and silt soils continual use without the compensating additions of manures or leguminous cover crops will cause potash depletion, which will result in unsatisfactory growth.

Because potassium is held" by soil particles and thus is not readily leached, large qua ities would apparently be available to the plant. Yet act y a comparatively small percentage is available. As a con quence it has been found that to maintain a high level of this element, frequent applications are necessary.

It is interesting to note that a soil high in colloidal matter (fine particles of soil of gel-like nature to which the property of adsorption is attributed) may come to such a shortage of potassium necessary for its maintenance that when potash is added, little or no effect is produced because of competition between the soil colloidal matter and the plants. Thus, frequently a heavier application of potash may be required on clay soil than on sand, this in spite of the fact that clay soils are usually considered to have more potash than the sandy types. Considering all these matters, the use of potash cannot be overlooked. Likewise, it must be borne in mind that growers of by-gone days did not seem to need to apply potash—they were content with the use of manures and bone meal. And therein lay the story. (1) The soils themselves—more virgin than now—contained enough potash; (2) manure supplies potash in high amounts; and (3) the calcium in bone as well as its nitrogen had and have a capacity to liberate potassium. Thus, frequently, when we apply lime or nitrate of soda, we liberate potash; but a limit is eventually reached, and replacements must be made.

Magnesium is lacking in the soil, potassium magnesium sulphate may be substituted. Since the first two mentioned contain about other materials bearing potash may be used in proportion. As an example, hardwood ashes containing. Potassium deficiency is readily recognized by an initial mottling of the foliage, followed by marginal browning and dying of the lower leaves. This occurs because of the mobility of potassium and its translocation to the younger leaves when a deficiency occurs. Excess of potassium is evidenced by plants of dwarf nature with short internodes. Yellowing of the foliage begins at the bottom and progresses upward. The yellow leaves turn brown and finally shrivel. Extreme overdoses will cause a complete collapse of the plant in a short time.