Phosphoric-acid Requirements.--Soil analyses show that the content of phosphoric acid in most soils of this country is relatively small. The results of experiments with the various constituents of fertilizers are in accord with this fact. Fertilizer experiments at the various stations and on farms are nearly a unit in showing that if any need in plant-food exists, phosphoric acid is deficient. When crop-producing power decreases, and the farmer begins to seek a commercial fertilizer to repair the loss, he finds that bone-dust or acid phosphate is serviceable. The resulting increase in yield often leads to such sole dependence upon this fertilizer that clover and manure are disregarded, the percentage of humus is allowed to drop, and finally the fertilizer is brought into disrepute. The need of phosphoric acid is so common that it is the sole plant-food in much fertilizer, and the dominant element in practically all the remainder on the market. [Illustration: Plat experiments.] The Need of Potash.--Land which is deficient in organic matter ordinarily is lacking in available potash, and responds with profit to applications, provided the nitrogen and phosphoric-acid requirements have been met. Clay soils contain far more potash than sandy soils, and in a farming scheme for them that permits the use of manure and clover, it may not become necessary to buy much potash. The liberal use of straw in the stables, and the saving of all the liquid manure, are helps. Farms from which the hay and straw have been sold for a long period of time develop an urgent need of potash. Much muck land is very deficient in this constituent. Fertilizer Tests.--Every farmer should conduct some fertilizer tests for himself. It is only the soil itself that can make an adequate reply to a question regarding its needs. The test should be made under conditions furnishing evenness in the soil, and it should be continued for years. There is pleasure to an intelligent farmer in such questioning of his soil, and only in this way can assurance be obtained that the investment in fertilizers is the wisest that can be planned for the farm. There are only three plant constituents to be tested, but they must be used in combination as well as singly. A soil that is deficient in the three may not give any return from potash alone, and usually does not, although it may give a marked increase from use of phosphoric acid alone. The plats may be eight rods long and one rod wide, containing each one twentieth of an acre, and having strips two feet wide separating them. The following chart suggests quantities of fertilizers to be used on the one-twentieth acre plats, 10 in number: