The method of making this test is simple, and consists in placing a measured quantity of soil in a flask, to which is added a solution composed of 20% calcium chlorid and 2% zinc sulfid. The mixture of soil and chemical solution is heated to the boiling point by means of an alcohol lamp, and the boiling continued for a minute for the purpose of driving off the carbonic acid gas, which is liberated first. The boiling is continued and a piece of moistened paper, previously impregnated with lead acetate, is placed over the mouth of the flask. If the soil contains acid, a chemical reaction occurs between it and zinc sulfid, and hydrogen sulfid gas is liberated. The quantity of acidity in the soil determines the quantity of gas which comes in contact with the lead acetate paper, and this determines the depth of color produced on the paper. A slight brownish color indicates the presence of very little acidity, while an intense black signifies the presence of injurious amounts of acidity. There are various degrees of coloration between these two extremes, and each gives an accurate indication as to the quantity of lime required to correct the acidity. This test is simple and inexpensive, and at the present time most county agent offices are equipped with this apparatus or a similar one for testing soils for farmers. Some newer methods are being devised, and doubtless this method will be improved upon as time passes, but the Truog test has qualities of accuracy and simplicity which will always make it valuable. CHAPTER VI SOURCES OF LIME _Nature's Provision._ Soils are composed of pulverized stone and organic matter. Much of the original stone contained little lime, and the human race would become nearly helpless if there were no stores of supply in the form of limestone, chalk, marl, etc. The day would come when the surface soil could not produce our staple crops if its loss of lime continued, and a means of replenishing the stock were not at hand. The huge deposits of limestone that have not been disintegrated by processes of weathering are assurance that the soil's need can be met forever. The calcium and magnesium in the stone are in chemical combination with carbonic acid forming carbonates, and there is an additional mixture of other earthy material that was deposited by the water when the stone was being formed, but much limestone possesses an excellent degree of purity. [Illustration: Lime Favors Clover at the Ohio Experiment Station] [Illustration: Lime Affects Growth of Corn at the Ohio Experiment Station]