_Naturally Thin Soils._ Nature may be prodigal in supplies of nearly all the elements of plant food to land and yet skimp its supply of lime, but naturally poor soils are quite surely in the acid class. The exceptions in our humid region are not extensive. When improvement is planned for, involving additions of organic matter and plant food, the application of lime to correct acidity is the first requirement. If such land could be given the characteristics of a limestone soil so far only as the lime factor is concerned, the building up of fertility would be relatively easy. Liming must form the foundation of a new order of things. The ability to grow the clovers and to furnish rich vegetable matter to the soil, which naturally is poor in humus, rests upon lime application first, and then upon any supply of plant food that may continue to be lacking. CHAPTER V TESTS FOR ACIDITY _The Litmus Paper Test._ A method of testing soils for acidity, which has been in use for many years, is the simple litmus paper method. Because of its simplicity and fair degree of accuracy, the litmus paper test is still used to a considerable extent in estimating the degree of acidity of certain soils. The best manner of using litmus is to place a strip of the blue paper in the bottom of a glass saucer, covering it with filter paper or other paper which is neutral--that is, paper which is neither acid nor alkaline. A small quantity of the soil to be tested is moistened with rain or distilled water and placed on this paper. If the acid is present the blue paper will be changed to a reddish color, varying in intensity according to the degree of acidity in the soil. Two objections to the use of litmus paper are to be noted: One of these is that the red color may be produced by carbonic acid gas without a trace of more powerful acids being present, and this may give a wrong impression to the operator. Another objection to the use of litmus is that the degree of acidity is not accurately indicated, and therefore the farmer is sometimes at a loss to know just how much lime should be applied to make soil conditions favorable for growing crops. _A More Accurate Method._ Within the last few years improved methods for determining the presence of acidity in soil have been developed. Some of these are suitable only for the chemist with his complete laboratory equipment, while others are more simple and can be used by anyone willing to exercise reasonable care. One of the simplest and most accurate tests to date is that devised by Professor E. Truog of the agricultural experiment station of the University of Wisconsin. This test not only detects positively the presence of soil acidity, but also gives definite information as to the degree of acidity. The test is based upon the principle that when zinc sulfid comes in contact with the acid, hydrogen sulfid gas is formed, and when this gas comes in contact with lead acetate, lead sulfid, a black chemical, is formed.