EXTENSION COURSE IN SOILS. 63 
derived by glaciation from limestone rocks are found to be more or 
less acid. Liming is the only practical way for correcting acidity. 
It is also beneficial to the soil in several other ways. Nearly all of 
the vast area of sandy lands of the South, and much land of the 
North and Northeast, as well, need liming. Besides other related 
topics, there will be taken up in this lesson the means for detecting 
soil acidity, together with the different kinds of lime, the quantity 
of each to be used on the soil, and the methods of application. 
Lime is not generally considered as a fertilizer, although calcium, 
the mineral element of lime, is present in certain soils to such a limited 
extent that some substance containing this element needs to be 
added as a plant food. 
Reasons for soil acidity. — While the question of acidity is still 
under investigation, the following reasons for this condition in soils 
seem to be fairly well established: (1) The abnormal breaking down 
of large quantities of vegetable matter in lowlands poorly drained 
causes acidity. (2) Because of the greater water solubility of the 
basic compounds of the soil than of the acid silicates, the bases are 
removed more rapidly from leaching than are the acid compounds. 
Since cultivated areas leach more readily than wooded and pasture 
lands, they thus develop acidity more rapidly. (3) In cropping, the 
basic elements of plant food are taken more rapidly from the soil 
than are the acid elements. When crops are removed from the land, 
therefore, instead of being fed and the manures returned, an acid 
condition eventually results from this cause. Moreover, the basic 
elements are carried from the subsoil to the surface to some extent 
by the roots of plants, and it is a common experience to find the sub- 
soil acid when the surface is still neutral or alkaline. (4) An acid 
residue is left in the soil from some fertilizers. Ammonium sulphate, 
for example, when applied to the soil gives up ammonia as a plant 
food and leaves sulphuric acid as a residue. It is thought, likewise, 
that when potassium sulphate is used in fertilizers, a part of the 
potassium becomes liberated, leaving an acid salt of potassium in 
the soil. 
Objections to acidity (Ref. No. 7, p. 141). — Nitrification of organic 
matter does not take place readily in acid soils, although in the case 
of acid marsh soils which are well drained, the nitrification may be 
sufficient to supply nitrates for the rapid growth of most crops. 
Acidity in the soil, moreover, has much more detrimental effects in 
its influence on nitrogen fixation by certain tubercle-forming organ- 
isms than on the process of nitrification. The bacteria which form 
tubercles on medium red clover, alfalfa, sweet clover, soy beans, and 
some other legumes do not develop at all rapidly in acid soils, and 
when the soils are quite acid it is necessary to use lime to correct 
