LIMESTONE 177 



organic acids, or possibly by the long-continued weak action of 

 drainage waters charged with carbonic acid, do exist in the soil, 

 and the evidence thus far secured indicates that they account for 

 most of the acidity of soils that are at the same time strongly acid 

 and very deficient in humus. 



Calcium bicarbonate may be formed by the action of carbonic 

 acid on silicates containing calcium, even though no limestone is 

 present. It is well known that plants have power to secure calcium, 

 as plant food, from acid soils containing calcium in silicates but 

 not containing limestone. 



Of course, it is not necessary to apply limestone to soils that 

 already contain abundance of calcium carbonate, but it should be 

 applied to soils that show acidity in the top soil and subsoil. 

 Not infrequently slight acidity exists in the surface, and sometimes 

 in the subsurface also, where the subsoil contains very large amounts 

 of limestone. From present information we cannot strongly advise 

 the application of limestone to such soils, although it would cer- 

 tainly do no harm, and for some crops might be beneficial. But 

 where the subsoil also is strongly acid, liberal applications of lime- 

 stone should be made. While a small amount of aridity in the sur- 

 face may not be a serious injury when the rainfall is abundant, 

 there is apparently in humid regions some rise of acidity from 

 strongly acid subsoils in times of partial drouth, corresponding 

 somewhat to the "rise of alkali " in arid regions, where the water 

 leaves the soil only by evaporation from the surface. If, however, 

 the subsoil contains abundance of limestone, some calcium bi- 

 carbonate will be brought upward into the subsurface or surface 

 soil with the capillary rise of the soil moisture, and this will be left 

 as normal carbonate when the water evaporates, and may serve to 

 reduce the acidity of the subsurface or surface soil, at critical 

 times, as in time of drouth. 



Clover and alfalfa are plants that are very sensitive to acid 

 conditions when dependent for most of their nitrogen upon the 

 bacteria, Pseudomonas radicicola, but these crops are grown very 

 successfully upon such soils as the brown silt loam of the Early 

 Wisconsin glaciation and the brown silt loam, yellow-gray silt 

 loam, and yellow silt loam, of the Late Wisconsin; whereas, they 

 are complete failures on the Lower Illinoisan gray silt loam prairie, 



