248 SOIL ACIDITY AND LIMING 



show an average annual loss of about 433 pounds of carbonate of 

 lime per acre to a depth of four feet when the land is cropped, and 

 a loss of 918 pounds per acre when the land is left bare without any 

 vegetation. The equivalent of thirty-three pounds of carbonate 

 of lime was removed by the crops. Thus the equivalent of 452 

 pounds of carbonate of lime was conserved by cropping the 

 land. These experiments show that cropping conserves the lime 

 in the soil as compared with the same soil left bare, and that the 

 annual loss of carbonate of lime per acre to a depth of one foot is 

 probably not much more than about 108 pounds when the land 

 is cropped. 8 



Deep Plowing Cannot Be Substituted for Liming. It is some- 

 times reasoned that the lime which leaches out of the soil is caught 

 and held in the subsoil, and that if the land is plowed deeply, this 

 lime can again be brought to the surface. When soils are medium 

 to strongly acid, this is impossible, since to a depth of a foot or 

 more the subsoil is just as acid as, or more acid than, the first 

 six or eight inches of surface soil. 9 



Residual Soils from Limestone Become Acid. It is usually 

 difficult for farmers in certain sections in which are found soils 

 derived from and underlaid by limestone to understand how it is 

 possible for them to become acid. Frequently in such cases the 

 carbonate of lime has been so thoroughly leached out of them that 

 the subsoils to depths of fifty inches or more, or to within two or 

 three inches of the rotten, underlying limestone, possess extremely 

 high degrees of acidity (Fig. 159). 



The Surface Soil the Critical Zone. It is a common belief 

 that soil acidity has a direct injurious action on the roots of 

 plants. Alfalfa, if any, would be most affected, since it so often 

 fails because of acidity. On the contrary, no such injury has ever 

 been found in ordinary acid soils. 10 Again, it is often thought 



8 Illinois Station concludes that the equivalent of 540 to 760 pounds of 

 carbonate of lime is leached from the surface 21 inches of soil annually. 

 111. Bui. 212, 1919. 



9 Based on the Truog test for acidity. Author's conclusions based on the 

 tests of more than 300 samples of soil collected in 6-inch zones from more than 

 50 widely scattered fields. ' 



10 Hundreds of alfalfa roots extending into deep, strongly acid subsoils 

 have been examined by the author on widely scattered fields. Recent investi- 

 gations at the Wisconsin Station show that the acidity of the plant sap is 

 sometimes higher than that of most acid soils. The acidity within the root, 

 therefore, is often greater than that outside in the surrounding soil. 



