362 



to be from 500 to 1,000 pounds per acre per year. There is one important 

 difference in the Rothamsted soil and the average Indiana soil, and that 

 is in the fact that the Rothamsted soils in the experiments reported have 

 from two to four per cent, calcium carbonate. The Indiana soils shown in 

 these analyses, on the other hand, have no calcium carbonate. The calcium 

 and magnesium in these Indiana soils are in the form of more or less 

 insoluble silicates. The inference to be drawn, therefore, would be that 

 there is no great loss of calcium or magnesium in acid soils in which these 

 elements are in the form of silicates. This does not mean that these soils 

 do not need lime for, as a matter of fact, they respond readily to the appli- 

 cation of lime, which is needed for the proper growth of clover. The need 

 for lime is greater now than it was in the virgin soils because the organic- 

 matter has been burned out of the cropped soil. Given two soils with the 

 same calcium and magnesium content and the same degree of acidity but 

 with different amounts of organic matter, the one with the greater organic- 

 matter content will grow better crops of clover and will not be in so great 

 a need of lime as the other. 



The virgin and cropped soils show no great difference in the content o2 

 sulphur. Experiments in Wisconsin and Kentucky have shown that in a 

 number of instances sulphur has been reduced in soils by cropping. 



Manganese shows quite a loss in the cropped soil. The effect of manga- 

 nese on soil fertility is attracting more or less attention among soil inves- 

 tigators, and although nothing definite seems to be known about its action, 

 it is possible that it does play an important part in agriculture. 



The changes in the content of silica, iron and aluminum are believed 

 to be of no importance as plant foods. They do, no doubt, have a very 

 important bearing upon the physical constitution of the soil. The writer 

 believes that the constitution of the silicates of iron, and especially of 

 aluminum, has more to do with injurious soil acidity than any other 

 factor. 



The method of determining soil acidity (limestone required) in this 

 work is that given in Bulletin 107 (Revised edition), Bureau of Chemistry, 

 U. S. Department of Agriculture. This method shows a relative acidity in 

 soils that is believed to more nearly represent toxic acidity than any other 

 method, especially in soils containing much organic matter. It is interest- 

 ing to note that while the acidity of the cropped soil has increased, the 

 acidity of the cropped subsoil has decreased. 



