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- 
‘ation 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 of 
sulphur. Experiments in Wisconsin and Kentucky have shown that in a 
Dumber of instances sulphur has been reduced in soils by cropping. 
Manganese shows quite a loss in the cropped soil. The effect of manga- 
ese on soil fertility is attracting more or less attention among soil inyes- 
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. 
