PHOSPHORUS AND LIMESTONE 93 
these bacteria, it is impossible for clover, soy beans, cowpeas, 
ete., to secure nitrogen from the air. If soil is very acid 
(sour), legumes can not be grown until it has first been 
sweetened by the application of limestone. Thousands of 
acres of land in southern Illinois are now growing clover 
where it was once thought such crops could not be grown. 
In these cases, clover crops were made possible by the appli- 
cation of limestone. 
To determine the acidity of soil, place blue litmus paper 
between two layers of soil to be tested. If the paper turns 
red in a few minutes the soil may be considered acid and the 
application of from one to three tons per acre would prob- 
ably be a very profitable investment. Ground limestone costs 
from one to three dollars per ton, delivered at most Illinois 
points. This difference in price is due largely to the differ- 
ence in freight charges. (The state penitentiary at Chester 
is the source of a considerable supply of crushed limestone.) 
Our soil is only very slightly acid. For this reason we 
have never used any limestone on our own farms. However, 
we intend to apply it to our alfalfa fields next year. 
Potassium 
So far, we have said nothing about the plant food element, 
potassium, for the reason that the common prairie soils con- 
tain enough of this element to last for generations to come. 
On the other hand, bottom lands, subject to overflow, already 
show a shortage of potassium. Potassium is usually supplied ° 
by applying muriate of potash. When muriate of potash is 
applied to land that is not deficient in potassium it acts as 
a erop stimulant rather than as a soil builder. Germany 
is the principal source of potash. 
