PHOSPHORUS AND LIMESTONE 91 
a soil, it is safe to say that the soil is not greatly deficient 
in phosphorus, although a 1,000 pound application might still 
be a profitable investment. At any rate, it can never do 
any harm. 
How to Buy PHosPpHorusS 
Phosphorus can be purchased in the form of bone meal, 
acid cut rock phosphate, and raw rock phosphate. The results 
of various experiments made at the Illinois Agricultural Ex- 
periment Station at Urbana, prove conclusively that the most 
economical method of increasing the phosphorus content of 
the soil is by the application of finely ground rock phosphate 
with stable manure, preferably spread on clover land. 
A good grade of rock phosphate should contain from ten 
to twelve per cent of phosphorus. Since the phosphorus is 
the only part of the rock that is of value to the soil, it should 
be purchased on the basis of the phosphorus it contains. 
The phosphorus is chemically locked up in the rock and 
requires the action of some acid to liberate it and make it 
available for plant use. For this reason it is best to apply 
it with manure on clover fields. 
The acids in the manure and clover plant will set free 
the phosphorus much more quickly than when applied to 
some grain crop. The rock phosphate should be ground fine 
enough so that at least 90 per cent of it will pass through 
a one-hundred mesh screen. The finer it is ground the more 
quickly it will be acted on by the acids in the soil. 
APPLICATION 
A good time to apply rock phosphate is in the fall. If 
as much as twelve tons are needed, it ig well to purchase it 
in bulk by the carload. By getting a minimum carload, it is 
much cheaper than buying in bags. <A good method of 
