48 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
soil by crops grown on it is continually removed from the farm by 
the sale of crops, the supply will become depleted and phosphorus 
will become the limiting factor in crop production. In many 
instances this is now the case. 
Phosphorus taken from the soil. — Most of the phosphorus absorbed 
by plants in their growth goes to the seed, so that when grain or seed 
is sold much of this element is removed from the soil. Likewise, 
when crops are fed to animals much of the phosphorus goes into 
the bones and milk, and if the animals or milk are sold from the 
farm considerable phosphorus is lost. It is evident also that the 
amount of phosphorus sold from the farm will vary greatly with 
the type of farming practiced. Grain raising is most apt to deplete 
the supply of phosphorus, since large quantities of this element are 
removed in the seed. The handling of live stock, especially if young 
animals born on the farm are raised and sold when they reach matur- 
ity, also removes considerable phosphorus. Dairy farming, in which 
it is customary to use a good deal of feed brought from outside of 
the farm and to sell butter fat which contains only a small amount 
of this element, removes much less; but even in dairy farming it 
must be recognized that there is some loss in the bones of old cows 
and in the milk, as well as by unavoidable leaching in the manure. 
In practice, these losses can be made good only by the purchase of 
phosphorus-bearing materials or of feeding stuffs which contain 
this element. 
PHOSPHORUS-BEARING MATERIALS. 
(Ref. No. 1, pp. 201-208; or No. 3, pp. 511-518; or No. 5, pp. 183-193; or No. 7, pp. 
261-277.) 
Besides what is naturally in the soil, the principal phosphorus- 
bearing materials are, (1) the bones of animals, (2) natural beds of 
calcium phosphate, and (3) phosphatic iron ores. 
Bone phosphates. — A very limited supply of phosphorus for soil 
improvement comes from the bone meal prepared by packing houses. 
This, of course, comes originally from the soil. Raw bone contains 
from 9 to 11 per cent of phosphorus, but in preparing it for use on 
soils it is now usually steamed or otherwise treated to remove the 
bulk of the organic matter, and then ground. Steamed bone meal 
contains from 12 to 14 per cent of the element phosphorus. Sul- 
phuric acid is sometimes added to bone meal. The resulting acidu- 
lated bone phosphate or so-called dissolved bone is more readily 
available than the raw bone. This product contains about 7 per 
cent of phosphorus and 2 per cent of nitrogen. Bone tankage, a by- 
product of the packing houses, contains 2\ to 9 per cent of phos- 
phorus. Unacidulated bone phosphate is not readily soluble in 
water and becomes available to crops slowly, so that rather larger 
