36 ELEMENTS OF FARM PRACTICE. 



form of commercial fertilizers. A supply of vegetable mat- 

 ter in the soil is important in aiding the liberation of 

 phosphorus. Even after all precautions to conserve phos- 

 phorus, there is almost certain to come a time when the 

 supply of available phosphorus in the soil will be so small 

 as to seriously limit the yield of crops. It will no doubt 

 have to be furnished to the soil in the form of commercial 

 fertilizers. The more important sources of phosphorus fer- 

 tilizers are ground bones, either steamed or raw from slaugh- 

 ter houses, and mineral phosphate mined in the United 

 States. This is mostly used in a form called acid phosphate. 

 In this form the phosphorus is easily available for plants. 

 Raw, finely ground phosphate rock is now used in large 



Figure 14. — Cattle and clover maintain soil fertility. 



quantities. It is cheaper and, where a good supply of veg- 

 etable matter is maintained, the decomposition of the 

 vegetable matter in the soil renders the phosphorus in the 

 raw rock soluble, so that plants can use it. The finer the 

 raw phosphorus rock is ground the more easily is the phos- 

 phorus made soluble. 



Potassium is much more abundant in most soils than 

 phosphorus or nitrogen. Most of the potassium used by 

 plants is in the stems or straw. Most of the straw and hay 

 produced on farms is used as feed or bedding for live stock 

 and returned to the soil in the form of manure. For these 

 reasons there are very few soils that are deficient in potas- 

 sium. It is seldom necessary to use fertilizers containing 

 potassium other than barnyard manure especially if a prac- 

 tical rotation of crops is followed. Occasionally it pays to 



