SOIL MANAGEMENT 25 



attack the fine rock particles and help to liberate the phosphorus. 

 Experience has shown that the raw ground rock can he used 

 with j^rofit when plowed under with green manuring crops like 

 clover or soy beans and that it has a high fertilizing value when 

 mixed with stable manure. 



The mineral phosphates are usually found on the market in 

 the form of acid phosphate w^hich is made by treating the raw 

 ground phosphate rock with acid for the purpose of making the 

 phosphorus soluble in water and immediately available for the 

 use of the crop. The better grades of acid phosphates contain 

 from fourteen to sixteen per cent of phosphoric acid. 



Bone phosphates. — Most of the bone sold for fertilizer pur- 

 poses is in the form of a meal that is made by grinding the bones 

 that have previously been treated with steam to remove the fat 

 and a part of the nitrogen compounds. Steamed bone meal con.- 

 tains from one to two per cent of nitrogen and from twenty- 

 eight to thirty per cent of phosphoric acid and is considered to 

 be more valuable than the raw bone meal for fertilizer purposes, 

 because the steamed bone can be much more finely ground and 

 the removal of the fat causes the particles to decay more rapidly 

 in the soil. Tankage, mentioned under nitrogen carriers, is 

 sometimes called bone tankage if it contains a large proportion 

 of bone. Tankage varies greatly in the amount of phosphorus 

 carried, the amount depending upon the quantity of bone present 

 in the waste material. 



Phosphorus is present in the soil in much smaller quantities 

 than potash and experience has shown that it is more likely to 

 become exhausted than is the potash. In some sections of the 

 country phosphates are practically the only commercial fertilizer 

 used. 



Mixed fertilizers. — In the making of conmiercial fertilizers 

 the manufacturer mixes two or more of the foregoing carriers 

 in the correct proportions to give the desired percentage compo- 

 sition of nitrogen phosphoric acid and potash. The fertilizer 

 unit is one per cent of a ton, or twenty poundSo A three-ten-four 

 fertilizer is one that contains three units, or sixty pounds of 

 nitrogen or ammonia, ten units, or two hundred pounds of avail- 

 able phosphoric acid, and four units, or eighty pounds of potash. 

 The trade or commercial value of such a mixture should be 

 determined solely by the number of pounds of plant food 

 materials w^hich it contains. The agricultural value of a ferti- 

 lizer is determined by the extent to which the mixture supplies 

 the needs of the soil and meets the requirements of the crops to 



