COMMERCIAL FERTILISERS 387 



materials must be mixed. The mere mixing re- 

 quires little skill, but it is very important to get the 

 right proportions of the different plant foods in the 

 mixture. We will suppose that excellent results 

 have followed the use of 1,000 pounds per acre of 

 a certain brand of fertiliser containing 4 per cent, 

 of nitrogen, 8 per cent, of potash and 6 per cent, 

 of phosphoric acid; but it is found that the plant 

 food in this fertiliser costs more than it can be 

 bought for in raw materials. This means that 

 for each acre, a mixture containing 40 pounds of 

 nitrogen, 60 pounds of potash, and 80 pounds of 

 phosphoric acid is needed. By referring to the 

 figures of the amounts of plant food in each fer- 

 tilising material, we find that the 40 pounds of 

 nitrogen may be obtained in 250 pounds of nitrate 

 of soda, or 200 pounds of sulphate of ammonia, 

 etc. The 60 pounds of potash may be obtained 

 in 120 pounds of sulphate of potash, or in 114 

 pounds of muriate of potash, etc. The 80 

 pounds of phosphoric acid may be obtained in 

 533 pounds of dissolved South Carolina rock, or 

 in 250 pounds of Florida phosphate, etc. If 

 sulphate of ammonia is found to be the cheapest 

 source of nitrogen, sulphate of potash, of potash, 

 and dissolved South Carolina rock, of phosphoric 

 acid, the mixture would be: 



200 Ibs. sulphate of ammonia 



120 Ibs. sulphate of potash 



533 Ibs. dissolved South Carolina rock 



853 Ibs. for an acre; larger quantities in proportion. 



It often happens that some fertilising materials 

 which can be bought to advantage contain more 

 than one kind of plant food. Thus a good sample 



