MAINTAINING SOIL FERTILITY 285 



removes 96 Ibs. of nitrogen, 33 Ibs. of phosphoric 

 acid and 68 Ibs. of potash each year per acre. A 

 crop of 1^ tons of hay per acre removes 35 Ibs. of 

 nitrogen, 7 Ibs. of phosphoric acid and 39 Ibs. of 

 potash; of wheat, at the average yield of 14 bushels 

 per acre, 33 Ibs. of nitrogen, 10 Ibs. of phosphoric 

 acid and 17 Ibs. of potash. 



Compare 1 these small amounts of plant food 

 removed by average crops with the vast amounts 

 that are in ordinary soils. What are they when 

 compared with the 4,000 Ibs. of nitrogen, 6,000 

 Ibs. of phosphoric acid and 20,000 Ibs. of potash 

 that the upper 8 inches of an average soil contains ! 

 In addition to all this is the undeveloped 

 richness of the subsoil, which becomes of use 

 from year to year. The average soil ought to 

 produce bumper crops for hundreds of years with- 

 out adding any fertiliser, if we considered but two 

 facts the great amount of plant food in the soil, 

 and the very small amount removed by crops. 

 But other things must be considered. If only 

 a small part of the 4,000 Ibs. of nitrogen, 6,000 Ibs. 

 of phosphoric acid and 20,000 Ibs. of potash is 

 available to plants as is usually the case the 

 drain of the crop upon this amount of available 

 plant food may be quite heavy. This is especially 

 true on the light and leachy soils, from which the 

 soluble plant food is quickly lost by leaching. In 

 other words, the amounts of plant food drawn from 

 the soil by farm crops makes little impression upon 

 the total amount that is in it, but it often does make 

 a decided impression upon the amount of soluble 

 or available plant food in the soil, which is, after 

 all, the kind of plant food that is of chief interest 

 to the farmer. 



The gradual decrease in yields on soils that have 



