FEETILITY 171 



which are left in the soil, and which must be taken into 

 account in estimating the quantities of plant food annu- 

 ally required by the crops. It appears, therefore, that, 

 in order to provide for the requirements of the crops, the 

 soil must be able to supply, annually, in round numbers, 

 about 100 Ibs. each of nitrogen and potash and 30 Ibs. 

 of phosphoric acid in an available state. Taking the mass 

 of the soil, as before, at 2,500,000 Ibs. per acre, these 

 quantities are equivalent to 0*004 per cent, of nitrogen 

 and of potash, and 0'0012 per cent, of phosphoric acid. 

 Of course, in order to provide the quantities required by the 

 crops, the soil must contain a considerable excess probably 

 many times as much as is actually abstracted by the plants. 



Loss of Plant Food by the Sale of Produce. The figures 

 given above show the quantities of plant foods required to 

 produce the crops. If they were removed from the land 

 and nothing put back, the annual loss would be at the 

 average rate of about 73 Ibs. of nitrogen, 24 Ibs. of phos- 

 phoric acid, and 79 Ibs. of potash. In that case the pro- 

 cess of exhaustion would be very quickly accomplished. 

 It is, however, impossible to abstract the plant foods from 

 the soil at that rate because, if nothing were put back, it 

 would be impossible to obtain crops of the magnitude 

 assumed. 



If only the grain of the corn crops and the meat pro- 

 duced by feeding the roots and hay were sold the loss would 

 be greatly reduced. It may be estimated approximately 

 as shown in the table on p. 172. 



The loss is thus reduced to less than half the quantity 

 of phosphoric acid and a third of the nitrogen containe'd 

 in the crops. The loss of potash is very small. The loss 

 of plant foods would, of course, be much greater if any 

 portion of the hay or roots were sold, or if milk were 

 sold instead of meat. If only butter were sold instead 

 of milk the loss would be less, and so on. 



