Chemical Food in Soils. 



79 



arc dissolved and carried away by the water for each 2 

 pounds of soil formed, the chief materials carried away 

 being the lime, magnesia and carbon dioxide. 



91. Amount of Essential Plant Food Removed from the 

 Soil by Crops. It is very important, in the management of 

 soils, to know something of the draught upon them which 

 crops of different kinds make, and in the table which fol- 

 lows is given the amount of materials removed from the 

 soil in 1,000 pounds of fresh or air-dried product. 



Table showing Ibs. of plant food in 1000 Ibs. oj air-dried 

 product. 



(WOLFF.) 



From this table it appears that each ton of clover hay 

 withdraws from the soil 39.4 Ibs. of nitrogen; 37.2 Ibs. of 

 potash ; 12.6 Ibs. of magnesia ; 40.2 Ibs. of lime ; 11.2 Ibs. of 

 phosphoric acid; and 14.2 Ibs of sulphuric acid, making 

 an aggregate of ash ingredients alone of 154.8 Ibs. 



92. Amount of Plant Food in an Acre-foot of Soil. If we 



tako 4,000,000 pounds as the dry weight, of an acre-foot of 

 all soils, except the humus and that at 2,000,000 (149), 

 and the percentages of essential plant food given in the 

 tables on pages 74 and 75, the amount of plant food per 

 acre-foot may then be computed, giving the results in the 

 table below : 



