282 



THE PRINCIPLES OF SOIL MANAGEMENT 



When land is put under cultivation, a very different 

 condition is presented. Crops are removed from the 

 land, and only partially returned to it in manure or 

 straw. This withdraws annually a certain small propor- 

 tion of the total quantity of mineral substances, but, 

 what is of more immediate importance, it withdraws 

 all of this in a readily available form. 



The following table, computed by Warington, shows 

 the amounts of nitrogen, potassium, phosphorus and 

 lime removed from an acre of soil by some of the common 

 crops. The entire harvested crop is included. 



Table XLIII 



145. Amounts of plant-food materials contained 

 in soils. Comparing the figures given above with those 

 showing the total amounts of the fertilizing constituents 

 in certain soils, it is evident that there is a supply in 

 most arable soils that will afford nutriment for average 

 crops for a very long period of time. The following 

 table shows the amount of nutrients contained in the 

 chief divisions of soil as given on page 30. 



