674 SOILS : JPBOPJSnTXES AND MANAGEMENT 



1. Single-element system. — This was one of the iSrst 



to be suggested, and was advocated because each 

 particular crop was supposed at that time to 

 respond largely to one element. Thus, nitrogen 

 was supposed to dominate wheat, rye, and oats; 

 phosphoric acid, to dominate corn, turnips, and 

 sorghum; and potash to dominate potatoes, 

 clover, and beans. Present knowledge of the 

 balancing effects of fertilizers shows this idea to 

 be fallacious. 



2. Abundant supply of minerals. — This system had 



its origin from the fact that potash and phosphoric 

 acid are relatively cheap and are slowly leached 

 from the soil, while nitrogen is expensive and easily 

 lost. Such a plan, therefore, provides always plenty 

 of potash and phosphorus, which is to be balanced 

 each season with sufficient nitrogen to give paying 

 yields. 



3. A system based on the plant-food taken out by the 



crop. — According to this plan, as much plant- 

 food is added each year as will probably be taken 

 out by the plant, this being determined by chemi- 

 cal analyses. This system overlooks the fact 

 not only that different plants feed differently on 

 the same soil, but that the same crop exhibits 

 marked variability with change of season and 

 change of soil. Moreover, no allowance is made 

 for losses by leaching, which are known to equal 

 at times the losses due to plant growth. 



4. Irrational system. — This is the plan followed by 



many farmers where fertilizers are an important fac- 

 tor in soil management. The formula is changed 

 from year to year, in a vain attempt to strike a 



