278 ROCKS AND SOIL 



allows plant diseases or harmful insects to get a strong foothold. 

 When the crop is changed, the new plant is usually immune; that is, 

 not affected by the disease. Certain weeds act in a similar way. 

 They grow readily in fields containing some one crop, and not so easily 

 along with other crops. Thus, the cockle is a weed that grows with 

 wheat, but not with corn. If the crop rotation is wheat, grass, corn, 

 the weed is killed before the next wheat crop. 



302. Artificial Fertilizers. Artificial, or commercial, 

 fertilizers are used when natural manures are not abun- 

 dant enough, or not of the right sort. As commonly used 

 they contain one, two, or all three of the elements nitrogen, 

 phosphorus, and potassium. One having all three ele- 

 ments is called a "complete" fertilizer. 



The potassium compounds needed by the soil are ob- 

 tained from wood ashes (when these are to be had; cf. 

 217), or from deposits, as at Stassfurt, in Germany. 

 Recently some lavas, and some deposits in the West, have 

 been found that give potassium compounds useful for 

 fertilizers. 



Phosphorus is usually sold, as a fertilizer, in the form 

 of small fragments of bone (bone meal) or in the form of 

 powdered rock phosphate (cf. 221). Rock phosphate 

 is found as a deposit in South Carolina, Florida, and 

 Tennessee. 



Calcium phosphate is very slightly soluble, so the 

 manufacturers of fertilizers often change it to acid calcium 

 phosphate by treating it with sulphuric acid (cf. 214). 

 The acid phosphate is called " superphosphate." In 

 purified form it is used for "acid phosphate" baking pow- 

 ders (cf. 130). Sulphuric acid and calcium phosphate 

 give, besides the acid phosphate, a fertilizer called land 



