CEREAL CULTIVATION EASTERN AREA 307 



of the essential foods, nitrogen, potash, and phosphorus. 

 (2) These foods may exist, but not in the form that 

 is available to the plant. (3) The soil may be acid. 



(4) Substances may be present that are toxic to the 

 crop. (5) A condition such as the presence of imper- 

 vious clay may exist that impedes water movement. 

 Note that only one of these refers to the actual food 

 supply. A soil may be well supplied with plant-food, 

 and the crop not be able to get it. The principal amend- 

 ments employed are liming, application of fertilizers, 

 green manuring, and crop rotation, in addition to good 

 cultivation methods. 



333. Liming. An application of lime to the soil may 

 be effective in a number of ways : (1) As a direct plant- 

 food it is as necessary as potash or nitrogen, but being 

 nearly always present in some form, its value in this 

 respect is not apparent. (2) It renders available potas- 

 sium and phosphorus much more rapidly than would 

 naturally take place. (3) It neutralizes the acidity in 

 " sour " soils. Whether the soil is acid or not may be 

 determined by placing a piece of blue litmus paper in 

 close contact with the moistened soil for a half hour. If 

 the paper changes color from blue to pink or red, the soil 

 is acid. (4) It counteracts the poisonous effects of salts 

 of magnesium and sodium. The carbonate of the latter 

 is best corrected by the sulfate of lime, known as gypsum. 



(5) It promotes the nitrification of humus which cannot 

 take place in acid soils. (6) Its presence is favorable for 

 the action of those organisms which fix atmospheric 

 nitrogen. (7) A very important effect of lime is its floc- 

 culation of fine clay, changing it from a stiff, impervious 

 condition into a friable, mellow soil, warmer and better 

 aerated. 



