Plants as Affected hy Parasites. 153 



phosphoric acid is indicated by scanty crops of light or 

 shrunken seed on plants of normal size. Lack of potash 

 is indicated by small crops of inferior fruit, accompanied 

 by satisfactory growth. 



268. Crop Rotation Economizes Pfant Food, because 

 some crops use more of a. given food constituent than 

 others. The alternating of crops having different food 

 requirements tends to prevent the exhaustion of special 

 food substances. 



269. A Growing Crop Tends to Conserve Fertility, be- 

 cause it reduces drainage by taking up water from the 

 soil, and at the same time, appropriates the available plant 

 food, thus pre\'enting loss of the latter from leaching. 



270. Manures are, in part, the Raw Material from which 

 the cultivator turns out valuable products. They should, 

 therefore, be most carefully preser\'ed and applied. 

 Leaching of the manure pile by undue exjiosure to rain 

 and over-rapid fermentation, by which nitrogen escapes 

 as ammonia or other gaseous nitrogen compounds, should 

 be stringently avoided. All refuse organic matter should, 

 so far as possible, be made to increase the always-too- 

 small stock of manure. 



Section VII. Plants as Affected by Parasites 



The only instance of a beneficial plant ijarasite (24) 

 of special interest to the cultivator, is the micro-organ- 

 isms in the roots of leguminous plants, which we have 

 already considered (260). Many parasites of harmful 

 insects are beneficial, but these are beyond our scope. 

 We need therefore to treat here only those parasites that 

 are directly injurious to economic plants. 

 10 



