19 



soil organic carbon was increased by application of farm yard manure and 

 by rotation including both green manure and legume crops. 



The preceding crop species can have beneficial or detrimental 

 effects on yields of succeeding crops. For example, onion and lettuce 

 planted after sweet corn with a winter crop of vetch ( Vicia sativa L.) 

 developed a severe root rot gradually reducing the yield and often 

 killing the vegetables (8?, 88). They postulated that during the de- 

 composition of corn residue under cool temperatures of spring, a toxin 

 was formed which injured plant roots (88). Mack et al. (102) observed 

 that average crop yield indices following cabbage, onion, summer pumpkin 

 ( CuGurbita pepo L. ) , and carrot were significaiitly greater than those 

 crops following sweet com, potato, and tomato. The low yields follow- 

 ing sweet corn, potato, and tomato might have been the result of lew 

 soil fertility after growing these crops. Some vegetables planted 

 after rice respond favorably to N application but not to residual N. 

 For instance, yields of sweet potato and tomato planted after rice 

 significantly increased when N was applied directly to the vegetable 

 crops (6). Jones (91) also reported xhat corn yields were higher when 

 preceded by peanut than by coi'jpea. The differences were larger without 

 N application and decreased at the optimum application of Ss- kg/ha. 

 Detrimental effects have also been observed with grain legumes. Expe- 

 riments proved that mungbeans have a depressing effect on yield, parti- 

 cularly at low levels of N (33). Apparently, mungbeans secrete certain 

 toxins v/hich depress growth. 



