240 THE LIVING ORGANISMS OF THE SOIL [chap. 



setting free the acid. To this action is due the acidity 

 produced by the long-continued use of ammonium salts 

 as manure, as seen on the experimental plots at Rotham- 

 sted and Woburn. At Woburn the soil is light and 

 sandy, containing but little lime, and the application of 

 ammonium salts containing 50 lbs. ammonia per acre 

 every season for twenty-four years, has rendered the 

 land practically incapable of carrying the crops. A 

 moderate dressing of lime, however, restores the fertility. 

 The following crops were obtained in 1900 on the barley 

 plots : 



The soil had become acid to litmus paper where the 

 lime had not been used : it is interesting to note that 

 though barley would not grow, oats flourished freely on 

 this sour soil. There are, however, two special organisms 

 which merit further consideration the fungus which 

 clothes the finer rootlets of many , classes of plants, 

 forming mycorhiza, and the slime fungus, or Plasmodio- 

 phora which causes the disease known as " finger-and- 

 toe" or "club" in turnips and other cruciferous plants. 



The term "mycorhiza" is applied to the symbiotic 

 combination of the filaments of certain fungi, whose 

 complete development is as yet unknown, .with the 

 finest rootlets of certain plants. Sometimes the fungus 

 forms a sort of cap on the exterior of the short root- 

 lets, which are generally without root-hairs ; in other 

 cases it penetrates the cortical tissue of the root itself, 

 which may also be furnished with root-hairs. Accord- 



