1906.] LIMING SOILS AND PLANTS. 93 



very decided influence especially upon the virulence of the dis- 

 ease," but that wood ashes " had no apparent connection " with 

 it. No explanation for the action of the mortar and cement 

 was offered and it was even looked upon as possibly incidental. 



Professor Thaxter found in his pure cultures of the fungus 

 that it grew best in a neutral medium, and this report sug- 

 gested the idea that if the soil were alkaline the fungus might 

 be aided by it in overcoming the acidity of the tuber while 

 growing upon its surface. If on the contrary the soil were 

 acid it seemed reasonable that it would be more difficult for the 

 fungus to vegetate upon the tuber which the soil surrounded. 

 With these ideas as a starting point the questio^n was studied 

 exhaustively for four years. In brief it was found that stable 

 manure did promote potato scab provided the fungus was 

 present ; that the addition of common salt or of acid to the 

 manure lessened the tendency, and that the addition of an al- 

 kaline substance, such as sodium carbonate, made the condi- 

 tions still worse. Slacked lime, wood ashes, carbonate of lime, 

 and substances such as calcium acetate and calcium oxalate 

 which yield calcium carbonate quickly in the soil ; or in other 

 words alkaline substances or other compoirnds which change in- 

 to such readily, favored without exception the development of 

 scab upon the tubers. On the contrary sulphate of lime (land 

 plaster) and calcium chloride, two compounds that under 

 ordinary circumstances can neither increase the alkaline re- 

 action of the soil nor practically lessen its acidity, both failed 

 to increase the scab. In short the experiments seemed to 

 establish beyond a doubt that even if the scab fungus is 

 present, no considerable amount of scab will develop in a very 

 acid soil if care is taken to avoid alkaline manures. On tht 

 contrary if the fungus is already in the soil or if it is intro- 

 duced into it upon the tubers used for planting, and alkaline 

 manures are used, the crops may after one or two years be 

 practically ruined by the scab. 



In the view presented here, the product of potato tubers 

 obtained where air-slacked lime had been used is to be seen at 

 the left ; the lot at the right, grown also with the aid of chemi- 

 cal manures but without lime, was entirely free from scab 

 even though badly scabbed tubers were planted in each case. 



