224 CU LT U R E OF T II E G RAP E. 



the fungi causing blight, or mildew, is caused by a surplus 

 of carbonic-acid gas, which gas would not exist as such 

 were there a sufficient supply of potash in the soil. " We 

 may now easily account for facts mentioned by your cor- 

 respondent, that old vines are much more liable to mil- 

 dew than young. They have exhausted the potash from 

 the soil ; raid, when their leaves absorb carbonic acid, the 

 plant has no potash with which to form a healthy salt by 

 union with it, and the diseased plant invites the fungi. 

 A humid summer is favorable for the generation of car- 

 bonic acid, and hence the reason why 'T ' found his young 

 vines attacked during such a season. 4 T' is correct 

 when he says, 'Soap-suds are always beneficial, and can 

 be used freely.' The reason is, soap-suds contain potash." 

 To this Mr. A. J. Downing adds, " There is some point in 

 these notions regarding mildew. Young and healthy 

 plants are seldom attacked by mildew, while old and 

 feeble ones are very liable to it. Our own observation 

 has led us to believe that wood-ashes are one of the most 

 beneficial fertilizers for the grape, giving it the appear- 

 ance of extraordinary luxuriance and health. The great 

 productiveness and longevity of the vineyards abroad, 

 which are formed upon a soil composed mainly of the 

 spent ashes of volcanoes, and the acknowledged superiority 



