110 AERATION AND AIR-CONTENT. 



over-irrigation and defective aeration, perennial and especially woody 

 plants exhibit damage less readily, with the result that they often 

 gradually develop diseases obscure in origin and impossible of refer- 

 ence to a specific parasite as a cause. In all such cases it is neces- 

 sary to consider as a possible cause the defective aeration that regu- 

 larly arises as a consequence of applying too much water. In older 

 districts the gradual rise of the water-table brings in its train the 

 evils of an oxygen deficit in the soil, and this must frequently be 

 the real cause of the troubles that are often referred to the pres- 

 ence of alkali. 



