104 AERATION AND AIR-CONTENT. 



pointed out that water-logging during September reduced the pro- 

 duction of wheat at Pusa somewhat more than 100 per cent. Defi- 

 cient aeration handicaps the deeper-seated roots, and also exerts 

 an unfavorable effect upon the development of the root itself. The 

 saving of irrigation water with the consequent improvement of soil- 

 aeration is further considered in the report for 1916-17. 



Clements (1916 : 90) has emphasized the importance of oxygen in 

 wet habitats in which plant remains accumulate so abundantly as 

 to make the access of air difficult. The decomposition is slow and 

 partial, and the water or soil becomes more or less acid. Lack of 

 oxygen seems a very necessary condition, and the possible effect 

 of the acid upon plant growth is compHcated by that of deficient 

 aeration. Both, apparently, act together in diminishing the absorp- 

 tive power of roots, probably in consequence of decreased respira- 

 tion. So far as succession is concerned, the production of acid in 

 swamps marks a series of stages which dominate for a time, owing 

 to a favorable response to poor aeration. A recent study of the 

 transpiration and growth of plants in aerated bog-water indicates 

 that the acid is a concomitant only and not a cause. The acid is 

 evidently a by-product of decomposition in the absence of oxygen, 

 and deficient aeration is to be regarded as the effective factor. As a 

 consequence, the measurement of the primary reaction in acid habi- 

 tats must be directed toward the effect upon the oxygen-content, 

 i. e., upon aeration. 



Coventry (1917) stated that for practical purposes it may be 

 assumed that the failure of natural regeneration in the deodar is 

 due to the accumulation of humus and other organic substances, 

 which have interfered with the proper aeration and drainage of the 

 soil. In support of this is the well-known fact that deodar shows a 

 distinct Hking for ridges and spurs and similar well-drained places. 

 It is usually found on light, well-aerated soils and does not grow 

 in the heavier clay soils. Some of the best natural reproduction 

 takes place on grassy slopes, which had formerly been subjected to 

 fires, and are consequently well-aerated and drained, owing to the 

 absence of humus. 



Hesselmann (1917) has shown that in the pine heaths of Sweden 

 operations which increase the soil-air and the organic matter avail- 

 able for energy promote nitrification, and consequently tree-growth. 

 This may be accomplished by the mixture of decaying leaves or 

 wood with the mineral soil, and especially by logging, which works 

 the surface layer into the soil. A marked increase of nitrification 

 has been secured by mixing sod with the soil or by stirring the 

 latter with the hoe. 



Howard (1918 : 187) emphasizes the neglect of aeration as a fac- 

 tor in growth, and reviews briefly the work of Hall on the effect 

 of increased aeration on the root development of barley, and that 



