174 MANURES. 



if allowed to accumulate, and remain unchanged, ia 

 supposed to be injurious to plants ; but, .probably, 

 the oxygen and carbonic acid of the air in the soil 

 change it to an inoffensive form, and even make it 

 again useful to the plant. 



6. It may also improve the mecJianical condition 

 of the soil, as it causes its particles to crumble, thus 

 making it finer ; and it roughens the surfaces of par- 

 ticles, making them less likely to become too com- 

 pact. 



These properties of oxygen claim for it a high 

 place among the atmospheric fertilizers. 



WATEli. 



Water may be considered an atmospheric ma- 

 nure, as its chief supply to vegetation is received 

 from the air in the form of rain or dew. Its many 

 effects are already too well known to need further 

 comment. 



Supplying water to the soil by the deposit of dew 

 will be considered in Section IV. 



CHAPTER XI. 



R E C A. P I T U L A T I O N . 



Manures have two distinct classes of action in the 

 soil, namely, chemical and mechanical. 



