136 



SOILS AND MANUEES 



of oxygen takes place very rapidly. The proportion of 

 oxygen in the air of the soil may thus be reduced to less 

 than half that found in the atmosphere, i.e., to about 

 10 per cent., and the proportion of carbon dioxide corre- 

 spondingly increased. 



The proportion of carbon dioxide found in the air of soils 

 under different conditions was as follows : 



Carbon Dioxide. 



The atmosphere . 

 Sandy forest subsoil 

 Loamy forest soil 

 Arable soil 

 Vineyard soil 

 Pastureland soil . 

 Sandy soil freshly manured (dry weather) . 

 ' , (wet ) . 



Per cent. 



04 



38 



1-24 



1-30 



1-46 



2-70 



3-33 



14-13 



The composition of the air of sandy soils generally re- 

 sembles that of the atmosphere more closely than does that 

 of other soils, because, owing to the rapidity with which 

 oxidation takes places in the first, they generally contain 

 but little organic matter. For the same reason, i.e., the 

 rapidity of oxidation, when they contain much organic 

 matter, e.f/., when freshly manured, a larger proportion of 

 carbon dioxide is often found in the air of sandy soils than 

 in any other. The rapidity with which organic matter is 

 oxidised in sandy soils results from their open texture 

 which promotes rapid diffusion, and, to a lefss extent, from 

 the fact that they are generally warmer. On the other 

 hand, the comparative dryness of sandy soils -tends to 

 retard the action of micro-organisms, and the progress of 

 the oxidation is, therefore, a,s a rule, greatly accelerated by 

 wetting. 



