280 SOILS. 



nitrates of the soils are reduced partly with the evolution of 

 nitrogen gas, partly to ammonia; while their oxygen is con- 

 sumed to supply the demands of the roots. Ferric oxid is 

 reduced to ferrous carbonate, sulfates to sulfids; thus de- 

 ranging the whole process of plant-nutrition and absorption of 

 plant-food. If continued for any length of time these condi- 

 tions end in the death of the plant. Too much importance can- 

 not therefore be attached to the proper aeration of the soil and 

 subsoil. 



Excessive Aeration; Compacting the Soil. On the other 

 hand, excessive aeration of the soil may be injurious in caus- 

 ing a serious waste of moisture; especially in arid climates, 

 where the hot, dry winds may readily destroy the germinating 

 power of the swollen seed when the seed-bed is too loose and 

 open, and later may injure or destroy the feeding roots. The 

 abundant growth of grain often seen in the tracks of a wagon 

 carrying the centrifugal sower, when the stand in the general 

 surface is very scanty, is usually due to the consolidation of the 

 seed-bed, and suggests at once the well-known efficacy of light 

 rolling to insure quicker germination and a better stand. Simi- 

 larly, the rolling of grain fields in spring is often the saving 

 clause for a crop in dry years. But such needful consolidation 

 must not, of course, be carried to the extent of creating a sur- 

 face crust which would subsequently serve to waste the subsoil 

 moisture. Hence, the soil-surface should be rather dry when 

 rolling is resorted to. 



The pressing of the earth around transplanted plants, simi- 

 larly, is a needful precaution, not only with respect to the dry- 

 ing-out of the soil, but also to insure close contact between the 

 roots and the soil. 



The Composition of the I-rce Air of the Soil usually differs 

 from the air above, in that besides being saturated with mois- 

 ture, its nitrogen-content is slightly increased (by one-half to 

 over one per cent) ; the oxygen-content on the other hand, is 

 diminished, being in part (sometimes nearly to the extent of 

 one-half of its volume) replaced by carbonic gas, de- 

 rived partly from its secretion by the roots, partly from the 

 oxidation of organic substances. It naturally follows that the 

 richer the soil in the latter, the more carbonic gas will be 

 formed under favoring conditions; so that in freshly-manured 



