ABSORPTION OF NITROGEN BY SOIL 4B 



Besides the rapid movements of salts due to a move- 

 ment of the water in the soil, they have also a slow 

 movement due to their molecular diffusive power, by 

 which their particles continually pass from a stronger 

 solution to one weaker. This movement is always in 

 action in moist soil, and tends to the equal distribu- 

 tion of all soluble matter. If a dressing of nitrate or 

 chloride of sodium is applied to moist soil, the manure 

 will dissolve and slowly spread downwards even before 

 rain falls. Again, when a heavy rain has washed alJ 

 soluble salts out of the surface soil, they will slowly 

 rise again by diffusion as soon as rain ceases. 



Of the soluble and diffusible salts occurring in soil 

 the nitrates are of the greatest importance as plant 

 food. The quantity of nitrates in a surface soil will 

 vary much, depending on the richness of the soil in 

 nitrogen, the previous conditions as to tillage, tempera- 

 ture and moisture, the extent of recent washing by 

 rain, and on whether the soil is or is not under crop. 

 Where a crop is growing the nitrates will be kept 

 nearer the surface, the evaporation of water from a 

 growing crop being far greater than from a bare soil. 

 The nitrates will also be constantly taken up by the 

 roots and employed as plant food. The loss of nitrates 

 by drainage is thus far less when the land is under 

 crop than in the case of bare fallow. 



11. Absorption of Atmospheric Nitrogen, — The 

 behaviour of soils to atmospheric nitrogen still re- 

 mains ill defined. A soil may, in some circumstances, 

 give up ammonia to the air, in other circumstances it 

 may absorb ammonia from the air. The free nitrogen 

 of the air does not combine with the inorganic or with 



