L 53' 3 



tional quantity liberated from the organic matter present, 

 If MgO is used instead of K 2 O, much smaller quantities of 

 NH 3 are obtained. The actual proportion of NH 3 in soil 

 is only about '0005 percent. There is a constant interchange 

 of NH 3 between water, air and soil and the sea appears to 

 act as the final reservoir for much of the NH 3 washed away 

 by drainage and percolation. But the sea also gives off 

 NH 3 to air, more in the hot weather than in the cold. The 

 air of hot countries and of towns contains a larger propor- 

 tion of NH 3 . The subsoil contains less NHs than the sur- 

 face soil and there is no NHs below a depth of about 6 ft. 



851. Nitric acid is formed in the air by electricity 

 and in some cases by the action of O 3 and then brought 

 down by rain and dew, and other meteoric waters, nearly 

 always combined with NH 3 as Ammonium nitrate. Priestley 

 first noticed the formation of HNOa in the atmosphere and 

 Liebeg found it in rain water. Nitrates and Nitrites are 

 formed in the soil also and in manure heaps and their for- 

 mation and removal by plants and drainage are questions 

 of very great importance to agricultural science. Some 

 chemists are of opinion that nitrates are formed from the free 

 N of air by the intervention of soil only, but there is no 

 certain proof of this ; but that they are formed by the in* 

 vention of bacteria has been proved. Nitrous and Nitric 

 anhydrides are, formed by electricity or ozone. NH 3 is also 

 converted into HN0 3 by the same agency, but not free N of 

 the air. NHs is to a certain extent fixed by soils, but Nitrates 

 are very soluble and they are washed away chiefly in com- 

 bination with lime as CaaNOa. Soils containing much 

 Ferric hydrate ^FesOa 3^0) are better able to retain nitrates 

 as basic Ferric nitrate. Soils containing much organic matter 

 to which air has not free access have their nitric acid reduced 

 to NH 3 , partly to free N, and Nitrous oxide. In the last two 

 forms N is useless to vegetation. This is one reason why 

 humus soils should be kept well cultivated whether 



