122 The Living Plant 



in that very solubility which renders them absorbable by plants, 

 for it leads to their constant drainage away with the superfluous 

 water; and were it not for a constant renewal of the nitrate sup- 

 ply plant life would soon be starved to extinction. This renewal, 

 known as the nitrification of soils, is a matter of such biological 

 and economic consequence that we must now consider it with 

 some care. 



The natural nitrification of soils takes place in four ways. 

 First, there is a constant return of combined nitrogen to the soil 

 from the excretions of animals, and the decay of plant and animal 

 bodies. Second, a small amount of combined nitrogen is added 

 to the soil with the rain which falls during thunder showers, for the 

 lightning acts as a kind of gigantic natural sparking machine which 

 forces the nitrogen and oxygen of the air into combination; 

 thus is formed the soluble nitrous acid, which is caught and taken 

 into the soil by the rain. Third, nitrates are constantly though 

 slowly added to the soil by the natural decay of the rocks which 

 contain them. In moist climates they must drain away about as 

 fast as they are formed, but in dry climates the drainage is slower 

 than their formation and they accumulate in the soil. This is 

 a reason for the richness of the finer soils of the deserts, which 

 blossom as the rose when water is added by aid of irrigation. 

 Fourth (and far the most important) of the natural methods of 

 soil nitrification is bacterial activity. Everybody knows that a 

 soil in order to be rich must contain a proportion of humus, 

 the material which is dark in color and supplies the open char- 

 acter. This humus consists chiefly of decaying vegetable matter, 

 which provides both the home and the nourishment for countless 

 numbers of tiny organisms, chiefly Molds and Bacteria. These 

 Bacteria, popularly known as Germs, are of several kinds, of 

 which some, in the course of their own processes, incidentally 

 work over the less valuable nitrogen compounds of the soil to 

 more valuable ones, while still others, and these the most impor- 

 tant, actually force the nitrogen and oxygen of the air to unite 



