56 NITROGEN. DECOMPOSITION OF NITROGENOUS COMPOUNDS. 



process, it is evident enough, from simple mechanical reasons, that 

 vegetation would soon cease, since there would be no room left in 

 the soil for new plants. When we realize, in addition, that the 

 very processes which purify the soil of these cumbersome bodies are 

 bringing them toward a condition for further use, we can appre- 

 ciate the extreme significance of these decomposition bacteria in 

 agriculture. 



All the types of decomposition which we have mentioned take 

 place in the humus of the soil; sugars, starches, cellulose, woody 

 tissues, proteids, and all other kinds of organic bodies are attacked 

 by microorganisms and eventually thoroughly decomposed. Since 

 this decomposition is the first step in the conversion of the products 

 of one generation of living things toward the condition in which they 

 can again be used, the conditions of the soil should be such as to 

 favor such decomposition. The thorough decay is possible only in 

 the presence of oxygen, and hence a cultivation of the soil facilitates 

 decomposition. Hard packed soils are inferior to looser soils 

 for this reason. The presence of large amounts of carbohydrates, 

 sugars, starches, straw, etc., is apt to give rise to acids, and soils 

 containing them may become sour. In such soil the nitrogenous 

 decomposition is checked, since decomposition bacteria cannot 

 stand much acid. It is evident, therefore, that in sour soils the 

 addition of lime to neutralize the acid will make it possible for 

 the bacteria to carry on an active decomposition that will soon place 

 its food materials once more within the reach of plant life. The 

 more vigorous the decomposition changes, roughly speaking, 

 the higher the fertility of the soil. Black marsh soil shows the highest 

 amount of decomposition; clay shows less, as a rule, and sandy 

 soil the least. The number of bacteria in any soil is directly 

 proportional to the activity of the decomposition changes going on 

 within it. 



