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. 
