62 NITROGEN. DECOMPOSITION OF NITROGENOUS COMPOUNDS 
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 appreciate 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 facili- 
tates decomposition. The inferiority of hard-packed soils is 
largely due to this factor. The presence of large amounts of 
some kinds of organic matter may 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 in 
general directly proportional to the activity of the decomposition 
changes going on within it. 
