150 THE SOIL: ORGANIC MATTER 



(b) Inactive Organic Matter, or Humus, serves as the 

 reserve nitrogen supply, decomposing but slowly, and thus 

 preventing the loss of nitrogen as nitrates by leaching. Its 

 decomposition is ordinarily so slow that it does not serve to 

 any great extent as a source of organic acids, carbon dioxide, 

 and inorganic salts. Its principal function is physical, in 

 that it improves the water holding capacity very materially ; 

 increases the heat absorption and thus warms up the soil 

 earlier in the spring; improves the structure of the soil by 

 loosening heavy clays, and making sandy soils more compact. 



In other words, active organic matter has a decided 

 chemical effect in the soil, while humus has an important 

 physical effect. This distinction is not absolutely definite, 

 but is generally true. 



127. Loss of Organic Matter. — The active decomposition 

 of organic matter in the soil is of vast importance to the 

 farmer. It is not wholly a question of piling up reserves of 

 organic matter, but rather of continually renewing the supply 

 which is undergoing constant decomposition, thus rendering 

 mineral particles soluble, freeing plant food from the organic 

 matter, and making nitrogen available. In addition, of 

 course, there must be a fair amount of humus, particularly 

 on sandy soils, for physical reasons. 



Active decomposition and loss of organic matter — and 

 this includes humus — takes place most rapidly under in- 

 tensive cultivation, proper drainage, and application of 

 lime and commercial fertilizers. This is just what should 

 take place, but the supply must just as surely be renewed 

 by good applications of manure, and by plowing under grass, 

 clover stubble, and green manure crops. Manure by its 

 rapid decomposition does not ordinarily form humus to such 

 an extent as do the fine, numerous roots of grass. Humus 

 accumulates in pastures not only because of the fine roots 

 thoroughly spread throughout the soil, but also because the 

 soil is not cultivated and the organic matter is hence not so 

 completely oxidized. Even very heavy applications of 

 manure do not result in increased content of organic matter 

 and humus, and consequently are not economical. It is 

 better to put on reasonable applications (say 6 to 10 tons 



