146 Humus: Nature and Formation 



considered the granary of plant nutrients in the soil. The benefits 

 to plant growth resulting from an increase in the concentration of 

 carbon dioxide about the leaves cannot be overemphasized. In addi- 

 tion to being an important source of plant nutrients, notably nitrogen 

 and phosphorus, humus also has solvent effects upon relatively in- 

 soluble elements. 



The major effects of humus upon the soil and upon the growing 

 plant are not associated with its direct fertilizing value, however 

 important this may be. The physical and biological functions of 

 humus are of great significance. The improvement in the physical 

 condition of the soil as a result of addition of humus is associated 

 with improved texture, structure, and tilth, better water and air 

 relations, influence upon soil temperature and reaction, retention of 

 plant nutrients, and neutralization of toxic effects of certain com- 

 pounds formed in the soil. Humus tends to make soils granular, 

 causing the individual particles to form aggregates, thus preventing 

 baking when the soil is dry and stickiness when it is wet. The effects 

 of humus are particularly evident on sandy soils and on heavy silt or 

 clay soils. In the latter, a more open structure develops, which 

 favors increased circulation of the air and more rapid movement of 

 the water. In the sandy soils, humus exerts a binding effect, thus 

 retarding rapid percolation of water and giving to the soil the prop- 

 erties of heavier texture with higher moisture-holding capacity. 



Soils receiving the proper amount of organic matter hold water to 

 better advantage, because sufficiently large pore spaces are created 

 to permit drainage of the excess water, while at the same time the 

 moisture-holding capacity of the organic matter is sufficiently high 

 to keep the soil from drying out too rapidly. This enables the plants 

 to resist drought not only because of the increased moistiue content 

 at the surface but also because of the deeper root penetration favored 

 by the improved soil structure. Air circulation in the soil is essential 

 for good root growth and plant development. Root penetration may 

 be favored by making the soil more porous; by improving the gas 

 relationships in the soil, whereby the water table is lowered, root 

 persistence is made possible. Soils receiving organic manures are 

 also found to be less subject to seasonal variations than those receiv- 

 ing artificial fertilizers only, as brought out by the investigations of 

 the Rothamsted Station, 



In the presence of sufficient humus, plant nutrients are washed 

 out from the soil less readily by the percolating waters. This is par- 

 ticularly true of the basic elements comprising a number of important 



