1 66 AGRICULTURAL ECONOMICS 



2. It can withdraw various ions NH 4 , K, PO 4 from their solu- 

 tions. 



3. It causes the soil to puff up or "ferment," and so leads to an 

 increase in the pore space and a marked improvement in the tilth and 

 general mechanical condition. 



4. It increases the water-holding capacity of the soil. 



5. It swells when wetted. 



6. Although the group is essentially transitional, it has a certain 

 degree of permanency and only slowly disappears from the soil. 



The group of substances possessing these properties is generally 

 called "humus," and so long as the word is used in a collective sense 

 as a convenient label it may be retained. But the practice has been 

 responsible for a good deal of loose thinking, because it obscures the 

 fact that the group is an indefinite and complex mixture, and gives 

 instead the impression that it is a single definite substance. 



The constitution of the soil. The components of the soil do not 

 form a mere casual mixture. A much more intimate mingling prevails, 

 amounting almost to a loose state of combination. It is unfortunate 

 that so little is known about the compound particles, because they 

 play a great part in determining the relationships between soil and 

 plant growth. They can be disintegrated by various cultivation 

 methods, such as plowing the soil when wet, or by allowing the stock 

 of organic matter and calcium carbonate to fall too low, and when 

 this has happened the "clay" properties become emphasized, so that 

 the soil loses its fine, crumbly state and is very apt to become sticky 

 when wet, and to dry into a hard cake through which young plants 

 can force their way only with difficulty. The compound particles 

 can be re-formed by careful cultivation and by adequate additions 

 of organic matter and calcium carbonate, but the process may take 

 years, nor can it be hastened until it is better understood. 



The rc.vlrr ^ annul fail to have noticed how many of the important 

 soil properties are due to colloids. The formation of these compound 

 particles, the absorption of soluble manures, the retention of water 

 (in part), the swelling of the soil when wet and its shrinkage when 

 dry, are all colloidal phenomena. Lf we regard the mineral particles 

 as the skeleton of the soil, we must look upon the colloids as clothing 

 it in many of its essential attributes. 



Air supply and temperature of the soil.- -The percentage of the 

 volume of a soil which is occupied by air is perpetually varying 

 inversely as the amount of water varies. While the yield of a crop 



