HOW THE SOIL WAS FORMED 57 



These are nitrogen, phosphoric acid, potash, and lime. In 

 a soil that brings poor crops there may sometimes be enough 

 of all of these, but they may be held so tightly by the iron 

 or clay that water cannot dissolve and carry them into 

 plants. Such insoluble substances are spoken of as un- 

 available. These can be changed into soluble forms, or 

 available plant-food, by the oxygen of the air, and the de- 

 cay of vegetable matter. To make the plant-food in the stiff 

 soil or subsoil useful, therefore, the soil must be loosened 

 with the plow, and the surplus water drained, thus letting 

 in the oxygen of the air. Oxygen is called the restless 

 element, because it is continually seeking change, and 

 causing other elements in the soil to change also. 



How decay of vegetable matter prepares plant-food. The 

 decay of vegetable matter in the soil helps to make the soil 

 elements more soluble, partly by loosening the soil, so that 

 the oxygen of the air can reach all parts of it. It also 

 helps because the carbonic acid formed during its decay is 

 absorbed by the water in the soil ; this mixture of water 

 and carbonic acid has a much stronger dissolving power 

 than pure 'water alone. The rotting of vegetable matter 

 helps to soften or rot the hardest rock and stiffest soil. 

 Of course the decay of former generations of plants also 

 furnishes plant-food directly to later generations. 



Soils not permanently exhausted. Even in a rich soil 

 only a small part of the nitrogen, phosphoric acid, potash, 

 and lime is in a soluble condition. They become soluble 

 very slowly and gradually, so that roots have near them 

 a small but continuous supply of newly prepared food. 

 This is well, for if all of these elements in the soil were in 



