32 Mineral Nutrition of Plants 



The liquid phase 



The liquid phase of soils varies in volume, of course, with the 

 moisture content of the soil. Immediately after a heavy rain the pores 

 of the upper portion of a soil may be entirely or nearly filled with water. 

 If the soil has adequate drainage, nearly one-half of this water is usually 

 removed quite rapidly by the force of gravity. When a point is reached 

 at which the attractive force of the soil for water equals the force of 

 gravity, the removal of water by drainage ceases. At this point the pores 

 of the soil may be about one-half filled with water; this water is not 

 held in the pores as droplets, however, but as films on the surface of 

 the soil particles. At this point, also, conditions are favorable for plant 

 growth: aeration is adequate, the plant is able to draw on the water 

 to supply its vast needs, and conditions are favorable for chemical and 

 biological activity so that nutrients are brought into solution and thus 

 made available for plant growth. 



It will be noted in Table I that the concentration of soluble salts 

 usually ranges from ioo to iooo p.p.m. of the dry soil. Nutrient ele- 

 ments contained in these salts are, of course, available for the nourish- 

 ment of growing plants. However, as will be seen later, plants are not 

 dependent on just those nutrients which are dissolved in the soil solu- 

 tion. If the concentration of these dissolved salts reaches 2000 p.p.m. of 

 the dry soil (10,000 p.p.m. of the soil solution in a soil containing 20 

 per cent of moisture), the salts may become toxic to many plants. Thus, 

 it is seen that a proper regulation of the liquid phase as regards content 

 of dissolved substances is a matter that requires much consideration 

 and attention in practical agriculture. 



Lack of adequate supplies of water probably limits crop growth more 

 than any other factor. Because the subject of soil moisture is given 

 special attention in papers that follow, it is not given further consider- 

 ation here. 



The gaseous phase 



It has already been indicated that the gaseous phase of soils, that is, 

 the soil air, varies inversely in amount with the amount of water pres- 

 ent, and that when the pore space is no more than one-half filled with 



