CHAPTER 



1/ Soil Moisture and the 

 Mineral Nutrition 

 of Plants 



C. H. WADLEIGH and L. A. RICHARDS 



T, 



he mineral nutrition of plants is related in many 

 ways, directly and indirectly, to soil moisture. It is the purpose of this 

 paper to discuss some of these relations and it seems appropriate to 

 introduce the subject with a review of some of the modes of behavior 

 of water in soil. Particular attention will be given to the physical con- 

 dition of moisture as it relates to the readiness with which roots may 

 absorb moisture present at the soil-root interface, and also the readiness 

 with which additional moisture at some distance from the roots can 

 flow toward the roots to replace that which is absorbed. 



PHYSICAL CONDITION OF MOISTURE IN SOIL 



Soil particles are wetted by and have considerable attraction for 

 water and, therefore, soils exhibit a capacity to take up and retain 

 moisture. This moisture is distributed throughout the pore system of 

 the soil and over the surface of the soil particles. Energy is involved in 

 this adsorption because work is required to remove moisture from soil. 

 The amount of work per unit amount of water removed will depend 

 on the moisture content of the soil. This work can be expressed in 

 terms of the pressure difference through which a unit volume of water 

 must be transferred to effect removal of water from the soil. In saturated 

 soil the pressure in the soil water approaches zero at atmospheric pres- 

 sure. Water drops will form and drip from soil which is very wet, so 

 the work that must be done against surface force action in order to 

 effect water removal from very wet soil approaches zero. As soil dries 



