l82 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



a number of factors. These factors may be grouped as 

 follows : 



(1) The quantity of the element present at the beginning of 

 the growing season in forms of combination which can be partly 

 or completely absorbed by the plant. This may be called chem- 

 ically available plant food. 



(2) The condition of the soil particles. Compounds chem- 

 ically available may be enclosed in the soil particles so as not to 

 be exposed to the action of plant roots. Such compounds are 

 physically unavailable. If the encrusting substance is removed, 

 such bodies become chemically available. 



(3) The amount of the plant food transformed during the 

 growing season into forms of combination which can be absorbed 

 by plants. This factor is certainly of importance with respect 

 to nitrogen; its importance in the case of phosphoric acid and 

 potash is apparently not so great but the matter requires study. 

 This factor may be called weathering availability. 



(4) The nature of the plant. Plants differ in both their 

 capacity for absorbing food and their need of it. Whatever the 

 cause of these differences, there is no doubt but that they exist. 

 We will call this factor physiological availability. 



The character of the soil, its chemical composition, the condi- 

 tions which prevail during the growth of the plant, and perhaps 

 other factors influence the amount of plant food taken up. 



Factors Influencing the Composition of the Soil Extract. 1 The 

 amount of phosphoric acid extracted from the soil by a given 

 solvent is the difference between that dissolved from the phos- 

 phatic or potash mineral and that absorbed by the fixing particles 

 of the soil. That is to say, the soil extract does not necessarily 

 represent the solubility of the mineral exposed to the action of the 

 solvent, but is the resultant of the solvent and fixative 

 forces. Furthermore, the quantity of phosphoric acid exposed 

 to the action of the solvent depends upon its con- 

 dition in the soil and the solubility of protecting material in the 

 solvent used. If the phosphate mineral is enclosed within quartz, 

 1 Texas Bulletins 126-145. 



