PLANT NUTRIENTS AND THEIR ABSORPTION 107 



rather than direct, for the displacement elements may combine to form 

 more soluble salts and thus be rendered more available. 



As an instance, according to Loew:^-* "Lime and gypsum can also in cer- 

 tain cases release such potash in the soil as is still unavailable. This, as well as 

 the enhanced root-hair production under the influence of the increased amount 

 of ume, accounts for the greater absorption of potash by the plant on soils rich 

 in lime." 



Displacement would of course be of little value to the plant if the 

 elements released were washed from the soil as a result of the greater 

 solubility of their salts. 



Availability of Ash Constituents. — The soil constituents are of use 

 to the plant only when combined in certain specific chemical compounds. 

 Thus, sulphur must be present as sulphate, phosphorus as phosphate, and 

 the various bases as relatively soluble salts. 



Availability and Solubility Distinguished. — Solubility, however, is 

 only the first prerequisite to availability and absorption; it is not an 

 absolute criterion of the crop-producing power of soils, as is indicated by 

 studios on many soils in this country^^ and by investigations on the red 

 soils of the "djati" forests of Java.^^ Nevertheless, "in general it can 

 be said that a very heavily fertilized or extremely rich soil gives a greater 

 solubility product than an unfertilized or poor soil."^^ Conversely 

 "as a result of laboratory studies it appears that the constituents of 

 soils which have been cropped for a long period of years go into solution 

 at a somewhat slower rate than do those of the corresponding virgin 

 soils. "^^'^ 



Factors Influencing Solubility. — The solubility of soil ingredients is 

 affected by such factors as temperature, moisture content, chemical 

 composition of the soil and root activity. According to McCool and 

 Millari3o the rate of solution is more rapid at 25°C. than at 0°C. The 

 concentration of the soil solution apparently depends also on the 

 relative masses of the soil and water. 



"At the ratio of 1 of soil to 5 of water the rate of solubility of natural soils 

 is also slow and the extent of solubility extremely small. In fact, the amount of 

 material that went into solution at this water content is only about half as much 

 as that at the water content of 1 of soil to .7 of water, and yet an apparent 

 equilibrium was attained. . . . The amount of material that goes into solution 

 seems to increase as the ratio of soil to water is increased up to about the opti- 

 mum moisture content and then it decreases."^* 



The effect of chemical composition on solubility is discussed by 

 Bouyoucos.^^ 



"As a whole it appears that the phosphates tend to depress solubihty and that 

 they probably act as conservers of bases under field conditions." Other salts, 



