EXPERIMENT STATION BULLETINS. 289 



RATE AND EXTENT OF SOLUBILITY OF ROCKS AND ROCK- 

 FORMING MINERALS UNDER DIFFERENT 

 TREATMENTS AND CONDITIONS* 



Technical Bulletin No. 50. 



BY GEORGE J. BOUYOUCOS, SOILS SECTION 



HISTORICAL 



In a former publication (1) the results of an extensive study on the rate 

 and extent of solubility of soils under different treatments and conditions 

 were presented. Using principally the freezing point method as a means 

 of measuring solubility, it was shown that when different classes of soil 

 were treated with N/iO salt solutions of C^a(N03)2, NaNOs, KNO3, KCl, 

 K2SO4, (NH4)2S04, MgS04, KH2PO4, CaH4(P04)2 and NaC2H302, and then 

 washed free of all soluble salts, and made up to a moisture content of 1 of 

 soil to 0.7 of water and kept at room temperature, the rate of solubility of 

 all the salt treatments; except (HN4)2S04, was slow and gradual and that 

 the process continued for a long time, (120 days), but usually for about 

 50 days. In the case of (NH4)2S04 the initial velocity was quite rapid but 

 soon slowed down. At the end of about 60 days there was an apparent 

 constancy or equilibrium in the solubilitj^ 'The extent of solubility at this 

 point was found to be very appreciable in all the salt treatments, and in 

 all the soils, with few exceptions. All the soils except sand, treated with 

 NaNOs, KNO3, KCl, K2SO4, (NH4)2S04, MgS04, and NaC2H302 yielded 

 quite a large amount of material to solution, the depression in many cases 

 rising from 0.005 to about 0.110°C or from 125 to 2,750 parts per million 

 of solution. The onlj^ salt treatments which did not cause a large solubility 

 product were Ca(N03)2, KH2PO4 and CaH4(P04)2, the depression in some 

 of these treatments rising only from about 0.007 to 0.015° or from 175 to 

 355 parts per milhon. As a general rule NaNOs and NaC2H302 in all the 

 soils, except sand, tended to vield the greatest solubihty product, Ca(N03)2, 

 KH2PO4, and CaH4(P04)2 the smallest, and KNO3, KCIK2, SO4, (NH4)2 

 SO4 and MgS04 an intermediate product. In many soils CaH4(F04)2, and 

 in a few cases, Ca(N03)2 and KH2PO4 did not only give the smallest con- 

 centration but even a smaller concentration than the check, indicating that 

 these salts have an indifferent or depressing effect upon the solubility of 

 soils. As a whole it appeared that the phosphates tend to depress solubility 

 and that they probably act as conservers of bases under field conditions. 



The results of solubility of these singly salt-treated soils were taken to 

 indicate that a salt or fertilizer treatment leaves a residual effect upon the 

 soil, and this residual effect continues to be manifested in increased solu- 

 bility and in increased crop-producing power. These data also go to indicate 



*The writer is very grateful to Mr. Ralph A. Paton for much valuable aid. 

 37 



