1118 EXPERIMENT STATION RECORD. 



reasonable certainty be considered as not in need of additional amounts of as- 

 similable potash. The presence of a relatively high content of carbonate and 

 humate of calcium (not less than 0.2.5 per cent) affords assurance that the soil 

 is not deficient in easily soluble lime compounds. 



Absorption of vapors and gases by soils, H. E. Patten and F. E. Gal- 

 lagher (U. 8. Dept. Agr., Bur. Soils Bid. 51, pp. 50, figs. 19). — This bulletin 

 reviews previous investigations on the absorption of water vapor and of gases 

 and gives the results of original investigations by the authors on the absorp- 

 tion of water vapor by various types of soils under varying conditions. The 

 data contained in the bulletin are summarized as follows: 



" In the absorption of water vapor by quartz flour, a soil separate, and typical 

 soils, the rate of approach to equilibrium between soil and water vapor has been . 

 followed at various degrees of humidity, and these equilibrium points deter- 

 mined. 



" The amount of water absorbed increases with the hinnidity, but not in a 

 simple mathematical relation. 



" Toluene and other vapors were compared with water vapor, using the same 

 soils, and show in general an absorption of the same order of magnitude. 



" The velocity of absorption decreases regularly as absorption proceeds, but 

 it is hot described l)y a simple logarithmic equation. 



" Relatively small decrease in vapor pressure produces a marked increase in 

 the rate at which the soil dries out. At the moisture content known as 

 ' optimum ' for plant growth, a rapid decrease in the rate of evaporation talces 

 place, and at this particular moisture content there are likewise changes in 

 other physical characteristics of the soil, such as specific volume, resistance to 

 penetration, etc., which altogether point strongly to a purely physical reason for 

 the existence of a narrow range of water content in a soil at which plants thrive 

 liest [E. S. R., 19, p. SIS]. 



" The content of water in a soil at which plants begin to wilt is greater than 

 the quantity of water the soil can take up from a saturatetl atmosphere. A 

 heavy soil containing a high percentage of moisture may hold the water so ab- 

 sorbed that while the plant can get some waten- still it is by no means fully 

 supplied. 



" The absorptive capacity of soil for water vapor is generally higher the finer 

 the texture of the soil and the greater the content of humus. In general, pro- 

 ductive soils have a very considerable capacity for water vapor. 



" Gases are absorbed to a ditt'erent degree by each solid substance; the differ- 

 ent soil coniix»nents all possess a considerable absorption capacity, and mixtures 

 of these constituents absorl) gases additively: that Is, each soil material exerts 

 its absori»tive effect indei)endently of the rest of the soil about it. Soil c(mstit- 

 uents moistened with water absorb gases in greater quantity than would the 

 same mass of water alone. I'asily condensed gases are in general absorbed 

 more easily. 



" Heat is evolved during the process of absorption, and this heat is greatly in 

 excess of that given out by the condensation of the vapor to a liquid. 



" For equilibria between soils and atmosphere saturated with water vapor 

 over a temperature range from 25° C. to 100° C, the amount of water absorbed 

 decreases with increasing temjjerature. This confirms the results obtained by 

 earlier investigators for the absorption of water vapor as well as for gases in 

 general. 



"Absorbed gases are held with remarkable tenacity, (ilass and minerals re- 

 tain hygroscopic water up to 500° to 800° C. 



