THE ABSOKPTIVE PROPERTIES OF SOILS 273 



143. Importance of absorption. — ^Absorption is impor- 

 tant, not only becanse it allows tbe soil to retain certain nntri- 

 ents against excessive leaching, but because it facilitates the 

 condensation and concentration of gases within the soil.^ Rus- 

 sell and Appleyard^ have shown that the inner soil air is 

 held very tightly and must in consequence be under consid- 

 erable pressure. Such gas absorption tends to force reactions 

 which otherwise would be very slow. A part of the catalytic 

 power of the soil may be accounted for in this way. Moreover, 

 the absorption of water by the soil is by no means unimportant. 

 It is because of such phenomena that the moisture of the soil 

 occurs in various forms and possesses distinctly different re- 

 lationships to the plant. 



The selective absorption of the basic ions by soils of every 

 type is important in a number of ways. In the first place, 

 potassium, calcium, magnesium, and iron function in the soil 

 as bases. Selective absorption tends to conserve these nutri- 

 ents to the exclusion of their acid radicals, which are readily 

 lost in drainage. Phosphorus, however, has a different status, 

 for although it is held as a part of an acid radical (PO4) , it is 

 saved from leaching by the insolubility of the compounds 

 which tend to form. In the second place, selective absorption 

 apparently produces residues when fertilizers are added and 

 these residues are almost always acid. Sodium nitrate, am- 

 monium sulphate, potassium chloride, and potassium sulphate 

 will leave an acid residue in the soil solution unless influenced 

 by extraneous factors, such as the addition of lime or the ac- 

 tion of plants. 



^Patten, H. B,, and Gallagher, P. E., Adsorption of Vapors and Gases 

 hy Soils; U. S. Dept. Agr., Bur. Soils, BuL 51, 1908. 



McGeorge, W,, Absorption of FertiliBer Salts dy Bawaiian Soils; Haw. 

 Agr. Exp, Sta., BuL 35, p. 32, 1914. 



Cook, E. 0., Factors A ff eating the Absorption and Distribution of 

 Ammonia Applied to Soils; Soil Sci., "Vol. II, No. 4, pp. 305-344, 1916. 



^Eussell, E. J., and Appleyard, A,, The Atmosphere of the Soil: Its 

 Composition and the Cannes of Variation; Jour, Agr. Sci., Vol. VII, 

 Part 1, pp. 1-48, 1915. 



