628 PRINCIPLES OF SOIL MICROBIOLOGY 



Colloids are coagulated, or flocculated, by means of electrolytes; 

 the univalent ions have a less intensive effect than the bivalent and the 

 latter less than the trivalent. Clay is an electro-negative colloid and 

 can be flocculated by positively charged ions; at a certain point it is 

 deflocculated by the negatively charged hydroxyl ion. This defloccu- 

 lation may be due to the lessened solubility of the di- and tri-valent 

 cations in the soil rather than to the direct effect of the hydrogen-ion 

 concentration. 18 Of particular importance is the action of calcium in 

 flocculating colloids. Sodium salts, notably sodium carbonate, serve 

 to deflocculate the soil colloids, although a sufficient concentration of 

 sodium ions may even cause flocculation. The flocculation of soil 

 particles is similar to the flocculation of suspensoid sols and is amenable 

 to the isoelectric theory, except in the case of lime. 19 Soil organic 

 matter has a protective effect on the flocculation of clay. 20 



Calcium carrying a double positive charge precipitates the negative 

 soil colloids, bringing about a change in the plastic properties of the 

 soil. The soil structure is thus changed entirely; the resistance to 

 penetration of moisture is reduced, an increase in pore space is brought 

 about, and there is an increase in the water holding capacity of the 

 soil. This improves the physical condition of the soil as a medium for 

 the activities of microorganisms. Drying of the soil also causes a 

 precipitation of colloids, but the change produced in the colloidal 

 soil is reversible. The change produced by lime is not reversible. 



The soil organic compounds contain reversible and irreversible 

 colloids. The addition of lime brings about an increase in the water- 

 soluble carbon compounds of the soil which favorably influences bac- 

 terial activities (in addition to favorable effect of reaction). This 

 leads to a greater decomposition of the soil organic matter with the 

 formation of C0 2 , NH 3 , nitrates and soluble phosphates. Thus the 

 application of lime leads to a neutralization of the soil acids, an in- 



18 Dayhuff, W. C, and Hoagland, D. R. The electrical charge on a soil col- 

 loid as influenced by hydrogen-ion concentration and by different salts. Soil 

 Sci., 18: 401-408. 1924. 



19 Comber, N. M. The mechanism of flocculation in soils. Trans. Faraday 

 Soc, 17: 349. 1922; Jour. Agr. Sci., 10: 425-436. 1920; Mattson, S. E. Die 

 Beziehungen zwischen Ausflockung, Adsorption, and Teilchenladung mit besond- 

 erer Berucksichtigung der Hydroxylionen. Inaug. Diss. Breslau. 1922; Kol- 

 loid Chem. Beihefte, 14: 227-313. 1922. 



20 Wolkoff, M. I. Studies on soil colloids. I. Flocculation of soil colloidal 

 solutions. Soil Sci., 1: 585-601. 1916; Oden, Sven. Die Koagulation der Tone 

 und die Schutzwirkung der Humussiiure. Jour. Landw., 67: 177-208. 1919. 



