THE COLLOIDAL MATTER OF SOILS 165 



colloid development is high adsorptive power. This 

 power extends not only to condensation of gases, but also 

 to water and to materials in solution. The water of 

 condensation on dry soil particles when exposed to a 

 saturated atmosphere is largely determined by the col- 

 loidal content. In other words, the surface exposure of 

 colloidal matter is so preponderant in water condensation 

 as in a general way to allow the one to be a relative meas- 

 ure of the other. Again, colloids exert adsorptive power 

 for material existing in the soil water, and to a limited 

 extent compete with the plant for food. Until the col- 

 loids are satisfied the soil solution may not reach its 

 maximum concentration for crop growth. This adsorp- 

 tive power is exerted especially on the basic materials, 

 such as calcium, and unless the existing colloids are fully 

 satisfied the soil tends to become lacking in available 

 bases. This condition is generally termed soil acidity. 

 It may readily be seen that the concentration of the soil 

 solution is governed to a considerable extent by the col- 

 loidal content of the soil, and that the adjustments in 

 concentration are always toward an equilibrium between 

 the two. Colloidal matter does not exert the same adsorp- 

 tive power for all materials, but is capable of what might 

 be called selective adsorption. For example, if ammonium 

 sulfate is added to a soil, the ammonia is strongly taken 

 up, which tends to release the sulfate. The continuous 

 use of such a fertilizer on a soil poor in lime will ultimately 

 result in the presence of free sulfuric acid. This example 

 is sufficient to emphasize the relationship of adsorptive 

 powers to fertilizer practice. 



110. Factors affecting colloids. — It must not be in- 

 ferred from the preceding discussion that the generation 

 of colloids is detrimental to soil conditions. In light 



