170 PHARMACEUTICAL BACTERIOLOGY 



yield, due to the coagulation of some of the colloids. The time honored 

 practice of burning over soils excessively rich in organic colloids (the rice 

 lands of India and other countries) results in increase in yield through a 

 reduction in the organic colloids. As is well known the rich soils of 

 California (adobe soils) are much improved by adding lime which has the 

 effect of coagulating colloids. The addition of sand to heavy rich soils is 

 beneficial because it dilutes the colloid^ and renders some of them harmless 

 through adsorption upon the sand particles. Adding phoshates to soils 

 rich in colloids is apt to result in souring the soils because of the increased 

 solution of the colloid (by the process of hydration). Following the phos- 

 phates with lime will prevent the objectionable changes through the coagu- 

 lation of the colloids, as already indicated. 



As the colloids increase beyond the optimum in wet soils, mold and 

 rotting bacteria gain the ascendency and soil souring follows, and this 

 souring still further increases the rate and degree of objectionable col- 

 loidal hydration. It is indeed true that the water content of the soils is 

 regulated through those colloids (organic as well as inorganic) which are 

 hydratable, that is which have a very marked affinity for water. As is 

 well known, gravelly and sandy soils do not hold the rain moisture, nor do 

 they readily draw the moisture from the depths below, unless perchance 

 the sand is exceedingly fine. 



One of the important agricultural qualities or properties of colloids 

 is their power of absorption; that is, the power of accumulating and 

 holding upon their surfaces, the various substances with which they are 

 more or less closely associated and which are required by the soil bacteria, 

 such as water and chemicals inclusive of the smallest colloidal particles. 

 Unfortunately, they adsorp noxious gases, soil toxins, bacterial toxins, 

 harmful chemicals, and other agriculturally objectionable substances, 

 with equal facility, should any be present. It is the work of the farmer to 

 so regulate and adjust the cultural operations as to render impossible, or 

 to reduce to a minimum, the solution and adsorption of harmful substances. 



The growing crop plant cannot utilize the larger colloidal particles 

 direct. The rotting bacteria, assisted by a variety of enzymes, transform 

 the colloidal masses and larger particles into smaller particles, which in 

 turn are seized upon by the bacteria of groups (2) and (3), converting them 

 into colloidal particles small enough to be used as food material by the 

 growing plant. The efforts of the modern agriculturist have to do, either 

 directly or indirectly, with the formation and transformation of soil 

 colloids, rendering these colloids available for the growing crop plant. 

 The change from mature plant represented by straw, hay, oats, corn etc., 

 to manure, and from manure to available plant food, and from available 

 plant food back into mature crop plant, is one continuous kalaidoscopic 



