THE MONTHLY BULLETIN. 255 



lands. It becomes very necessary to keep the soil in such condition 

 that bacteria will thrive, and especiall}^ to preserve the top layer of 

 soil which, according to present teachings, is literally the cream of 

 the land. The theory that the soil which lies deep needs to be brought 

 to the surface has been given many hard knocks by the bacteriologist 

 for he has proved that the home of bacteria is humus, the debris of 

 decayed organic matter, and surface soil alone contains humus. 



Several years ago the supposition arose that soil-sterility is often 

 determined by the excretions of growing plants, wliich may poison the 

 land for some crops Ijut whieli may be innocuous to other plants that 

 follow. "While there is much to show that plants do give off harmful 

 excretions, or in some way cause soils to become more or less toxic, yet 

 the statement made by some iiivestigators that toxicity is a prime factor 

 in soil-fertility has not yet gained general credence. The importance 

 of toxicity of soils is denied by chemists in particular. 



Just now the worldwide audiences in soil-fertility have closed their 

 ears to the ni'guments of ch(Mnists and physicists over their differences 

 to listen to the zoologist who has brought his science into play to solve 

 the problems of soil-fertility. According to the zoologist the few inches 

 of humus-filled crust on the earth's surface is veritably a jungle — a 

 jungle filled with one-celled plants, bacteria, which are preyed upon 

 by one-celled animals, protozoans. Now those voracious protozoans 

 may devour so many of the bacteria engaged in changing the nitrogen 

 of the air and soil into available plant food that their number is 

 lessened to a harmful degree. The predaceous protozoans, however, 

 can be removed by suitable treatment, as by chemicals or heat to steril- 

 ize the soil so that the protozoans, but not all the bacteria, are killed. 

 Delivered from the mouths of their enemies, the ammonia-producing 

 bacteria increase with tremendous rapidity and a higher level of soil- 

 fertility is ffuickly attained. Here at last we have a reason as to Avhy 

 sterilizing the soil, an operation practiced in one way and another since 

 before Christ's time, is beneficial to the soil — another linking of a 

 traditional practice of agriculture with a new discovery in science. 

 The value of soil-sterilization in flower and vegetable-o'rowing under 

 glass has long been demonstrated ; it is possible that we shall sometimes 

 nse it in the fields in our efforts to increase soil-fertility. 



Even more recent than the discoveries of the biological aspects of 

 the soil are those which deal with the phenomenon of adsorption. The 

 classical illustration of adsorption is made by pouring a solution of a 

 dye in a long tube filled with chalk. The dye is separated from its 

 solvent and is held by the chalk, the clear solvent showing at the lower 

 end of the tube. It has long been known that when solutions of min- 

 erals are poured on soils, the dissolved salt is adsorbed by the soil 

 particles. One sees at once that the economic use of commercial fertil- 

 izers depends largelj^ on adsorption. But the physical chemist has 

 discovered new and even more important phases of adsorption. 



It is now well demonstrated that soil particles not only adsorb sub- 

 stances but also exercise selective adsorption whereby the constituents 

 of chemical compounds are adsorbed at different rates. Thus, if 

 Potassium chloride be added to a soil, potassium is adsorbed more 

 rapidly than hydrochloric acid. That is, the soil particles possess 

 themselves of the potassium and the hydrochloric acid reacts with other 



