THE MONTHLY BILLETIN. 21 



Why the Rate and Extent of Clay Formation Are Important in Regu- 

 lating Soil Depth. 



But as QUO of tlie well beloved instructors of my undergraduate days 

 was wont to remark this is "all porch to my liouse." Admitting the 

 interesting reactions just described which contribute respectively to 

 the slow and rapid formation of clay substances in the arid and humid 

 regions, how does that affect the depth of our soils and therefore those 

 radical differences Miiich distinguish them so sharply from soils of the 

 humid region? "Well, that is easily enough answered. Clay is a 

 substance of great lunding or cementing power besides being possessed 

 of many other striking characteristics of the utmost importance in 

 soils. But if that is true we should expect that the soil particles would 

 be more slightly cemented in the arid than in the humid region, and 

 that the spaces between the particles which admit of an easy flow of air 

 and water would be larger. As a nuitter of fact our expectations come 

 true. In soils in which clay forms more slowly the particles are much 

 less cemented than in those in which clay forms rapidly. Therefore, 

 soils of the former type must be looser and must partake, when con- 

 sidered by and large, of more of the sandy soil structure than of the 

 clay soil structure. Here also our reasoning serves us truthfully and 

 field observations confirm it. But why, you will ask, does a greater or 

 lesser amount of cementation or binding of the soil particles through 

 more or less clay formation so markedly influence the resulting soil 

 structure? The answer to that question is also very simple. It de- 

 termines to a very considerable degree the amount of air which can be 

 held by a soil, and still more markedly the amount of air which can 

 penetrate into the greater depths thereof. Yet, again, you may ask 

 how does this difference in air penetration account for the striking 

 differences observed between soils of the humid and arid regions? In 

 just this way. Air makes possible the proper activation of three of the 

 essential agencies to successful and healthy plant growth, viz, (1) proper 

 root development of plants, (2) proper development of the denizens 

 of the dark recesses of soils in numbers and efficiency — the bacteria, 

 fungi and others, and (3) the proper oxidation and solution of soil 

 minerals which are used as plant food. In just such measure, there- 

 fore, as air is admitted to the soil depths will conditions therein, other 

 things being equal, be made congenial for the development of these 

 powerful agencies in the regulation of a plant's development. More- 

 over, in just such measure as we contribute by artificial means to the 

 greater or lesser penetration of air into the soil depths do we contribute 

 to the greater or lesser efficiency of the agencies named; in the former 

 case augmenting, in the latter, defeating the purposes to whose end the 

 natural agencies described above are operating. From the foregoing 

 remarks we should expect, therefore, that in soils of the arid region, 

 owing' to slow clay formation, the interests of many generations of plants 

 are best subserved, because deep root development is made possible, 

 not only for the reason that the roots themselves can obtain the neces- 

 sary air supply, but because the other agencies named, and which are 

 concerned in large measure with the welfare of the plant, are also made 

 to operate efficiently. Conversely we should expect but a poor activa- 

 tion of these agencies in soils of the humid regions in which clav forms 



