Sources of the Constituents of Minnesota Soils. 393 



The part of plants in soil formation. Plants are builders of 

 soils ; such is not always their office. They are also destructive 

 in their action, disintegrating rocks by keeping the surface moist, 

 — water being a medium of chemical action, and in itself a uni- 

 versal solvent. Plant decay furnishes an important source of 

 organic acids. These acids in their action upon minerals and 

 rocks are as vigorous as sulphuric or nitric acid of soils. Roots 

 of plants — especially of trees and shrubs — pry off blocks of rock, 

 separating them from the ledge and thus extend the rock sur- 

 face over which corroding influences can work. 



Effect of moisture in soils. — A practical note may be added 

 touching the presence of moisture in soils : Cultivation varies that 

 ingredient in all soils ; experiment has shown this. The Michigan 

 Agricultural College during the past few years, has been con- 

 ducting a series of experiments upon different kinds of soils, and 

 the following result is reached : The steady loam soil that is cul- 

 tivated has three per cent, more moisture than that which is mere- 

 ly raked ; and that which is raked, has one per cent, more moisture 

 than unworked soil. This is a strong argument put in a strong 

 way in favor of industry and care on the part of the farmer who 

 has planted his crop. 



The chemical elements in soils. — Dr. T. Sterry Hunt calls 

 attention to the providential fact that those substances which the 

 waters percolating through the superficial rocks take out and car- 

 ry away are the substances which plants do net want, and the very 

 substances which the waters leave are those which plants want 

 and must have in their growth. ''Drainage waters of soils, " he 

 says, "like that of most mineral springs contain only carbonate?, 

 chlorides and sulphates of lime and magnesia and soda, the 

 ammonia, potash, phosphoric and silicic acids being retained by 

 the soils."''' Briefly stated, those elements out of which are made 

 the salts so essential to growing plants is in part the air, but in 

 larger part the porous rocks of the earth's superficial crust 

 through which the waters percolate, and from which they dissolve 

 some essential ingredients. The number of these elements is not 

 large. They are all grouped together imder the general name 

 foods. Some are used directly, others indirectly; some perform 

 one office in the plant economy, others another and widely dift'er- 

 ent one ; some can be used immediately by the plant protoplasm 



• Chemical and geological cssars, The Chctnistry of Mctamorphic rocks, 2nd 

 cd., 1878, p. 22. 



