2 THE SOIL SOLUTION 



no organic substance in the nutrient medium is necessary to the 

 maintenance of plant growth, nevertheless organic substances 

 are probably always present under natural conditions. They 

 may or may not be absorbed by the plant and may affect it 

 beneficially or otherwise. 



The study of the soil solution is of the first importance in the 

 investigation of the relation of the soil to plant growth, and in 

 the following pages there is given an outline of our present 

 knowledge of the chemical principles involved, with such dis- 

 cussion of the physical and biological factors as is essential to 

 an orderly presentation of the subject. 



To understand clearly the relations of the soil solution to the 

 soil as a whole and to the plant which it nourishes, it is desir- 

 able to consider some attributes of soils in general. Every soil, 

 no matter of what type it may be, is a complex system. In it 

 various processes are continually in operation, excepting possibly 

 in the extreme case when it remains frozen for a time at some 

 definite temperature. The resultant or summation of these 

 processes, whether expressed in plant production or otherwise, 

 will vary from time to time, both quantitatively and in direction ; 

 for instance, as to the amount and kinds of plant growth it 

 produces. That is to say, any particular soil area is seemingly 

 an organic entity, functioning according to its own inherent 

 properties, but subject to the modifying influences of environ- 

 ment, as by exceptional climatic extremes, flood, fire, and especi- 

 ally by artificially imposed agencies of control. 



From the practical point of view the problem of the soil in 

 its relation to crop production is like the problem of the factory 

 or of any other industrial endeavor, in that it is a problem of 

 management or control. The soil possesses this distinction, 

 however, that it is both the raw material and the factory. 1 The 

 processes involved are physical, chemical and biological, are 



1 According to S. W. Johnson Some points of agricultural science, 

 Am. Jour. Sci. (2), 28, 71-85 (1859) "The soil (speaking in the widest 

 sense) is then not only the ultimate exhaustless source of mineral (fixed) 

 food, to vegetation, but it is the storehouse and conservatory of this 

 food, protecting its own resources from waste and from too rapid use, 

 and converting the highly soluble matters of animal exuviae as well as of 

 artificial refuse (manures) into permanent supplies." 



