18 THE SOIL AND THE PLANT 



able to make use of the vast store of gaseous elementary nitrogen, 

 but even here they require the cooperation of soil organisms. 



The elements phosphorus and sulfur are present in the soil 

 both in organic and in inorganic forms. They undergo various 

 transformations in the soil through the activities of microbes, 

 either directly or indirectly, before they can be assimilated by 

 plants. Here again, plants cannot assimilate appreciable amounts 

 of phosphorus either in the form of organic compounds or in the 

 form of insoluble rock phosphate. In the first case, it has to be 

 decomposed by microbes; in the second case, it is made soluble 

 by the various inorganic and organic acids formed by the soil 

 microbes. 



Potassium, iron, magnesium, and calcium are basic constituents 

 of soils which are required by plants in certain amounts. Potas- 

 sium frequently occurs in available forms in amounts in- 

 sufficient to satisfy the requirements of plants. Calcium and 

 magnesium are recjuired by plants in smaller quantities, but they 

 play important roles in the neutralization of soil acids and in the 

 coagulation of the dispersed soil particles. All of these elements 

 are obtained by the plants from the soil, and all of them are 

 changed from one form to another, directly or indirectly, by the 

 soil organisms. 



Higher plants may be grown in the absence of other forms of 

 life and in the absence of organic materials under artificial con- 

 ditions when they are supplied with the necessary salts as they are 

 required. Such conditions never develop in the natural habitats. 

 Here the soil organic matter acts as a reservoir from which many 

 of the elements required for development of the plant are supplied 

 throughout the growing season. The mineral fraction of the soil 

 slowly furnishes other elements to the soil solution, partly as 

 a direct result of microbial activities. This may be brought about 

 either through the solvent action of carbonic or other acids or by 

 the oxidation or reduction of these minerals. 



Summary. — Four distinct relationships between soils, plants, 

 and microbes should be considered: namely (1) the role of microbes 

 in soil formation and soil transformation; (2) the role of microbes 

 in the liberation of nutrients for plant growth; (3) the soil as a 

 medium for the growth of microbes; (4) the role of higher plants 

 in supplying nutrients for microbes. 



Through their activities, microbes liberate large quantities of 



