THE SOIL AND ITS IMPORTANCE TO PLANTS 31 



The solvent power of soil water is increased by the presence 

 within it of carbon dioxide, liberated in the respiration of plant 

 roots and of the lower organisms. Thus reinforced, water not 

 only attacks the surfaces of the rock particles but absorbs any 

 soluble material which may appear in the humus or as a product 

 of bacterial activity. 



There is a great variety of substances present in the soil 

 solution, and we know from chemical analyses of the ash* of 

 plants that very many of them may be taken into the plant body. 

 Compounds of nitrogen, sodium, potassium, calcium, magnesium, 

 iron, manganese, aluminum, phosphorus, sulphur, chlorine, and 

 silicon are commonly absorbed by the roots, and many others 

 may be taken up occasionally. Certain of these elements are 

 far more important to the plant than others, however, and it has 

 been clearly proven by experiment that seven are essential for 

 normal plant growth: Sulphur, phosphorus, calcium, magne- 

 sium, potassium, iron, and nitrogen. The actual amount of these 

 mineral nutrients taken up by the plant is exceedingly small in 

 proportion to the size of the plant body, but in the activities 

 of protoplasm each plays a very necessary part, and a soil which is 

 deficient in any one of them will be unable to support vegetation 

 successfully. 



The removal of large amounts of nutrient materials from agri- 

 cultural soils, in the form of crops and in other ways, reduces the 

 available supply of certain chemical elements, notably nitrogen, 

 phosphorus, and potassium, to such an extent that a fresh supply 

 must be returned to the soil if abundant plant growth is to be 

 maintained permanently thereon. This necessitates the common 

 practice of adding to the soil various tj^pes of fertilizers which 

 renew the supply of essential salts there available to plants. 



Organisms. — Aside from its service as a medium for the root- 

 growth of higher plants, the soil provides a dwelling-place for a 

 great variety of other organisms, whose activities have a profound 

 effect on the composition of the soil and on the processes which go 

 on therein. Rodents, insects, and angleworms all modify the 

 physical character of the soil by their abode within it. Those 

 most minute and lowly of living things, however, which we 

 group together as micro-organisms are of far greater importance, 



* The ash of plants is the residue left after complete combustion of the 

 plant tissues, and an analysis of it indicates the amount and character of 

 mineral substances present in the plant. 



