142 ENVIRONMENTAL CONDITIONS. 



those that are effective below. All of our categories of external condi- 

 tions are effective both above and below the soil surface, excepting 

 light alone (as far as we now know), but it seems especially profitable 

 to consider this classification with reference to chemical conditions. 



The chemical conditions above the soil surface are characterized by 

 a striking and almost complete uniformity and symmetry. The air 

 of different regions and of different habitats comprises practically 

 always the same gases, and these, with the exception of water-vapor, 

 occur with but little variation in their partial pressures. Of the sub- 

 stances influencing plants, other than water, carbon dioxid exhibits 

 the greatest variation, but even this variation appears to be, compara- 

 tively speaking, of but little account. We may safely conclude that 

 few plants in nature are ever appreciably influenced by variations or 

 differences in the quality or intensity of the chemical environment 

 above the soil surface.^ Small supplies of ammonia and inorganic 

 salts may reach the plant from its aerial environment, but with these 

 generally insignificant phenomena of absorption we need not deal here. 



When we turn our attention to the soil, we find a very different 

 state of affairs. Every soil differs chemically from every other soil, 

 the soil solution varying between wide limits in the nature and amount 

 of solutes present.^ At one extreme of the series are thoroughly 

 washed sands, in which are almost no dissolved material; at the other 

 extreme are alkali soils, which are highly impregnated with soluble 

 inorganic salts. In the middle region between these extremes, in 

 most ordinary soils, it appears that the quality and concentration of 

 the soil solution are without very great differences as far as inorganic 

 compounds are concerned, but that these ordinary soils show very 

 great differences in the kinds and amounts of organic matter present.^ 



In spite of the great amount of work that has been devoted to the 

 problems of the soil, the whole question remains as one that has 

 hardly been really touched in a way to be of any present aid in problems 

 of plant distribution. Of course, our general knowledge of the paucity 

 of soluble matter in a few sands and of the superabundance of certain 

 compounds in alkali soils is of definite value in this regard; but even 

 here the strictly quantitative aspect of our problems remains wholly 

 for the future to develop. 



' If atmospheric ionization should prove an important chemical feature influencing plants in 

 nature, and if this varies from place to place and from season to season, then this statement may 

 require modification in this regard. See report of Spoehr's work in MacDougal, D. T., Annual 

 Report of the Director of the Department of Botanical Research, Carnegie Inst. Wash. Year 

 Book No. 13, 87-88, 1915. 



^ Cameron, F. K., The soil solution, the nutrient mediiun for plant growth, Easton, Pennsyl- 

 vania, 1911. 



' Livingston, Britton, and Reid, 1905. — Livingston, 19076, Schreiner, O., Organic compounds 

 and fertilizer action, U. S. Dept. Agric, Bur. Soils Bull. 77, 1911. — Schreiner, O., and E. C. Lath- 

 rop, Dihydroxystearic acid in good and poor soils. Jour. Amer. Chem. Soc, 33: 1412-1417, 1911. 

 Also numerous other papers from the Bureau of Soils, U. S. Department of Agriculture, deal 

 with this matter. 



