148 THE STU DY OF PLANT COMMUNITIES • Chapter VII 



dug some four to six feet long, and wide enough to stand in com- 

 fortably. One face is kept vertical and cut cleanly to observe the 

 horizons— and possibly the root distribution. Depth of the pit is, 

 of course, determined by local conditions and position of the par- 

 ent material. 



Soil-Plant Relationships.— Soil must provide plants with an- 

 chorage, a supply of water, mineral nutrients, and aeration of their 

 roots. Not all plants require these essentials to the same degree, but 

 unless all are present to some extent the average plant cannot be- 

 come established. On this basis, soil has four major components : 

 (1) mineral material derived from parent rock, (2) organic sub- 

 stances added by plants and animals, (3) water, and (4) soil air. 

 These components vary in amount and proportion from place to 

 place, and the variation may be a significant factor in determining 

 the distribution of species and vegetation types. 



Local Soil Variations.— Size of soil particles (soil texture) and 

 shape of particles, which determines how they fit together (soil 

 structure), may vary markedly within short distances. Texture 

 and structure primarily affect the plant through their influences 

 on air and water in the soil. Organic materials, in addition to 

 modifying soil structure, are the source of plant nutrients that 

 may be quite unavailable from mineral sources. 



These variables are a product of the manner in which the soil 

 originated and the time involved in its development. Great areas 

 of the earth are covered with soils that overlie the parent rock 

 from which they were formed. These are sedentary soils, whose 

 materials are termed residual, if of mineral origin, or cumulose, 

 when deposited as organic matter. If soil material has been brought 

 to its present location by some agency such as wind, water, grav- 

 ity, or ice, it is said to be transported and will accordingly have 

 distinguishing characteristics. 



Soils Formed in Place — Residual materials are most weathered 

 at the surface and become progressively more like the parent rock 

 with increasing depth. Where parent rocks differ in hardness or 

 solubility, the resulting soils will differ. Fine-textured clayey soils 

 may represent the leached residue of easily soluble rock, such as 

 limestone, or may be the individual particles that made up a fine- 

 grained hard rock. When the parent rock contains a high propor- 



