Soil and Its Action on Plants 75 



of this substratum and the degree to which its physical characteristics 

 are bound up with its chemical and its physicochemical features. 

 Certain of the latter will be further discussed in relation to other 

 ecological factors considered in later chapters. 



When gross differences in the land substrata exist, factors limiting 

 plant growth can frequently be distinguished. On a solid rock sub- 

 stratum lichens and certain mosses are characteristically the only 

 plants that can survive. In situations with a coarse, shifting sub- 

 stratum, like a sand dune or a gravel slide, the vegetation is limited to 

 specially adapted forms. Dune grasses with their network of hori- 

 zontal rhizomes, and certain other plants, such as ParonycJiia, with 

 strong and extensive root systems are among the few plants that can 

 maintain a foothold. When the soil is very hard owing to a high silt 

 and clay content or to the development of a hardpan, the roots of many 

 species cannot penetrate. At the other extreme the presence of very 

 soft soils prevents the establishment of plants that require firm anchor- 

 age. Many evidences of this relation were seen in New England 

 after the hurricane of 1938 when whole groves of trees on loose soil 

 were uprooted but neighboring groups of the same species on hard 

 ground remained standing after the storm. 



In other situations a gross difference in such factors as the chemical 

 composition, moisture, or temperature of the soil may be distinguish- 

 able as the prime influence controlling the vegetation, and examples 

 of these will be considered later in the appropriate chapters. Too 

 often for the peace of mind of the ecologist, however, very compli- 

 cated or subtle diflFerences occur in the soil, and frequently two or 

 more interdependent factors appear to act mutually in limiting plant 

 growth. More detailed treatments of the ecology of soils, such as 

 those referred to earlier in this section, should be consulted for a 

 further discussion and examples of situations of this type. In some 

 habitats investigators disagree not only as to which of the soil influ- 

 ences is critical in determining the composition of the vegetation but 

 also even as to whether the climatic factors are not more important 

 than the edaphic (soil) factors. 



These ecological relations in the soil also frequently illustrate the 

 principle of partial equivalence: an increase in one factor may some- 

 times partially make up for a deficiency in another factor ( Allee et al, 

 1949, Ch. 16). The lack of moisture in a sandy soil may be compen- 

 sated for to some extent by a greater rainfall in certain localities, or, 

 conversely, plant species for which a given region is generally too 

 humid may find the effective moisture conditions sufficiently reduced 

 in a local area with a sandy substratum. 



