76 The Substratum 



Action of Organisms on Soil 



Having seen the many ways in which the nature of the land sub- 

 stratum may influence the hves of organisms, we now may inquire to 

 what extent the action is reversed. The fact is soon revealed that 

 animals and plants play a very important part in modifying their sub- 

 stratum on land just as they do in water. This activity on the part 

 of terrestrial organisms is particularly striking in relation to the forma- 

 tion and development of soil. It has been truly said that if it were not 

 for organisms there would be no soil— at least none of biological im- 

 portance. The soil is an outstanding example of the result of the 

 organism and the environment acting as a reciprocating system. 



Abundance of Organisms in Soil. The great abundance of organ- 

 isms which live wholly within the soil and the far-reaching extent of 

 the underground parts of organisms are not always appreciated. 

 Anyone who spades up a garden or transplants a shrub should be 

 impressed with the number of roots and the bulk of the root systems 

 of even small plants. The roots of the typical plant are so finely 

 divided and subdivided into rootlets and root hairs that a tremendous 

 surface is provided for the exchange between the organism and its 

 surroundings (cf Weaver, 1947). Roots are frequently sufficiently 

 abundant to produce a continuous mat or network extending several 

 feet into the soil. The root system of a maize plant may extend over 

 1 m laterally and 2^/4 m deep. The roots of 17-year old apple trees 

 were found to have occupied all the soil between rows 10 m apart 

 and to have grown to a depth of 10^ m (Weaver and Clements, 

 1938). 



In the animal kingdom the number of species that burrow through 

 the soil and thus influence it is also very large. Burrowing rodents 

 and moles of one kind or another exist almost everywhere, and they 

 are much more abundant than is generally realized. Although in 

 some instances the actual number of the larger forms may not be 

 impressive, their digging activities may be remarkably extensive. 

 Prairie-dog burrows more than 4 m deep have been reported. In 

 certain parts of California systematic trapping has shown that as 

 many as 50 mice inhabit each hectare (2^/4 acres) under normal con- 

 ditions. Periodically, as we shall see later, the rodent population 

 tends to increase greatly in numbers. Even when the population of 

 mice and other burrowing forms is at low ebb, a considerable in- 

 fluence on the soil may be produced in the course of a year. In addi- 

 tion to a variety of mammals, many kinds of reptiles and amphibians 



