10 PLANT SUCCESSION AND CROP PRODUCTION 



Man is by far the most important of the biotic agencies in altering 

 the appearance of the hindscape. Earthworms, burrowing beetles, 

 snails and plants with tap roots favor the circulation of air in soils 

 and its consequent more rapid oxidation to humus. Nitrogen gath- 

 ering bacteria are also to be included among the favorable organ- 

 isms whose effect can be recorded. 



In any of the broader groups of factors are a number of smaller 

 sets of factors. For example rainfall and humidity, (or saturation 

 deficit) of the air would be in the climatic group. Soil moisture and 

 water available for plant growth would be in the second group. In 

 the geographic distribution of plants the climate or the soils may 

 prove to be limiting factors which definitely determine areas be- 

 yond which certain plants are unable to advance. It is an abuse 

 of the word edaphic to suppose that it has reference to local condi- 

 tions. The effect of soils upon plant growth may be limited to 

 certain regions, however, since edaphic refers to the soil conditions. 

 The importance of the edaphic factors is often as great as the 

 climate group in detennining the success or the failure of certain 

 species. 



In the crop plants a fourth set of factors ; namely, the economic 

 group, is fully as important in determining geographic distribution 

 as the other three. For in addition to being limited to regions where 

 soil and climate are suitable or can be controlled to a certain degree, 

 such economic conditions as improvement in transportation facili- 

 ties, increase in population, land values and intensiveness of utili- 

 zation of land all affect crop production. Thus we see at once the 

 importance of removing the discussion from all static considerations 

 and using genetic and dynamic ideas of the development of our agri- 

 culture. In the natural vegetation the changes in conditions give 

 rise to successions or changes in the types of vegetation. In the 

 crop plants conditional control gives rise to changes in type of 

 farming. This analogy between vegetation and crops does not refer 

 to rotations. In rotations an attempt is made to prevent conditions 

 from retrogressing. The change in the type of farming is historical 

 just as successions are historical. 



The disappearance of plants from the vegetation as conditions 

 change may be referred to the operation of limiting factors. Some- 

 times moisture is the limiting factor, as with the desert vegeta- 

 tion. Or, it may be temperature as we progress northward. Two 

 or more limiting factors may work in combinations. With the crops 

 the limiting factors' may easily be from the economic group as well 



