40 MISC. PUBLICATION 3 69, U. S. DEPT. OF AGRICULTURE 



Summary of Effects of Soils on Plant Composition 



Investigations designed to study the effect of the single factor, 

 soils, on the composition of plants are rare indeed, which is unusual 

 when one realizes for what length of time the importance of this one 

 factor has been understood. Probably not more than one or two field 

 experiments and a small number of greenhouse experiments may really 

 be considered to have demonstrated that differences in soils alone are 

 responsible for differences in such constitutents as calcium and phos- 

 phorus in the plant. 



It is recognized, of course, that there are two approaches to any 

 study of the capabilities of soils to supply the proper amount of min- 

 erals to plants. First, a field study of the soil in its natural environ- 

 ment over a period of several years will indicate how that soil, in the 

 sense that the soil is a summation of its environmental factors, in 

 relation to all other soils in that or other localities, will affect plant 

 composition. In such studies the elimination of climatic and physical 

 differences is not desirable. The advantage of such a study is that 

 information is obtained as to the properties of the one soil being 

 studied in relation to all other soils. The disadvantage of such a 

 study lies in the fact that since several variables are not accounted 

 for or eliminated, no predictions based on any one or several factors 

 may be made concerning the behavior of this one soil type in another 

 locality, even though it is recognized that over a long period of time 

 the climate will not differ greatly on different areas of the same soil 

 type. Thus, if a plant grown on a certain soil be deficient in cobalt, 

 that deficiency may be due, among other causes, to an inherent defi- 

 ciency or unavailability of this element in the soil, or it may be due to 

 climatic factors. If the first factor be the cause, one would natu- 

 rally suspect that that soil type would be deficient in cobalt wherever 

 found. If the latter factor be the cause, one would expect that, over 

 relatively short periods of time, this soil type in some localities would 

 support plants with a normal quantity of cobalt. If one wished, 

 therefore, to determine the effect of soil alone on plant composi- 

 tion, that soil must be compared with others under exactly the same 

 conditions of climate, relief, and management practices. 



Climate and soils are, moreover, so interrelated that greenhouse 

 tests or other methods designed to bring soils together under the same 

 conditions cannot be depended upon to give accurate information as 

 to the ability of the soil in situ to supply the proper amounts of any 

 nutrients required for the development and health of the plant and the 

 animal or human consuming it. It must be recognized, however, 

 that in spite of their limitations, translocation tests of soils, whether in 

 greenhouse or open air, are the only means of differentiating between 

 effect of soil and climate, unless the soils are situated in such a way 

 as to eliminate climatic and other factors. While such tests are of 

 limited value in determining differences in soil, they are of much 

 greater value in studying the means of modifying with amendments 

 soils that fail to support plants of the highest nutritive value. 



There is an abundance of evidence that plants and their parts vary 

 greatly and significantly in the principal elements when grown on 

 different soils in different environments, or even in environments that 

 differ only in small degree. 



