42 SOIL CONDITIONS AND PLANT GROWTH 



simultaneously with those in sand gave a very different result 

 owing to interactions in the soil which will be discussed later. 

 It is evident, however, that the soil must be studied dynami- 

 cally and not statically. While the curve is still of the 

 general type it differs from those obtained in sand in that 

 there are now a number of factors concerned and the final 

 result cannot be expressed by a surface or by a series of 

 surfaces, but would involve something equivalent to projection 

 into an nth dimension, an operation not within the scope of 

 ordinary simple mathematics. 



The general result of the quantitative work is that the 



/ limiting Factor. 





Increment of Factor 



FIG. 7. General relation between any particular factor and plant growth. 

 An increment in the factor causes increases in growth up to the point when 

 some second factor sets a limit ; further increases then have no effect. 

 Finally, excess of the factor may cause positive injury. 



numerous soil factors 'involved in plant growth can be dis- 

 entangled and separately studied. The operation of each 

 factor is expressed by a curve which would probably be like 

 Figs. I or 3, flattening out and finally bending over when 

 larger amounts of the factor exert a harmful effect. In actual 

 natural conditions the case is complicated by the fact that two 

 or more factors are almost always concerned, one of which 

 acts as a limiting factor. The curves are of the general type 

 of Fig. 7. 



Any possibility of simple mathematical treatment of the 

 amount of growth in soil is ruled out by the circumstance that 

 the various factors act not only on the plant, but also on the 



