The Plant as Related to the Soil 49 



roots are said to spread through an area equal to that 

 shaded by the branches. Only in exceptional conditions 

 do the roots extend very deeply into the soil. Even in 

 forest trees, the most vigorous roots are found in the 

 first foot or two of soil. In young trees, the tap-root 

 is often noticed to grow directly down for some distance, 

 but, when the trees are old, the side roots will be found 

 to be many times larger. (See Fig. 31.) 



79. The Total Length of the Roots is very great. Hell- 

 riegel* noted that a vigorous barley plant in a rich porous 

 garden soil had one hundred and twenty-eight feet of 

 roots, while another growing in coarse-grained, compact 

 soil had only eighty feet of roots. One-fortieth of a cubic 

 foot sufficed for these roots. It may be readily under- 

 stood that all the soil was occupied. Professor Clark, 

 after making a number of measurements, estimated that 

 a vigorous pumpkin vine had fifteen miles of roots and 

 gained one thousand feet per day. Professor King, of 

 the Wisconsin Experiment Station, estimates that if 

 all the roots of a vigorous corn plant were put end- 

 to-end they would measure more than one mile in 

 length. 



80. The Vertical Distribution of Roots is affected 

 to a large extent by the depth of the plow line, particu- 

 larly so on stiff clay soils. The roots extend much deeper 

 in dry seasons than in wet ones. These facts have been 

 found out by carefully washing the soil away from the 

 roots, leaving them supported on poultry netting. 

 These observations are easily explained when we con- 

 sider the effect of tillage on soil conditions. Fig. 32 



*Herman Hellriegel (1831-1895) devoted his life to the study of the 

 chemistry of plant nutrition. He was the first to discover the relation of the 

 bacteria causing the tubercles on the roots of legumes to the fixation of free 

 nitrogen. He made many other important discoveries in agricultural science. 



