THE BOOT AND THE SOIL 



67 



103. The branching of roots in land plants appears 

 to depend much upon the amount of free oxygen (31) 

 and available plant-food which the soil contains, so long 

 as the moisture supply is sufficient. In cultivated ground 

 having a compact sub-soil the roots of annual crops usually 

 branch most freely just at the bottom of, or a little below, 

 the layer of soil stirred by the plow, this being the point 

 at which the supply of oxygen, plant-food and moisture 

 is probably best suited to root growth. As the depth 

 of tillage is increased, roots 



branch freely at a greater depth. 

 Masses of decomposed manure 

 beneath the surface of the soil 

 are usually penetrated through 

 and through with finely-branched 

 roots; and fragments of bone 

 in the soil are often inclosed in 

 a mat of delicate rootlets. These 

 materials furnish plant-food in 

 abundance. Roots that pene- 

 trate the deeper and more com- 

 pact layers of soil, on the other 

 hand, and those in poor and dry 

 soils, are usually little branched. 

 It is clear, therefore, that unless a soil is well aerated (93) 

 by a proper system of tillage, and by draining if need be, 

 and unless it contains abundant soluble plant-food in the 

 aerated part, the roots of plants growing upon it will 

 not branch freely and hence the plants cannot be well 

 nourished. 



104. Transplanting (400) and root pruning (416) 

 stimulate root branching. Removing the growing points 

 of either the stem or root (65) stimulates the develop- 



Fio. 31. — Pruning: show- 

 ing how root pruning stim- 

 ulates root branching. 



