HEART-ROT 119 



The discussion must thus be carried back a step farther, 

 viz. to a consideration of the agencies which facilitate the 

 aeration of the subsoil in the cases of cultivated land and 

 woodland. 



These agencies are notably divergent in the two cases. 

 In arable land the surface soil is very thoroughly aerated 

 to a depth of 5 to 7 in. by ploughing, and somewhat more 

 where a steam plough is used, but below this there is very 

 little disturbance of the soil particles. Three factors may, 

 however, assist, though slightly, in bringing fresh supplies 

 of oxygen to the subsoil. These are (1) worms, which 

 burrow deep in cold winters and in dry summers (a depth 

 of 5 ft. is recorded as not uncommon in Darwin's Vegetable 

 Mould and Earthworms, p. 112) ; (2) rain, which percolates 

 through to the subsoil carrying with it a small amount of 

 dissolved oxygen, and allowing room for more air when it 

 drains away ; and (3) frost, which very rarely affects the 

 subsoil in our climate. The combined action of these three 

 factors is very slight in most soils, and only in those with 

 very deep drainage is the subsoil at all effectively aerated. 



In woods, worms are comparatively infrequent, and frost 

 is entirely negligible. But there is a new factor which is 

 incomparably more efficient than any in arable land, viz. 

 roots, which act in a multiplicity of ways. By boring their 

 way into the subsoil and by their secondary growth in 

 thickness they force apart the soil particles, and if they 

 die and rot the space which the root formerly occupied 

 must become filled again. Also the organic remains will 

 improve the soil. Still more important than all these 

 mechanical disturbances is the absorption of water by the 

 roots. For when roots draw in water from between the 

 particles immediately surrounding them the equilibrium 

 in the soil is disturbed and water is attracted by capillarity 

 from the more distant, moister soil into that wliich has 

 been dried through its propinquity to the roots. This 

 system by which roots obtain water from a distance is 

 depicted in the well-known diagram in Sachs's Physiology 

 of Plants, which has been copied into most subsequent text- 



