36 THE FERTILITY OF THE SOIL [CH. 



space for the full development of its roots. It suffers 

 from the restriction of its supply of water and of 

 food, and apparently from other causes as well ; recent 

 experiments seem to indicate that new factors may 

 come into play when one set of roots runs up against 

 others because there is not space for both. In 

 Mr Pickering's experiments at Woburn growing grass 

 had a very detrimental effect on the fruit trees 

 planted in it, and there is also evidence that weeds 

 may have a directly harmful action on sown crops. 



Absence of injurious factors. However good its 

 food and water supply, a soil may remain infertile if 

 injurious substances happen to be present. Little 

 attention has been paid to these in England, but they 

 have been much studied in the United States where, 

 indeed, they have given rise to considerable contro- 

 versy. It seems beyond dispute that substances 

 harmful to plants do occur in wet soils poor in 

 calcium carbonate. Nothing is known about these 

 substances (in English soils, at any rate) but some 

 years ago the fashion arose of calling them acids 

 without any sufficiently rigid proof It is distinctly 

 unwise to prejudice future investigations by assign- 

 ing a name already used for a definite group of 

 substances to another that is yet unstudied, and we 

 shall therefore adopt the nomenclature of the prac- 

 tical men and speak of such soils as "sour." Whatever 

 the cause of "sourness" (and the name commits us 



