SOIL CONDITIONS AFFECTING PLANT GROWTH 55 



subsequently see, largely determines the nature and amount 

 of the organic matter of the soil. Tall vegetation keeps off 

 light from the lower growing plants and more or less sup- 

 presses them. Thus on the Rothamsted grass plots clover is 

 seriously reduced in amount by nitrate of soda which causes 

 tall growing grasses to flourish : Lathy rus, on the other hand, 

 in consequence of its tall growing habit is not adversely 

 affected but grows vigorously. Numerous other instances are 

 recorded in the Journal of Ecology}- The extreme case is 

 seen in wood-land where there is very little undergrowth and 

 where, therefore, organic matter has not accumulated in the 

 soil. Adjacent pieces of land at Rothamsted, both untreated 

 and differing only in the flora, showed the following differences 

 in composition : 



One of the most effective ways of suppressing weeds is to 

 grow a heavy crop which, in the farmer's language " smothers " 

 them by excluding light and by exerting certain root effects. 



Food. 



The nutrition of plants is complicated by the fact that 

 plants synthesise their own food from various substances 

 taken out of the air and the soil. It is common in farmers' 

 lectures to speak of these as the actual foods, but the student 

 must be perfectly clear in his own mind that they are only 

 the raw materials out of which the food is made. It is 



1 See e.g. E. Farrow, Journ, Ecology, 1916, 4. 



