182 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



percentage saturation and pH. These relationships are represented 

 schematically on page 181. 131 



Inhibition of Growth by Plant Products.— That certain plants 

 produce soil conditions inhibiting the growth of other plants is 

 probably true. 273 Over a hundred years ago it was argued that crop 

 rotation was necessary for this reason and that fallowing of land 

 favored the next crop because it permitted the leaching of harmful 

 excretions or by-products of decomposition resulting from the 

 previous crops. Today we cannot entirely ignore this line of rea- 

 soning, for explanations of the benefits of rotation and fallowing 

 based upon nutrient deficiencies are not always adequate. Like- 

 wise, there is some evidence that toxic substances are released in 

 the soil as excretions, 215 or when external root cells are sloughed 

 and decompose, 209 or when the plants disintegrate after death. 



A number of grasses inhibit growth of other plants. In lawns, 

 certain strains of bluegrass almost completely check the growth of 

 white clover. 1 Walnut inhibits the growth of a number of herbs. 

 Fairy rings of both fungi and higher plants may be the result of 

 toxic products produced by the plants, for other explanations do 

 not always suffice. If water, supplied in excess to flats of experi- 

 mental plants, is permitted to percolate through the soil and is then 

 used as the water supply for other plants, the latter are frequently 

 inhibited in growth even under the most favorable conditions. 17 

 Extracts from decomposing plant remains have produced similar 

 results. Apparently toxic or growth-inhibiting substances are pro- 

 duced by a number of plants, which may affect germination of 

 seeds and growth of seedlings, or even of mature plants of the 

 same or other species. Some species are affected, others are not. 

 Whether higher plants are affected directly is not always clear. 

 Perhaps effects upon soil organisms and their activity in turn af- 

 fect the higher plants. 



The subject is controversial, and some evidence is conflicting. 

 The limited information that is available is often derived from 

 observation of agricultural soils and cultivated plants. Cultivation, 

 probably because of better aeration, reduces the effectiveness of 

 inhibiting substances, and the problem is practically eliminated by 

 crop rotation and the compensating effect of fertilizer. It is, there- 

 fore, not surprising that investigators have turned to other things. 



