126 WEEDS OF FARM LAND 



favourable to the growth of most species, but Rumex acetosella 

 is quite indifferent to them. Consequently, while most plants 

 are hesitating to start boldly into growth the sheep's sorrel 

 pushes ahead, dominates the situation from the first and holds 

 its own to such an extent that many of the other weeds do not 

 have a fair chance to make a start. On ordinary soils, how- 

 ever, the competition is keen from the beginning. Sheep's 

 sorrel would be quite content with the soil conditions, but it 

 cannot put up with the competition of other species, and con- 

 sequently it gets crowded out of existence. This does 

 not, of course, imply that if competition were removed 

 sheep's sorrel would spring up everywhere, as seeds would 

 not be present in every soil. In the general distribution of 

 species the balance has been continually adjusted, until now 

 a few species are very specially associated with particular soils 

 while the great majority of plants will grow everywhere if 

 conditions are favourable. Thus the struggle for existence is 

 continually going on and the balance of dominating species is 

 shifting day by day in response to the perpetual changing of 

 the conditions which favour or inhibit growth. 



This being the case, very few, if any, arable weeds can be 

 regarded as symptomatic of the soil on which they grow i.e. 

 there are practically no individual species which indicate 

 infallibly the nature of the land where they occur. Individual 

 species are therefore of little use as decisive indicators, but 

 communities of species give much more accurate information. 

 For instance, the presence of sheep's sorrel indicates acidity 

 of soil, though it does not necessarily show whether the land 

 is heavy or light, but if it is associated with spurry and annual 

 knawel the community or association of species shows definitely 

 that the soil is very light or sandy in addition to being acid. 

 Again, though no single weed is exclusively associated with 

 clay land, the presence of such a mixture of weeds as black 

 bent, hoary plantain, corn buttercup, coltsfoot, and couch-grass 

 at once indicates a heavy soil. In considering the association 

 of weeds with soil, therefore, it is necessary to lay more stress 

 pn the particular communities in which the plants occur than 

 on the connection of one particular species with one definite 

 type of land. 



