ARABLE WEEDS. ASSOCIATION WITH SOILS 123 



chickweed, curled dock, and fat hen are found on all soils in 

 such abundance that they are often dominant. Again, a weed 

 may occur on all soils but it may be far more abundant on 

 some types than on others, as happens with ribwort plantain 

 which is occasionally dominant on chalky soils and not else- 

 where, and black bindweed which is often dominant on light 

 soils but not on heavier land. This local dominance of ubiq- 

 uitous weeds gives a good indication of the soils which are 

 really preferred by the species. Thus the frequency with 

 which a weed occurs is not necessarily associated with its 

 abundance. Naturally enough the weeds that are of universal 

 distribution are usually the most frequent in occurrence, as they 

 are not limited by the soil conditions, and so have a greater 

 choice of habitat, but on the other hand plants that are 

 specially localised, as spurry and white mustard, are generally 

 far more abundant in their particular situation than are the 

 weeds of general distribution. v 



In the following pages a weed is said to 



(1) be characteristic of a soil when it is usually frequent or 

 abundant on that soil but only occurs infrequently and in small 

 amounts on other types of land. 



(2) have a preference for a particular soil if.it is more often 

 frequent or abundant on that soil than on any other. 



(3) be generally distributed if it shows no preference for 

 one soil more than another. 



The chief arable weeds may be classified according to their 

 association with soils, as follows : 



A. Weeds of General Occurrence. 



(a) Plants that are equally distributed on all types of soil. 



(b) Plants occurring on all soils, but less frequently on 



heavy land. 



(c) Plants occurring on all soils, but less frequently on 



chalk. 



B. Weeds Specially Associated with Heavy Land. 



(a) Plants characteristic of heavy land (clay and heavy 



loam). 



(b) Plants common on heavy land, but also of general dis- 



tribution. 



(1) Plants which flourish equally well on chalk. 



(2) Plants which do not occur so frequently on chalk. 

 (a) With distinct preference for the heavier soils. 

 (/?) Equally plentiful on heavy and medium soils. 



