122 WEEDS OF FARM LAND 



soil to some extent, or it may be absent in one district from 

 the very soil on which it is so characteristic elsewhere. Con- 

 sequently exceptions will be found to nearly every statement 

 made, and it is only by gathering together and correlating 

 evidence from many sources that any approach to real accuracy 

 will be made. 



For purposes of convenience soils may be classified as 

 follows : 



(1) Clays and heavy loams. 



(2) Medium loams. 



(3) Light loams and sand. 



(4) Chalk. 



(5) Special soils, as peat. 



Such a classification is necessary for working purposes, but in 

 actual fact no hard and fast line can be drawn between the 

 different sections. Clays and heavy loams merge imperceptibly 

 into the medium soils,and there is no sharp dividing line between 

 medium loam and the lighter soils. Again, land that is 

 termed clay in one part of the country will pass as loam else- 

 where, the terminology depending to a large extent upon the 

 general nature of the soils in the surrounding district. Never- 

 theless, when an investigator is constantly working on various 

 soils in different districts, a recognition of the relative lightness 

 or heaviness of the land develops almost instinctively, and 

 experience enables the classification of the weeds according to 

 the soil they occupy to be made with a fair degree of accuracy. 

 In addition to determining the soil with which a weed is 

 associated, it is necessary to draw a distinction between the 

 frequency with which a plant occurs and the amount of it that 

 is present. A weed like field forget-me-not may be fre- 

 quently found on any soil, and yet it may occur in such small 

 quantities that it is often noted as being scarce in the fields, 

 while it may never be present in such abundance as to be 

 dominant. 1 On-the other hand, such plants as field speedwell, 



1 The relative abundance of weed species in any field is indicated by the 

 following terms : 



(1) Dominant. When one species is much more abundant than any other. 



(2) Sub-dominant. When a second species, less plentiful than the dominant 

 one, is also more plentiful than the other species of the flora. 



(3) Distributed. When a weed is rather plentiful over the whole of a field, 

 but is not relatively more abundant than other associated species. 



(4) Occasional or sparse. When occasional specimens of a weed are to be 

 found here and there. 



(5) Scarce or rare. When isolated individuals occur, sometimes only one or 

 two specimens being found. 



