CHAPTER VIII. 



ARABLE WEEDS. ASSOCIATION WITH SOILS. 



I. GENERAL. 



IN travelling about the country and inspecting fields in various 

 districts the first impression received is that the individuals of 

 the weed flora are very closely determined by the type of soil 

 on which they grow. Sandy land is often smothered with 

 poppies, spurry or corn marigold, boggy soil is carpeted with 

 willow weed, chalk shows abundance of chicory, and loam and 

 clay present a general mixture of weeds without any great 

 outstanding species. A closer investigation shows that this 

 first impression is to a large extent erroneous, that most 

 individual species of weeds may be found on different kinds of 

 soils in various parts of the country and that comparatively 

 few weeds are definitely associated with any particular soil. 

 Still closer examination of the weed populations shows that as 

 a general rule the weed communities are far more characteristic 

 of the different soils than are the individual species. The 

 communities vary within themselves in different localities, but 

 in their broad outlines they show some connection with the 

 character of the soil. It is probable that many factors interact 

 to determine the nature of the weed population, and that the 

 mere chemical or mechanical composition of the soil is of 

 comparatively little importance except in so far as it is bound 

 up with other factors such as drainage, rainfall, temperature, 

 season, acidity, and aeration. The interaction of all these 

 factors determines the relative competition of the species 

 growing on the soils, and so the personnel of the weed com- 

 munities is profoundly affected. The influence of factors other 

 than soil is often well shown by the difference in the propor- 

 tions between the weeds that grow in wet seasons and dry 

 seasons. If the season be very \vet some plants gain the 

 upper hand at an early date and maintain their position all 

 through the year, whereas if the season be dry quite another 

 set of plants comes to the front on the same soil. 



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