n8 WEEDS OF FARM LAND 



The whole question of weed population probably resolves 

 itself into one of competition between the different species. 

 The absence of a weed in any particular situation may be due 

 to one of several reasons : 



(1) The plant is so unsuited to the soil that it will pot 

 flourish on it under any circumstances. 



(2) Other species are so much more favoured by the con- 

 ditions of growth that the particular species concerned is 

 smothered out and is unable to make any headway. 



(3) The method of cultivation may be such as to hinder 

 the species from establishing itself. 



(i) It very frequently happens that a species is absent 

 from a particular type of soil in one place while it is present or 

 even abundant upon similar soil in other districts. The con- 

 clusion may be drawn that comparatively few plants are really 

 antipathetic to any soil, and that if other conditions are suit- 

 able most plants will grow, at least to some extent, on any 

 kind of soil. Naturally, there are exceptions to this rule, but 

 they are probably less frequent than they seem at first sight. 

 Salt marshes are characterised by a specialised set of plants 

 adapted to the peculiar conditions induced by the flooding 

 with salt water which occurs at high tides. These species, 

 as Salicornia spp., Suceda maritima and S. fruticosa, Aster 

 tripolium, Glaux maritima and several others are never 

 found as weeds of cultivation or in any inland area. Never- 

 theless it is quite possible to grow them in ordinary soil 

 and to obtain good plants. On the other hand, the salt 

 marsh conditions are so exceedingly abnormal that the 

 majority of land plants find it impossible to live in such situ- 

 ations, because the combination of waterlogging, flooding, 

 and an excess of salt render the position untenable. But, as 

 soon as the land is drained and the floods are kept back by 

 dykes the typical land plants soon find entrance and before 

 long oust the original occupants of the soil. 



Sorrel and sheep's sorrel are usually associated with acid 

 soil, and it is sometimes asserted that they will not grow in 

 the presence of a sufficient supply of lime. Rothamsted ex- 

 periments have shown that, in the absence of competition of 

 other plants, these species do better with a full supply of lime 

 than they do on an acid soil. Pot cultures were carried on 

 with a light soil, deficient in lime. The amount of chalk 

 necessary to correct the acidity of the soil was determined 

 and then calcium carbonate was added to the pots in varying 



