776 TRANSACTIONS OF SECTION M. 
3. The Weeds of Arable Land. 
By Wryirrep E. Brencuuey, D.Sc., F.L.S. 
The distinction between weed and crop is very sharp!y marked where arable 
land is concerned. Weeds may be defined as ‘ plants other than those intended 
to be sown, which grow up naturally with the crop, and if unchecked prove 
exceedingly detrimental to the crop.” The weeds may spring either from seeds 
already in the soil or from seeds accidentally introduced with those of the crop. 
Weeds are able to do damage to the crop in various ways :— 
(1) They utilise much of the raw food material and water of the soil, so 
lessening the supply available for the crop. 
(2) They steal light, as when they grow luxuriantly they overshadow the 
crop plants, and so hinder carbon assimilation. 
(3) The weed seeds render the crop seeds impure, and so entail much expense 
in the special processes of cleaning and separating thus rendered necessary. 
- (4) Weeds provide harbourage for various insect and fungus pests which 
would either be entirely absent or at least far more easily dealt with if the 
ground were clear of alien plants. 
An examination of the relations existing between the weeds, crops, and soils, 
carried on in various parts of England, shows that arable land forms a definite 
plant habitat, and carries a distinct weed flora which varies somewhat according 
to local conditions. In dealing with such a problem as this, the utmost care is 
needed in the interpretation of results, as so many factors come into play. 
Differences in climatic conditions, methods and times of cultivation, systems of 
manuring, &c., all play their part in influencing both the composition of the 
weed flora and the balance of its component species. 
On clay soils the weed flora is less rich in species than is the lighter loam, 
and though several plants have a decided preference for heavy land, yet no 
species can be said to be ‘symptomatic’ of clay, occurring on such soil and 
nowhere else. 
Sandy soils possess a much more characteristic weed flora, as they are 
colonised by a great diversity of plants, a good many species being definitely 
associated with light soils. Such plants as spurrey, corn marigold, sheep’s 
sorrel, and knawel appear to be characteristic of sandy soils which are deficient 
in chalk, ‘sour’ soils. 
Chalk provides quite a peculiar habitat for weeds, partly because of its texture 
and partly because of its chemical composition, as so much calcium carbonate is 
present. The weed flora of the chalk is relatively very rich in species, some of 
which are markedly characteristic of such soil. 
; It is now evident that a definite association exists between the species of weed 
plants and the soil on which they grow. This association may be either 
(a) Local, when a weed is symptomatic of a certain soil in one district, but 
is not so exclusively associated with it in another. : 
(6) General, when a certain species is symptomatic or characteristic of the 
same type of soil in different districts. 
The nature of the crop also plays its part in determining the weed flora. 
This influence is partly due to the different habit of growth of such crops as 
cereals, roots and leguminous plants, but it is probably more largely the result 
of the varying methods of cultivation applied to the crops—e.q., root crops are 
so thoroughly cultivated throughout their season of growth that comparatively 
few species of weeds can hold their own, whereas leguminous plants cannot be 
hoed during growth, so that many weeds have an opportunity to flourish if they 
can withstand the competition of the crop. 
It is evident that the problem of the weeds of arable land must be approached 
with a perfectly open mind, recognising that a relation obtained in one place 
does not necessarily hold good in another. Yet when all the factors influencing 
the flora come to light it seems probable that the apparent contradictions will 
find a satisfactory explanation and prove to be different expressions of a well- 
defined order. 
