52 WEEDS OF FARM LAND 



that the crop seed should be free from weed seed. This can 

 now be easily assured by the guarantee that must be issued 

 with all seeds sold, but hitherto much care has been necessary. 

 Cleavers (Galium aparine) was a common impurity in rape 

 seed, and in the districts bordering the North Sea it was usual 

 to clean the rape by running the seed over a cloth, when the 

 rape rolled off while the cleavers seeds were caught by the 

 cloth by means of their hooks. 1 



When all other methods fail two more drastic treatments 

 may be tried : the land may either be fallowed for a season or, 

 as a last resort, it may be laid down to grass for a term of 

 years. 



Under the old system of agriculture fallowing was usu- 

 ally regarded as an integral part of the rotation of crops, but 

 with improved methods of cultivation and the increased use of 

 manures it is now less generally used unless for such a specific 

 purpose as clearing land of specially bad weeds. If land can 

 be kept clean without a bare fallow it is usually more profit- 

 able to keep it cropped continuously, partly because the value 

 of the extra crop is obtained and partly because a definite loss 

 of nitrogen has been proved to occur when soil is left bare 

 and exposed without any green covering. Under some cir- 

 cumstances, however, and especially on heavy land, a bare 

 fallow proves the most effective and prompt means of reduc- 

 ing the weeds. For this purpose the land should be ploughed 

 and harrowed to remove as many weeds as possible, and then 

 be cultivated to provide a tilth in which the weed seeds in the 

 soil may be encouraged to germinate. Further ploughings 

 and cultivations must be carried out during the season, the 

 great object being to obtain as many weed seedlings as possible 

 in order that they may be destroyed forthwith. It is essential 

 that the intervals between the cultivations should not be long 

 enough to allow any of the weeds to seed, and careful watch 

 must be kept, as some weed species flower and seed with great 

 rapidity under all circumstances and, if the season be dry, the 

 longer lived weeds may be induced to hurry on their seeding 

 processes on account of the drought. Fallowing may be par- 

 ticularly effective when for any reason a succession of straw 

 crops has been carried on the same field. The Broadbalk field 

 at Rothamsted, which has carried wheat for seventy-five years, 

 since 1843, is badly infested with black bent (Alopecurus 

 agrestis), so badly that constant hoeing and hand pulling fail 



: Jour. Bd. Agric. (1912), XIX, p. 321. 



