GENERAL SCHEME 33 



hoeings, some weeds, like the "Black Bent" grass, Alopecurus 

 agrestis, are kept under with the greatest difficulty. 



The general scheme of the experiments in the Broadbalk 

 field has been to test the manurial requirements of wheat by 

 growing it continuously with various combinations of manures 

 repeated year after year on the same plots. At the outset 

 of the experiments it should be remembered that little was 

 then known as to the manurial requirements of any crop. 

 Liebig had just stirred the agricultural world by the general 

 statement that if a plant were supplied with the mineral 

 constituents left as ash when the plant is burnt, it will require 

 no further assistance in the shape of manure, but will draw its 

 carbon and nitrogen from the atmosphere. The first experi- 

 ments were designed to verify the truth of this statement, and 

 were extended to test the effect of each of the constituents 

 found in the plant. The effect of mineral manures alone is 

 compared with that of nitrogenous manure in various forms, or 

 of a combination of the two. The constituents of the mineral 

 manure phosphoric acid, potash, soda, and magnesia are 

 variously combined with nitrogenous manures, so as to ascertain 

 the part each of them plays in the nutrition of the crop. Thus 

 Plots 6, 7, 8, 9, 15, 16, 17, and 18 receive varying amounts and 

 combinations of nitrogen, together with the same mineral 

 manure containing all the elements present in the ash of the 

 wheat plant. Again, all the Plots 10, 11, 12, 13, and 14 receive 

 the same amount of nitrogen, but differ in the arrangement of 

 the accompanying mineral manure. Some of the plots also 

 test the question of the season at which the manures are 

 applied, and whether any of the residues are carried forward to 

 .another year. The long duration of the experiment serves to 

 eliminate many of the sources of error in field experiments, 

 such as initial variations in the condition of the soil of various 

 plots due to previous manuring, irregular attacks of insect and 

 other pests, and variations due to seasons which may favour 

 some manures and not others. Also by gradually exhausting 

 .the soil of particular constituents, the continuity brings to light 



