358 INVESTIGATION BY CULTURE EXPERIMENTS 



This is especially noticeable on the mineral section, where best 

 provision is made for rapidly exhausting the' nitrogen by removing 

 other limits to crop production. With barley under the fallow 

 system the yield for the last twenty years averages no more where 

 minerals are supplied than where no fertilizer is used, thus indi- 

 cating the same nitrogen limit for that crop, and emphasizing the 

 fact that no amount of phosphorus or other elements can increase 

 the yield of crops where nitrogen has become the limiting element. 

 In the case of wheat, the yield is still greater where the minerals are 

 supplied, because wheat is the first crop grown after the year of fallow 

 cultivation, the principal effect of which is to liberate nitrogen from 

 the residue still contained in the soil humus; and whatever weeds are 

 allowed to grow, during the fallow year or other years, will help to 

 save soluble nitrogen from loss in drainage water; and if the volun- 

 teer herbage includes any legume plants, some atmospheric nitrogen 

 would thus be added. Of course if any growth of this character 

 were larger on the mineral plots than on the unfertilized land, the 

 effect would be greatest on those plots in the increased growth of 

 the wheat, turnips, and barley. 



It is pointed out by Dyer (Results of Investigations on the Roth- 

 amsted Soils, Bulletin 106 of the Office of Experiment Stations, 

 United States Department of Agriculture) that where barley is 

 grown every year on Hoos field the most common weed on the plot 

 receiving minerals without nitrogen is yellow trefoil, which grows 

 even while the barley crop is supposed to occupy the land; and that 

 Sir Henry Gilbert had expressed the opinion that very appreciable 

 quantities of nitrogen were added to the soil by that leguminous 

 plant, which grows persistently as a weed on that plot despite the 

 efforts to eradicate it. 



Since the above was written, Director Hall, of Rothamsted, has 

 kindly furnished the specific information that the fallow portion of 

 Agdell field is kept plowed, and is therefore practically free from 

 weeds during that year; but when wheat is grown, " there is a good 

 deal of wild yellow trefoil, particularly in certain seasons, and 

 on the plots receiving mineral manures only." He states that this 

 trefoil was so abundant in 1907 that after the wheat harvest he 

 had it cut and weighed separately, and found that the amounts 

 per acre (including, presumably, the wheat stubble etc.) were 



