more rapidly than those which received no nitrogen : while the plots 

 receiving phosphates began as usual to ripen earlier than the others. 

 In all cases the crops were very uniform over the whole plot, and the 

 irregularities which showed in 1911 on plot 2A vanished entirely. 

 Several of the plots yielded over 60 bushels of grain, 30 cwt. of 

 straw and 7,000 lb. of total produce per acre : to find any parallel we 

 have to go back nearly 60 years. The season was of course very 

 favourable for barley : the spring being moist and the summer cool 

 and dry. But there was another circumstance which appears to have 

 contributed to the high yield. For 60 years in succession, barley 

 crops have been grown continuously in Hoos Field without any break, 

 but recently weeds had accumulated to such an extent that after the 

 harvest of 1911 it was decided to fallow the field for a year, cul- 

 tivating thoroughly to keep the land free from all growth during 

 the season, and, of course, withholding all manure. The fallow 

 ended in March, 1913. 



There can be little doubt that the fallow played a considerable 

 part in bringing about the high yield. It is difficult to account 

 for the result on our present views as to the effects of fallowing : 

 something more seems to be involved than the accumulation of 

 nitrate over the winter. Laboratory work, discussed later on, in- 

 dicates the existence of another factor : an apparent effect of a 

 growing crop on bacterial decompositions in the soil which is not 

 exerted during the fallow period. The important problems thus 

 opened up are under further investigation. 



Another very important problem is raised by these results. The 

 yield — 60 bushels of grain and 30 cwt. of straw — is extraordinarily 

 high for us, and has been obtained only three times before, viz., 

 1854, 1857 and, in Agdell Field only, in 1861. It seems to represent 

 the utmost our soil can do. There is remarkably little variation 

 between the best plots this year, seven of them varying only within 

 4 bushels, viz., from 60 to 64, and the variation does not become 

 much wider if one includes the three early years and the Rotation 

 experiments as well as the continuous crop. This result is readily 

 explained on physiological grounds: of the various plant require- 

 ments, all must be satisfied, or growth will not continue. If any 

 one is withheld, it sets a limit beyond which crop growth will not 

 take place. Lack of food, water, temperature may all constitute 

 limiting factors, any of which would prevent the crop from rising 

 indefinitely. The fact that our crop has not yet been pushed beyond 

 64 bushels during the 60 years of experiment shows that some 

 limiting factor is at work which is not overcome by any of the 

 manurial combinations or cultivation methods we use. 



The limit may be set by the sheer inability of the plant to grow 

 any larger, in which case, the plant breeder could come to the rescue 

 by producing more vigorously growing varieties. But this was not 

 the case here. Sixty-four bushels of barley is by no means a record 

 crop on good barley soils, and probably many farmers have obtained 

 more. The limit in our case seems to be set by the soil type; ours 

 is not a good barley soil, i.e., it is not perfectly adapted to barley, 

 and no mere addition of food stuffs will make it so. 



The barley on the Agdell Rotation Field did not yield anything 

 like as heavily as on the Hoos continuous plots, the highest crop 

 being 33 bushels of grain, 15 cwt. of straw, and 3,500 lb. of total 



