Published online by Cambridge University Press: 27 March 2009
Yields of the three ‘test’ crops, winter wheat, potatoes, spring barley in the Rothamsted Ley–Arable experiments 1949–69 increased by about 100%, 80%, 40% respectively, each in a period of 19 years. The wheat variety was changed once during the period and this change is enough to explain most of he increase of yield. For potatoes there was no change of variety; the estimated effects of changes in manuring, etc. explain less than one third of the observed increase. For barley two changes of variety explain about one third of the increase; most of the remainder may be explained by improved manuring of preceding crops, especially potatoes. After 1969 the cropping was drastically changed but a few recent yields from other experiments at Rothamsted indicate that the steady rates of increase may have continued at least up to 1980.