The Broadbalk wheat experiment 1968-78 : yields and plant nutrients in crops grown continuously and in rotation

Modifications made to the Broadbalk Wheat experiment in 1968 included growing wheat in 2 three-course rotations, potatoes, beans, wheat and fallow, wheat, wheat to compare yields of wheat grown in rotation with those of wheat grown contiuuously. The major change in manuring was to apply all N in spring as'Nitro-Chalk' and introduce a test of 192 kg N ha-l.
Average yields for all crops and all treatments during 1970-78 are given here. Wheat yields are now larger than at any time since 1843, even on soils which have been unmanured since then. Wheat after beans gave the largest grain yield. Potatoes and wheat responded differently to unbalanced manuring.
Nitrogen response curves were fitted to the 1970-78 average yields of wheat grown continuously, as first and second crops after fallow and following beans, It was possible to bring the curves for the three rotational wheats into coincidence with that for the contitruous wheat by vertical and horizontal shifts, Relative to wheat gown continuously, the vertical shifts were equivalent to increased yields for wheat after beans (0.51 t ha-r) and sccond wheat after fallow (0'38 t ha-l), but decreased yield for first wheat after fallow (-0'36 t ha-r), The horizontal shifts were equivalent to increases in available N for wheat after beans (23 kg ha-r) and first wheat after fallow (53 kg ha-1), but a slight decrease for second wheat after fallow (-9 kg ha-r).
The percentage of N, P, K, Ca, Mg and Na in grain and straw of wheat after beans and continuous wheat, potatoes and bean grain for each treatment averaged over 1970-75 (1978 for N) and offtakes of these nutrients in ths harvested crops are given. Wheat grain after beans and from contitruous wheat contained, on average, 1.94 and 1.92% N in dry matter respectively. Of the total offtake by wheat in grain plus straw the proportion in the grain was 82% for N, 85% for P, 44% or K, 16% for Ca, 7l% for Mg and 40% for Na.
Compared to earlier periods %N in grain has changed little but N offtakes are now larger, because yield has increased, and apparent recoveries of fertilizer N have improved. Percentage P in grain has decrcased by about 25% but offtakes are now larger, again because yield has increased. There has been no or very little change in %K in grain but there were large seasonal differences in %K in straw. The concentrations of Ca, Mg and Na in grain and straw have changed very little.

RESP-08657