X .] CA .V RO TA TIONS REP LA CE FER TIUSERS f 29 5 



particular crop is grown continuously on the same land. 



On the rotation field at Rothamstcd the yield of 



wheat on the unfertilised plot has been remarkably 



maintained; for the last five courses (lOth to 14th of 



the whole series) it has averaged 26- 2 bushels per acre, 



but it is below the yield of the fertilised plots on the 



Broadbalk field, which averaged 357, 32, and 397 



bushels for the same years, and also below the fertilised 



plot on the same rotation field, which averaged for the 



same period 37- 1 bushels per acre, although the fertiliser 



is only applied once in four years to the Swedes, which 



are followed by barley and either clover or a bare fallow 



before the turn of the wheat comes round. But with 



other crops than wheat no such maintenance of yield is 



to be seen on the unfertilised plot of the rotation field — 



the barley yield has been reduced to 158 bushels 



against 277 on the fertilised plot, the clover yield to 



94 cwts. against 378 on the fertilised plot, and the 



turnips to as little as 16 cwts. against 400 on the 



fertilised plot. Here we see that with the barley, clover, 



and particularly with the turnip crop, a rotation is quite 



unable to do the work of the fertiliser; the yield of 



turnips is reduced to a minimum on the impoverished 



soil, even though the crop only comes round once in 



four years and then grows so poorly that it can do little 



specific excretion to harm the succeeding crop. Many 



instances could be given of the incapacity of certain 



plants to grow in soil the fertility of which had been 



exhausted by other crops ; for example, at Rothamsted 



in 1903, Swede turnips were sown on Little Hoos field, 



which was known not to have been cropped with Swedes 



or any kindred crop for more than forty years, and the 



average yield from thirty-two unmanured plots was only 



9-3 tons per acre, although an exceptionally good start 



was made by the plant. In the following season barley 



