246 SCIENCE PROGRESS 



of ammonia which were then beginning to be available in 

 commercial quantities ; as the results became available he entered 

 the lists against Liebig. An animated controversy followed 

 which lasted as long as Liebig lived, for though the tide of 

 opinion finally settled down against Liebig's point of view, 

 certain outstanding difficulties were never explained until after 

 his death and he has a few faithful adherents even at the present 

 time. The main problem is solved, however. 



What the Rothamsted experiments demonstrated was that 

 for the cereals and some of the other farm crops growth up to 

 a certain point was proportional to the supply of nitrogen. The 

 leguminous crops proved to be an exception ; they did not 

 respond to nitrogenous fertilisers to any great extent and they 

 proved exceptions in another way, in that it was not possible to 

 maintain their growth year after year upon the same land as 

 with some of the other crops. One of the most unexpected 

 features revealed by the Rothamsted experiments has been the 

 possibility of dispensing to a large extent with a rotation of 

 crops, provided the supply of fertilisers is kept up. One of the 

 cardinal principles in the old conservative system of farming — 

 a principle embodied in the leases under which the land was 

 let — was that two corn crops should never be taken in suc- 

 cession and it would have been considered impossible to grow 

 wheat year after year without any break. Yet the sixty-eighth 

 successive crop of wheat upon the Broadbalk field has this 

 year been harvested and one of the plots has yielded more 

 than five quarters to the acre ; even the plot which has had no 

 manure of any kind since 1839 has produced over twelve bushels 

 per acre. For various practical reasons it is never desirable to 

 attempt such continuous cropping as this but one immediate 

 result of the Rothamsted experiments has been to encourage 

 a much greater freedom of cropping, until to-day it is a regular 

 thing to take two or three corn crops in succession ; a few 

 farmers have even adopted the plan of continuous corn-growing, 

 except in the occasional years that have to be devoted to 

 cleaning and restoring the tilth of the soil. 



It would be out of place here to discuss in any detail the 

 conclusions which Lawes and Gilbert drew from their field 

 experiments after the results had been confirmed by a few years' 

 repetition but it is enough to say that the accepted theory of 

 the manures appropriate to the different farm crops was estab- 



