HISTORICAL AND INTRODUCTORY 19 



estimated, and before he had finished, the essential facts of 

 plant nutrition were settled and the lines were laid down 

 along which scientific manuring was to be developed. The 

 water cultures of Knop and other plant physiologists showed 

 conclusively that potassium, magnesium, calcium, iron, phos- 

 phorus, along with sulphur, carbon, nitrogen, hydrogen, and 

 oxygen are all necessary for plant life. The list differs from 

 Liebig's only in the addition of iron and the withdrawal of 

 silica; but even silica, although not strictly essential, is ad- 

 vantageous to cereals. 



In two directions, however, the controversies went on for 

 many years. Farmers were slow to believe that "chemical 

 manures" could ever do more than stimulate the crop, and 

 declared they must ultimately exhaust the ground. The 

 Rothamsted plots falsified this prediction ; manured year after 

 year with the same substances and sown always with the same 

 crops, they even now, after sixty years of chemical manuring, 

 continue to produce good crops, although secondary effects 

 have sometimes set in. In France the great missionary was 

 Georges Ville, whose lectures were given at the experimental 

 farm at Vmcennes during 1867 and 1874-5 (288). He went 

 even further than Lawes and Gilbert, and maintained that 

 artificial manures were not only more remunerative than dung, 

 but were the only way of keeping up fertility. In recom- 

 mending mixtures of salts for manure he was not guided by 

 ash analysis but by field trials. For each crop one of the 

 four constituents, nitrogen compounds, phosphates, lime, and 

 potassium compounds (he did not consider it necessary to 

 add any others to his manures) was found by trial to be more 

 wanted than the others and was therefore called the 

 " dominant " constituent. Thus for wheat he obtained the 

 following results, and therefore concluded that on his soil 

 wheat required a good supply of nitrogen, less phosphate, and 

 still less potassium : 



