HISTORICAL AND INTRODUCTORY 13 



(3) Soil fertility may be maintained for some years at least by 

 means of artificial manures. 



(4) The beneficial effect of fallowing lies in the increase brought 

 about in the available nitrogen compounds in the soil. 



Although many of Liebig's statements were shown to be wrong, the 

 main outline of his theory as first enunciated stands. It is no detrac- 

 tion that de Saussure had earlier published a somewhat similar, but 

 less definite view of nutrition : Liebig had brought matters to a head 

 and made men look at their cherished, but unexamined, convictions. 

 The effect of the stimulus he gave can hardly be over-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 physiol- 

 ogists showed conclusively that potassium, magnesium, calcium, iron, 

 phosphorus, along with sulphur, carbon, nitrogen, hydrogen, and oxy- 

 gen 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 advantageous 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 ex- 

 haust 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 Vincennes 

 during 1867 and 1874-5 (286). 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 recommending mixtures of salts for manure he was not guided by 

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

 stituents, nitrogen compounds, phosphates, lime, and potassium com- 

 pounds (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 nitrogen was 

 the dominant for cereals and beetroot, potassium for potatoes and 

 vines, phosphates for the sugar cane. An excess of the dominant 

 constituent was always added to the crop manure. The composition 

 of the soil had to be taken into account, but soil analysis was no 

 good for the purpose. Instead he drew up a simple scheme of plot 



