ROTHAMSTED EXPERIMENTS 237 



per acre of the wheat lands of the world. On this field it was 

 found that mineral manures alone gave very little increase, 

 whilst nitrogenous manures alone gave a much greater increase 

 than mineral manures alone, but the mixture of the two gave 

 much more than either alone. It is estimated that the reduction 

 in yield, due to exhaustion, of the unmanured plot over 40 years 

 1852 to 1891 was, provided it had been uniform throughout, 

 equivalent to a decline of one-sixth of a bushel per acre. It is 

 related that a visitor from America, when being shown over the 

 Broadbalk field, said to Sir John Lawes, "Americans have learnt 

 more from this field than from any other agricultural experiment 

 in the world." 



Another set of field experiments of exceptional interest is 

 that relating to the "Mixed Herbage of Permanent Grass Land." 

 The land was divided into twenty plots. Two plots have re- 

 ceived no manure from the commencement of the experiment, 

 two have received a dressing of farm-yard manure each year, 

 whilst the remainder have each received a different kind of 

 artificial or chemical manure, the same kind being applied year 

 after year on the same plot, except in a few special cases. 

 Repeated analyses have shown how greatly both the botanical 

 constitution and the chemical composition of the mixed herbage 

 varied according to the kind of manure applied. 



The results of these experiments were given under three 

 headings agricultural, botanical and chemical, and show in an 

 exceptional manner the care of detail to which every investi- 

 gation was subjected by Gilbert. Some people have thought 

 that this minute attention to detail was carried to excess by 

 Gilbert, and resulted in a bewildering multiplication of numeri- 

 cal statements and figures. One can, however, but admire his 

 love of accuracy and absolute conscientiousness, and if his 

 caution appeared at times to be carried to an extreme, the result 

 has been to make "the Rothamsted experiments a standard 

 for reference, and an example wherever agricultural research is 

 attempted." 



One of the most important results of the Rothamsted investi- 

 gations has been the replacing of the "mineral theory" of Liebig 

 by the "nitrogen theory" of Lawes and Gilbert. Liebig held 



