124 The Rothamsted Barley Experiments. 



during the last four of the twelve years the manure-residue 

 half plot shows an average of about 24 bushels per acre per 

 annum less than the half plot where the application was 

 continued. The average yield per acre over the first twenty 

 years under continuous manuring was 48J bushels, whilst 

 over the succeeding twelve years it was, where the manuring 

 was continued, 51| bushels, but where it was discontinued 

 only 34| bushels, showing, therefore, an average annual 

 deficiency, under the influence of the residue only, of 17f 

 bushels, or of 33*6 per cent. Over the whole period of 

 thirty-two years the total produce (grain and straw) was 

 without manure less than 1 ton per acre per annum, whilst 

 with the farmyard manure it was 2| tons, and in some years 

 it reached from 3| to 3| tons. It must now be apparent 

 that, whilst there were gradual exhaustion and reduction of 

 produce without manure, there were gradual accumulation 

 and increase of produce with the annual application of 

 farmyard manure. But when the application was stopped, 

 although the effect of the residue from the previous applica- 

 tions was very marked, it somewhat rapidly diminished, 

 notwithstanding that calculation showed an enormous accu- 

 mulation of nitrogen as well as of other constituents. 

 Indeed, determinations of nitrogen in the surface soil, after 

 the twenty years' application of farmyard manure, showed it 

 to be nearly twice as high as on the unmanured plot. The 

 very large amount that accumulates within the soil is in a 

 very slowly available condition. 



It is now necessary to consider what is the character of the 

 exhaustion induced by the growth of the crop without manure ; 

 and to what constituent, or constituents, of farmyard manure 

 its effects are mainly due. These questions are solved bv 

 examining the results obtained on the plots dressed with 

 various purely mineral manures, purely nitrogenous manures, 

 and combinations of the two. A general view of the effects 

 of sixteen different conditions as to manuring is given in 

 Table XXVI., which is a summary table, recording the 



