20 



Thus, both in the experiments on the heavy land at 

 Kothamsted, and on the sandy soil at Woburn. it has appeared 

 that the effect of long continued previous applications of dung 

 is exhausted on land cultivated for barley or wheat in thirteen 

 or fifteen years. We may arrive at the same conclusion by 

 looking at another feature of these experiments. 



If a dressing of manure, continuously applied during 

 many years, is not excessive in quantity that is, if the 

 manuring power of the dressing and of its residues is in- 

 sufficient to produce the highest crops which the soil could 

 yield in an average season the produce of the land will be 

 found to increase year by year till the time is reached when 

 the earliest dressings of the manure cease to affect the crop ; 

 after this stage has been gained the annual produce will 

 remain unaltered, save by variations in the character of the 

 seasons, the influences of past residues having now become 

 a constant quantity. The following table shows the average 

 produce of wheat and barley at Eothamsted in successive 

 periods of five and ten years, the land receiving every year 

 14 tons of ordinary yard manure per acre. The produce of 

 wheat and barley in successive periods at Woburn, the land 

 receiving annually 7 to 8 tons of cake-fed manure, is also 

 given : 



TABLE III. PRODUCE OP WHEAT AND BARLEY AT ROTHAMSTED 



CONTINUOUSLY MANURED WITH FARMYARD MANURE. 



