Natural Sources 



resulted, but it is not clear that these increases were signifi- 

 cantly different from those produced by an equivalent 

 amount of N-P-K. 



The experiments of Hartley and Greenwood (1933) which 

 showed marked increases in crops after application of the 

 small dressings of i and 2 tons of farmyard manure, were for 

 some time believed to show a special value of farmyard manure, 

 since the equivalent amounts of nitrogen in the form of artifi- 

 cials failed to produce a similar effect. It was later shown, 

 however (Hartley, 1937) that increases similar to those given 

 by farmyard manure in these small dressings could be 

 obtained by the use of equivalent amounts of readily-avail- 

 able phosphate. 



The remarkably long-lasting yield responses to farmyard 

 manure on one of the Hoosfield (Rothamsted) classical barley 

 plots, still await a satisfactory explanation. Plot 7-1 annually 

 received farmyard manure at the rate of 14 tons per acre from 

 1852 until 1 871, but has been unmanured since. The yields of 

 both straw and grain on this plot continue to be appreciably 

 higher than those on the unmanured plots or on those 

 receiving certain artificials without nitrogen. The average 

 yield (since 1871) is about the same as that of the plot annu- 

 ally receiving 206 lbs. of sulphate of ammonia only, but is 

 less than that of the plots receiving 275 lbs. of nitrate of 

 soda. 



In this connexion, the rather neglected work of von 

 Liebenberg (19 16) may be recalled. For a rotation manured 

 once with a normal dose of farmyard manure and artificials, 

 Liebenberg showed that doses of 20 and 50 kg. of farmyard 

 manure per hectare, applied annually each autumn there- 

 after, produced appreciable, though small increases, in crops. 

 Considerable tabular details of yields and composition were 

 given. He wrote of the 50 kg. dose that nutrients as well as 

 organic matter were active, but in the 20 kg. dose "diirfte 

 wohl die organische Substanz allein oder in der Hauptsache 

 die ertragsteigernde Wirkung verursachen." 



By Mr. N. W. Barritt the author has been shown quince 



67 



