36 RESULTS WITH WHEAT 



balk wheat field the whole of the produce has been removed 

 from the land since 1840, without any return of farmyard 

 manure, save on plot 2. The condition of the land is thus 

 entirely different to that which occurs in an ordinary rotation. 

 In ordinary arable culture the soil is frequently being enriched 

 by the residues of leguminous crops, by the manure of animals 

 fed on the land, or by actual applications of farmyard manure. 

 The various nitrogenous matters thus supplied nitrify in the 

 soil, and the nitrates thus produced are at the disposal of 

 whatever crop the farmer is pleased to grow. The larger is 

 the quantity of nitrates furnished by the soil, the smaller is 

 the quantity of ammonia salts or nitrate of soda which the 

 farmer can profitably employ. The average produce without 

 ammonia or nitrate was in Broadbalk field 15 bushels. Had 

 the produce without these dressings been 24 bushels, it is 

 clear that only 2 cwts. of sulphate of ammonia could have 

 been profitably employed. If the natural fertility of the land 

 had been equivalent to a crop of 29 bushels (which is 

 about the average produce of wheat land in Great Britain) 

 only i cwt. of sulphate of ammonia could have been safely 

 used. The farmer must in every case have a just idea 

 of what would be the produce of his land without manure, in 

 order to answer the question, ' What quantity of additional 

 manure can I profitably apply ? ' 



Before proceeding further, we should mention that Lawes 

 and Gilbert, when discussing the earlier results obtained at 

 Rothamsted (Jour. Roy. Agri. Soc. 1864, 4^4,) > came to the 

 conclusion that an increase of one bushel of wheat might on 

 an average be expected for each 5 Ibs. of ammonia applied, 

 when the land was well supplied w r ith phosphates, potash, and 

 the other ash constituents of the wheat crop, and the quantity 

 of ammonia employed was not excessive. The average of the 

 40 years' experiments shows that 5-6 Ibs. of ammonia pro- 

 duced one bushel of wheat under the conditions just named. 



Influence of Ash Constituents. Turning now to 

 the results yielded by plots n and 100, shown in Table V., 

 we see clearly that i cwt. of sulphate ammonia will yield 

 5 bushels of wheat in the Rothamsted field only when 



