Yield of Wheat. 



15 



limiting' factors come into play, the increase being smaller for the 

 third and fourth increments of nitrogen. The curve of production 

 becomes convex, illustrating- the law of diminishing - returns. 

 There is an important practical application of this curve in districts 

 where it is customary to manure for wheat. So long- as the 

 increased crop is more than proportional or is simply proportional 

 to the supply of food, it may be profitable to go on adding manure. 

 But when the yield falls off, a point is reached where further 

 additions of manure are unprofitable. 



f?f>t,irn c C° rn 32/- 



mums - Street. .30/- 



Cost of Production 



RfiHimi Cornet 24t 

 ™tums. straw 3f 20/- 



43 86 /Z9 



Pounds of Nitrogen supplied per acre. 



Curve 2. —Returns from Plots receiving varying Quantities 

 ok Nitrogen. 



The diagram No. 2 shows the results obtained during a selected 

 period of 13 years, when there were four plots receiving regular 

 increments of nitrogen. The vertical distance between the dotted 

 line (cost of production) and the curve of returns shows the profit 

 or loss accruing from the varying quantities of manure. Up to 

 a certain point, the better the farming the higher the profit ; 

 beyond this the profit falls off. The curve illustrates the law of 

 diminishing returns and also Lawes' dictum that "high farming 

 is no remedy for low prices." 



The general principles which have just been illustrated that the 

 earlier increments of nitrogen may produce increased yields more 

 than proportionate to themselves, while later amounts are followed 

 by a constantly diminishing increase — i.e., that the curve x)f 

 production is first concave and then convex — is true not only of 

 nitrogen but of manure generally and of any of its simple con- 

 stituents, should the experiments begin with a deficiency and end 

 with an excess. The principle is also applicable to water supply 

 and to many other factors, each of which may limit the crop 

 production. For example, in arid climates the yield is generally 

 unaffected by the supply of nitrogen because it is determined 

 wholly by the water supply, enough nitrogen being always present 

 to satisfy the needs of a larger crop than the limited water supply 

 will permit of. 



B 2 



