284 THEORIES OF FERTILISER ACTION [chap. 



by the series of curves in Fig. 5 ; where the cost of 

 production forms a straight line that is always inter- 

 sected by the curves expressing the value of the returns, 

 which begin by rising more rapidly than the cost of 

 production, but tend to become horizontal. The point 

 of intersection, when profit ceases, is nearer the origin 

 the lower the range of prices obtainable for the crop, 

 as shown by the two curves representing the returns at 

 low and high prices respectively ; this demonstrates that 

 the expenditure on fertilisers or anything else required 

 by the crop must be reduced when prices of produce 

 are low, or, as expressed by Lawes, high farming is no 

 remedy for low prices, 



Liebig's law of the minimum must, however, be 

 extended to all the factors affecting the yield as well as 

 to the supply of plant food, e.g., to such matters as the 

 supply of water, the temperature, the texture of the soil. 

 Any one of these may be the determining factor which 

 limits the yield, or two or more of them may act 

 successively at different periods of the plant's growth. 

 On poor soils the water-supply is very often the limiting 

 factor — on very open soils because the water actually 

 drains away, on extra close soils because the root range 

 is so restricted that the plant has but little water at 

 hand and the movements of soil water to renew the 

 supply are very slow ; in either case for comparatively 

 long periods the plant will be sure to have as much 

 nutriment as is required for the small growth permitted 

 by the water present. It is only when the water-supply 

 is sufficient that the resources of the soil, as regards all 

 or any of the constituents of a fertiliser, are tested and 

 may become in their turn the limiting factors in the 

 growth of the crop. Hence it follows that fertilisers 

 may often be wasted on poor land, where growth is 

 limited by the texture of the soil, by the water-supply, or 



