XIII.] FERTILISERS FOR WHEAT 263 



amount of manure, we shall soon reach the stage when 

 the last addition has no effect at all. This "law of 

 diminishing returns," as it is called, means that a 

 farmer cannot recoup himself for low prices by forcing 

 big crops with the aid of fertilisers. The increase thus 

 bought is the dearest part of the crop. For every farm 

 there is a sort of level of expenditure on materials like 

 fertilisers, and the more profitable and the richer the 

 land, and the more valuable the crops that can be sold, 

 the higher will this level become. Bearing this fact in 

 mind, we find that in Britain the wheat crop rarely 

 receives any fertiliser, except perhaps a small top- 

 dressing of nitrate of soda or soot in the early spring. 

 Wheat has a long period of growth and an extensive 

 root system, whereby it is able to forage for itself pretty 

 thoroughly, and obtain all the phosphates and potash it 

 requires from soil in ordinary good conditions. As, 

 however, it makes its growth during the period of the 

 year when the soil is comparatively cold, and as the soil 

 has received very little cultivation before the wheat is 

 sown, and none at all while it is growing, the processes 

 which produce ammonia and nitrates out of the nitro- 

 genous residues of the soil cannot be very active at the 

 time the wheat chiefly requires its nitrogen. Hence the 

 value of nitrogenous fertilisers to wheat in the spring, 

 whereas phosphates and potash meet with very little 

 response. If nitrogen is the dominant fertiliser for 

 wheat, barley, on the other hand, chiefly demands 

 phosphates. It is a comparatively shallow-rooted plant, 

 and makes its growth in the late spring on land which 

 has been much more thoroughly prepared than that on 

 which the wheat crop is sown. In the United Kingdom 

 barley is very often grown on land which is already com- 

 paratively rich because the ground has been previously 

 occupied by the turnip crop, which may even have been 



