14 



Yield of Wheat. 



magnesia, soda, sulphuric acid, and silica also play their part, but 

 are supplied in sufficient quantity by all ordinary soils. 



Nitrogen. — At the time the experiments were commenced the 

 necessity for nitrogenous manures was denied by Liebig ; several 

 of the plots were therefore arranged to show the effects of different 

 amounts and various forms of nitrogenous manure. It was soon 

 demonstrated that nitrogenous manures were necessary, and that 

 the. yield was proportional to the nitrogen supplied. The action 

 of two sets of factors may be traced in the results. 



(a) If we have a series of plots, each receiving more phosphoric 

 acid and potash than the plant can possibly require, the yield on 

 each plot should be strictly proportional to the supply of nitrogen 

 if the wheat plant be able to deal with all the nitrogen it receives. 

 The amount of food a plant takes up, however, depends on the 

 extent of the absorbing root surface. At first an increase in the 

 amount of nitrogen in the soil increases the root system, and 

 therefore the absorbing surface, as well as the amount of material 

 that each unit of this surface can take up. Hence the yield is 

 more than proportional to the supply, i.e., the second increment 

 of nitrogen on Plot 7 produces a larger increase than the first 

 increment on Plot 6. When yield is plotted against supply of 

 nitrogen the curve begins by being concave instead of linear. 

 (Curve 1.) 



rote/ 

 produce 



43 



86 



J29 ■ 



Pounds of Nitrogen supplied per acre. 



Curve i.— Effect of increasing Supply of Nitrogen on the 

 Yield of Wheat. 



There are a few exceptional years in which this relationship 

 does not hold. 



(b) When further amounts of nitrogen are supplied, other 



