290 SCIENCE PROGRESS 



The crop on Plot 5 has nothing beyond the nitrogenous 

 material in the soil. To the other plots nitrogenous manure 

 has been added, so that the crops get respectively 43, 86 and 

 129 lb. of nitrogen over and above what is present in the 

 soil. 



It might be supposed that the increased yields thus obtained 

 should be simply proportional to the increased nitrogen supply, 

 but the figures show that this is not the case, nor would it 

 really be expected. The amount of food a plant takes up 

 depends not only on the concentration of the food-stuff in the 

 soil, but also on the extent of the absorbing root surface. The 

 first increments of nitrogen increase the root system and there- 

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

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

 growth is more than proportional to the added nitrogen, and 

 the second increment of nitrogen on Plot 7 produces a larger 

 increase than the first increment on Plot 6. This increase, 

 however, does not go on indefinitely. Some other factor, such 

 as the water supply, that sufficed for the smaller crop no longer 

 suffices for these larger ones, and thus sets a limit to the 

 increase ; further increments of nitrogen therefore give a 

 smaller proportionate crop return. Still further additions of 

 nitrogen introduce a new complication. The straw is not 

 strong enough to carry these heavier crops, and they do not 

 stand but become " laid," in which condition they are difficult 

 and expensive to harvest. 



The two general principles thus brought out hold not only 

 lor nitrogen manuring but for manuring generally. They have 

 an important economic application and show that, up to a certain 

 point, the better the farming the higher the profit ; beyond this, 

 the profit falls off. A scheme of manuring that pays when 

 wheat is selling at 355. a quarter may be unprofitable when 

 wheat is only 30s. Certain complications may arise from 

 interactions between the added manure and soil which modify 

 the water and air supply, or from secondary actions of manures 

 on the crop, but these details need not now be discussed. 



The same general principles can be traced in the effects of 

 water supply on the yield of wheat. Without water no crop is 

 possible : with successive additions of water more and more 

 crop is obtained ; beyond a certain point, however, the increased 

 yield is no longer proportional to the increased water supply 



