44 EXPERIMENTS UPON WHEAT 



search the soil for mineral plant food ; hence when grown 

 under ordinary farm conditions in rotation, it is rarely necessary 

 to supply it with any but nitrogenous manures. As it is also 

 grown during the cooler season of the year and with very little 

 cultivation of the ground, the natural nitrifying processes are 

 slow, hence the special need for an external supply of nitrogen 

 in the shape of manure. 



Plots 9 and 16 receive nitrate of soda and mineral manures, 

 so that Plot 9 has the same manuring as Plot 6, and Plot 16 

 as Plot 7, except that the ammonium-salts on Plots 6 and 7 

 are replaced by equivalent amounts of nitrate of soda. The 

 manuring of Plots 9 and 16 has however been changed 

 during the progress of the experiments, so that they are only 

 comparable with 6 and 7 since 1885. Taking the averages 

 from 1893 to 1902 as set out in the diagram Fig. 4, it will be 

 seen that nitrate of soda is a more effective source of nitrogen 

 than the ammonium-salts ; the single application yields 16 per 

 cent, more grain and 26 per cent, more straw than the corre- 

 sponding amount of ammonium-salts : the double application, 

 however, yields practically the same amount of grain and only 

 about 1 cwt. more straw. This superiority of nitrate of soda 

 for wheat is no doubt partly due to the fact that it remains 

 soluble, thus diffusing deep into the soil and encouraging a 

 greater range of roots, whereas the ammonium-salts are 

 retained near the surface. The injurious effects of continuous 

 applications of ammonium-salts, which are due to the removal 

 of the carbonate of lime from the soil and its resultant acidity, 

 now so strikingly shown on the corresponding permanent 

 wheat and barley plots on the Royal Agricultural Society's 

 farm at Woburn, are not apparent at Rothamsted, where the 

 soil started with a good supply of chalk. Analyses made in 

 1904 show that the soil of Plot 7 still contains more than 

 2 per cent, of carbonate of lime. 



It should be noticed that the increase of crop for each 

 application of nitrogen is not proportional to the extra nitrogen 

 supplied, but that each successive addition gives a smaller 



