SOIL CONDITIONS AFFECTING PLANT GROWTH 63 



period is so much longer than that of the other crops, con- 

 tinuing until the end of October. 



TABLE XVII. EFFECT OF VARYING SUPPLY OF NITROGENOUS MANURE ON 

 THE GROWTH OF CROPS. ROTHAMSTED. 



The actual increase of growth brought about by successive 

 increments of nitrogenous food depends on the amount of 

 water and other nutrients, on the temperature, and so on ; any 

 of these may act as limiting factors. Table XVIII. shows the 

 crops obtained on some of the Rothamsted mangold plots ; in 

 one case the supply of potassium is so small that it becomes 

 the limiting factor, in the other sufficient potassium is supplied. 



TABLE XVIII. INFLUENCE OF POTASSIUM SALTS ON THE ACTION OF 

 NITROGENOUS MANURES. ROTHAMSTED. 



The effect of varying water supply is more conveniently 

 studied in pot experiments than in the field, since any com- 

 parison between yields in wet and dry seasons is complicated 



1 From 400 Ib. ammonium salts. 



2 From 400 Ib. ammonium salts and 200 Ib. rape cake. 



