34 



SOIL CONDITIONS AND PLANT GROWTH 



white turnips show increases in leaf greater than those in root, but 

 mangolds show substantially the same increase both in leaf and root, 

 because their growing period is so much longer than that of the other 

 crops, continuing until the end of October. 



TABLE XI. EFFECT OF VARYING SUPPLY OF NITROGENOUS MANURE ON THE GROWTH 



OF CROPS. ROTHAMSTED. 



The actual increase of growth brought about by successive incre- 

 ments of nitrogeneous 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 XII. 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 XII. 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 comparison between 

 yields in wet and dry seasons is complicated by the great differences 

 in temperature conditions. Tucker and von Seelhorst (257) put up 

 three series of soil pots in which the water was kept at a definite 



1 From 400 Ib. ammonium salts. 



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



