i8o 



SOIL CONDITIONS AND PLANT GROWTH 



3. When the conditions are made wholly normal by allow- 

 ing vegetation to grow, some of the nitrate is taken up by the 

 plant and only a part is washed away, the division depending 

 on the favourableness of the conditions for plant growth. 

 The absorption of nitrate by the plant is much greater, and 

 the amount of nitrate in the drainage water is therefore much 

 less, on the Rothamsted wheat plots where ample supplies of 

 potassium salts and phosphates are present, than on the plots 

 where these nutrients are less abundant and the crops smaller 

 (Table XLIX.). 



Table XLIX. — Effect of Phosphates and Potassium Salts on the 

 Utilisation of Nitrates by Plants, 



Part of the absorbed nitrate remains in the root and 

 stubble, and is again added to the soil when the plant dies. 

 Hence the percentage of nitrogen in the soil is higher where 

 the conditions are favourable for the growth of plants than 

 where, by the operating of some limiting factor, plants cannot 

 make full growth and therefore leave untouched much of the 

 nitrate to be washed away. It is this that accounts for the 

 losses in fallow ground — losses that have been discussed by 

 Lawes, Gilbert, and Warington {\66b\ by Russell (241/) and 

 by von Seelhorst (258). 



Most of the data hitherto accumulated are incomplete, 

 because they refer only to crop results and take no account of 

 nitrates washed out in the drainage water : fuller data have, 

 however, been obtained in the lysimeter experiments of Gerlach 



