LOSSES OF PLANT FOOD FROM PLANTS 553 



finally yielded four tons of dry matter per acre, contained 87 

 pounds of potassium on June 17, but only 66.6 pounds at maturity. 

 The total plant, including the roots and stubble, contained 106.8 

 pounds per acre of potassium on June 17, but only 69.4 pounds at 

 maturity (July 27). 



Wilfarth, Romer and Wimmer's investigations with spring 

 wheat showed the following results: 



RELATIVE AMOUNTS OF PLANT FOOD IN WHEAT CROPS 



In the case of potatoes, the tops at maturity contained only 24 

 per cent of the maximum dry matter, the corresponding percent- 

 ages being 43 for nitrogen, 39 for phosphorus, and 13 for potas- 

 sium; but in the tubers the maxima for all constituents were 

 found at maturity. 



After discussing different suggestions, Wilfarth, Romer, and 

 Wimmer state that the only possible assumption for the loss of po- 

 tassium from the barley plants is that it returns to the soil through 

 the roots ("durch die Wurzeln in den Boden zuriickzuwandern"). 



In a letter dated March 13, 1908, the author expressed the 

 following opinion to his colleague, Professor J. H. Pettit, then 

 at the University of Gottingen: 



"I do not believe the return is by the internal route through the stem and 

 root, but by leaching, even before the plants are sufficiently mature to harvest." 1 



1 This opinion was based largely upon experience in the preparation of phar- 

 maceutical infusions and upon field observations, as where corn on peaty swamp 

 land shows markedly the effect of potassium leached from an oat shock which 

 stood through one or two heavy rains the previous season. In the letter to Professor 

 Pettit mentioned above, a complete outline was given for a laboratory and pot- 

 culture experiment to secure more exact data upon the problem, but subsequently 

 reported investigations render this unnecessary. 



