4 o 



SOIL CONDITIONS AND PLANT GROWTH 



TABLE XV. RESULTS OF WITHHOLDING PHOSPHATES, POTASSIUM COMPOUNDS, AND 

 NITROGEN COMPOUNDS FROM BARLEY. Hoos FIELD EXPERIMENTS, ROTHAMSTED. 



These results are plotted in Fig. 5. The effect of phosphate starva- 

 tion shows itself in depressing the yield of straw and of grain, the straw 

 being the first to surfer. Potash starvation takes longer to set in, 

 not because potassium is less necessary but because the soil contains 

 a larger quantity ; it also affects the straw first. Nitrogen starvation 

 sets in at once, rapidly bringing both grain and straw down to a very 

 low level. 



It is difficult to get behind these effects and ascertain their causes. 

 The function of phosphoric acid in the cell is not easy to discover ; even 

 when the problem is reduced to its simplest state by experimenting 

 with spirogyra in culture solutions little more has been ascertained than 

 that phosphates are wanted for mitotic cell division, doubtless because 

 phosphorus is a constituent of the nucleus, and also for the normal 

 transformations of starch. Loew (181) found that fat and albumin 

 accumulated in absence of phosphates, but the colour was yellow and 

 there was no cell division ; as soon as a trace of potassium phosphate 

 was added, however, energetic cell division took place. Reed (236) 

 showed that starch was formed in absence of phosphorus, but did not 

 change to sugars ; erythrodextrin was formed instead and also cellulose. 



