66 



SOIL CONDITIONS AND PLANT GROWTH 



pronounced; it fell a little to 41-3 per cent, in the sixth ten 

 years (1902-1911), but rose to 46*8 per cent, in the period 

 1913-1919. Even in sand cultures the difference is not very 

 marked: Hellriegel (130^) grew barley with varying supplies 

 of phosphate with results given in Table XIX. In absence 

 of phosphate no grain was formed ; when a little was added 

 grain formation proceeded normally, and the resulting grain 

 was nearly full weight per individual ; as the phosphate supply 

 increased the percentage of grain increased, but soon reached 

 a maximum beyond which it would not go. 



TABLE XIX. EFFECT OF VARYING PHOSPHATE SUPPLY ON THE GROWTH OF 

 BARLEY IN SAND CULTURES. HELLRIEGEL (130^). 



It is in the total growth of straw and of grain that the 

 effect of phosphate is manifested as shown in Table XX. 



The Rothamsted results are plotted in Fig. 12. The 

 effect of phosphate starvation shows itself in depressing the 

 yield of straw and of grain, the straw being the first to suffer. 

 Potash starvation takes longer to set in, not because potassium 

 is less necessary but because the soil contains a larger quan- 

 tity ; 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 essential for mitotic cell division, doubtless because phos- 



