38 SOIL CONDITIONS AND PLANT GROWTH 



less extent ; for this reason they are applied to the wheat crop in some 

 of the northern districts of England to bring on the harvest a few days 

 earlier and obviate risk of loss by bad weather. The northern limit of 

 growth of several crops may in like manner be extended. This ripen- 

 ing effect is well shown on the barley plots at Rothamsted ; crops re- 

 ceiving phosphates are golden yellow in colour while the others are still 

 green. 



But these effects, important as they are, are nothing like as striking 

 as those shown by nitrogen compounds. There is no obvious change 

 in the appearance of the plant announcing deficiency or excess of phos- 

 phate l like those changes showing nitrogen starvation or excess ; the 

 hastening of maturity is only seen when there is a control plot unsup- 

 plied with phosphates and does not even lead to an increase in the pro- 

 portion of grain borne by the plant. On the Rothamsted plots supplied 

 with nitrogen and potassium compounds, but no phosphate, the grain 

 formed 44*9 per cent, of the total produce during the first ten years ot 

 the experiment (1852-1861), and almost exactly the same proportion 

 (447 per cent.) during the fifth ten years (1892-1901) when phosphate 

 starvation was very pronounced. Even in sand cultures the difference 

 is not very marked, Hellriegel (131) grew barley with varying supplies 

 of phosphate with results given in Table XIV. In absence of phosphate 

 no grain was formed ; when a little was added grain formation pro- 

 ceeded 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 XIV. EFFECT OF VARYING PHOSPHATE SUPPLY ON THE GROWTH OF BARLEY 

 IN SAND CULTURES. HELLRIEGEL (131). 



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

 phosphate is manifested as shown in Table XV. : 



1 Barley grown in water cultures without phosphorus compounds acquires a red colour 

 in the stem, but this i not commonly seen in the field. 



