THE REQUIREMENTS OF PLANTS 37 



certainly true that it causes a much enhanced development of the under- 

 ground collective apparatus of the plant, especially of lateral and fibrou s 

 root, distributing a complete network to a considerable distance around 

 the plant, and throwing innumerable mouths to the surface ". Dress- 

 ings of phosphates are particularly valuable wherever greater root de- 

 velopment is required than the soil conditions normally bring about. 

 They are invaluable on clay soils, where roots do not naturally form 

 well, but, on the other hand, they are less needed on sands, because 

 great root growth takes place on these soils in any case. They are 

 used for all root crops like swedes, turnips, potatoes, and mangolds, 

 and also for shallow-rooted crops with a short period of growth, like 

 barley. Further, they are beneficial wherever drought conditions are 

 likely to come on, because they induce the young roots to grow rapidly 

 into the moister layers of soil below the surface ; probably, as Hall has 

 suggested, this explains the marked effect of superphosphate on wheat 

 in the dry regions of Australia. 



Later on in the life of the plant phosphates hasten the ripening pro- 

 cesses, thus producing the same effect as a deficiency of water, but to a 

 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 * 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 of 

 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- 



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

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



