SOIL CONDITIONS AFFECTING PLANT GROWTH 65 



radishes (Fig. 1 1 shows one of Lawes and Gilbert's photographs) : 

 the introduction of superphosphate as a fertiliser revolutionised 

 agriculture on some of the heavier soils by allowing better 

 growth of these crops. Phosphates are needed also for 

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

 barley. Further, they are beneficial wherever drought is 

 likely to set in, 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 processes, thus producing the same effect as a de- 

 ficiency 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, and the oat crop in the west, 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 ripening effect is well 

 shown on the barley plots at Rothamsted ; crops receiving 

 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 phosphate ^ like those changes showing 

 nitrogen starvation or excess ; the hastening of maturity is 

 seen only when there is a control plot unsupplied with phos- 

 phates : it leads to no increase in the proportion 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 ( 1 892-1 901) when phosphate starvation was very 



^ Barley grown in water cultures without phosphorus compounds acquires a 

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



5 



