THE REQUIREMENTS OF PLANTS 37 



series expressing the operation of a limiting factor ; they would more 

 properly be expressed by a surface. From the practical point of view 

 the important result is that a given increase in the food supply may 

 produce no increased growth, small increase, or a larger increase, ac- 

 cording to the extent of the water supply. 



Phosphorus. Phosphates are by far the most efficient phosphorus 

 foods known for plants. The relationship between phosphorus supply 

 and growth has been measured by E. A. Mitscherlich (p. 24) in a series 

 of experiments on oats grown in sand with each of the three calcium 

 phosphates. For equal weights of the three salts the relative efficiencies 

 corresponded with the basicity; for equal weights of P 2 O 5 , however, 

 the values were 2 -66 : 2*31 : 1-65. This was in sand cultures ; in soils 

 different efficiencies were found : thus for the mono-phosphate the 



values were : 



Sand. Soil i. Soil 2. Soil 3. 

 2'66 i'8o 174 2-40 



The effect of a phosphate on the crop is twofold. In the early 

 stages of growth it promotes root formation in a remarkable way. So 

 long ago as 1847 Lawes (160) wrote : " Whether or not superphosphate 

 of lime owes much of its effect to its chemical actions in the soil, it is 

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

 ground collective apparatus of the plant, especially of lateral and fibrous 

 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, 

 in their absence swedes and turnip roots will not swell but remain 

 permanently dwarfed like radishes : 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 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 



