64 SOIL CONDITIONS AND PLANT GROWTH 



by the great differences in temperature conditions. Tucker 

 and von Seelhorst's experiments have already been described 

 (pp. 38 and 44). 



From the practical point of view the important result is 

 that a given increase in the food supply may produce no in- 

 creased growth, small increase, or a larger increase, according 

 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. 33) 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 1*80 i'74 2*40 



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

 early stages of growth it promotes root formation in a re- 

 markable way. So long ago as 1847 Lawes (161) 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 con- 

 siderable distance around the plant, and throwing innumerable 

 mouths to the surface ". Dressings of phosphates are par- 

 ticularly effective wherever greater root development is re- 

 quired 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 



