THE SOIL AND THE PLANT 145 



If we group the figures to show their distribution we find in 

 the case of phosphoric acid the number of instances where there 

 are : 



Less than 5 parts per million of dry soil = 120 or 2797 per cent. 

 5 to 10 „ „ „ = 193 „ 44-99 



10 » 20 „ „ „ = 105 „ 24-47 



20 „ 30 „ „ „ = 8 „ r86 „ 



30 1, 40 „ „ „ » 3 n o*7o ., 



A similar distribution can be worked out for the other con- 

 stituents. Under the circumstances we cannot accept the 

 conclusion that the soil solution is of the same composition 

 in all cases. The difference in view is more fundamental than 

 it appears. Whitney tacitly assumes that the concentration 

 of the soil solution is uniform throughout the mass of soil. 

 On our view the concentration is subject to considerable local 

 fluctuations, depending on the local partial pressure of carbon 

 dioxide in the soil atmosphere which in turn depends on the 

 distribution of roots and other living agencies, on the dis- 

 tribution of calcium carbonate, of water and other factors. 

 Even if a quantity of the soil solution could be extracted we 

 cannot agree that the average composition it possessed would 

 have any real counterpart in the soil. 



However, these criticisms are met by the argument that 

 variations in concentration of the culture solution do not in 

 any case influence the rate of plant growth and consequently 

 any variations in the strength of the soil solution would be 

 without effect on fertility. Again we cannot accept the state- 

 ment. In an infinitely dilute nutrient solution, i.e. in pure 

 distilled water, plants make no growth ; the amount of growth 

 increases with the concentration till some limiting factor inter- 

 venes, when further increase in concentration produces no 

 increase in crop ; finally, if the concentration become too 

 great, secondary adverse effects set in and growth is depressed. 

 There is a fairly wide range over which the limiting factor 

 controls the situation and the nutrient solution may increase 

 in composition without increasing plant growth ; accordingly 

 we find a number of culture solutions in use for plant physio- 

 logical purposes, but we have no evidence whatever that the 

 soil solution is anywhere near this range of concentration. The 

 fact that plant nutrients such as nitrates, phosphates and 

 potassium salts are among the commonest and most effective 



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