Scientific A griculture. 



236 



[March, 1912. 



simple proportion. This complex is de- 

 composable by change in temperature, 

 concentration, etc. ; but the process is 

 continuous anil does not show the per 

 saltern character of chemical change ; 

 the complex can be acted upon by 

 various solutions (as for example added 

 fertilisers), absorbing certain substances 

 as a whole, e.g., organic substances, or 

 simply giving up to the solution an 

 amount of base equivalent to what it 

 has absorbed. These changes, he has 

 shown, follow the ordinary laws of 

 absorption of colloids [2, 3]. On this view 

 the concentration of the soil solution 

 directly depends on the composition of 

 the colloidal complex, and may be altered 

 within limits by addition of soluble 

 salts. 



Thus we are np against a question of 

 fact— whether the soil solution is or is 

 not constant in composition in the case 

 of different soils, Unfortunately the 

 difficulty of extracting the soil solution 

 has not been satisfactorily overcome, 

 and recourse must be had to devices 

 which give its composition only approxi- 

 mately. If we suppose that the water 

 of a well represents the soil solution, 

 then there is clearly no sort of constancy 

 of composition in passing from soil to 

 soil, as can be seen by looking through 

 the numerous recorded analyses. If we 

 suppose that the drainage water from 

 the top layer of soil, which is rathei 

 different in composition from well 

 water, more nearly represents the soil 

 solution we again find no evidence of 

 constancy, although in the case of a 

 particular field the drainage water may 

 show but little variation throughout 

 the whole season.* Unfortunately suffi- 

 cient data are not yet collected. But 

 perhaps it is fairest in this discussion to 

 accept Whitney's method and to analyse 

 the figures obtained by Taylor and 

 Mooney on which he based his state- 

 ments. Considerable variations are 

 found ; the highest and lowest values 

 are :— 



Highest, Lowest. 

 Phosphoric acid ... 40 60 59 

 Nitric acid ... 62 00 trace 



Calcium ... 102 85 trace 



Potassium ... 62-20 493 



* Von Seelhorst (Journal fur Landwirtschaft, 

 1901, 49, 251) made complete analyses, at first 

 each week and afterwards each fortnight, of 

 drainage water rum ii g ftom a fie d during the 

 year in August, 1899, to August, 1900. The re- 

 sults in parts per million are : — 



CaOMgO S0 3 KaONs05 

 Highest figures ... 184 46 4 59'2 3 7 8-2 

 Lowest figures ... 157 31-3 43 5 1-7 1-0 



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 27'97 p. e' 

 5 to 10 do do do = 193 ,, 44 99 „ 

 10 „ 20 do do do = 106 „ 24-47 „ 

 20 „ 30 do do do = 8„ 186 „ 

 30 „ 40 do do do =3 „ 0-70 „ 



A similar distribution can be worked 

 out for the other constituents. 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. Ou our view the con- 

 centration 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 distri- 

 bution 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 com- 

 position it possessed would have any 

 real counterpart in the soil. 



However, these criticisms are met by 

 the argument that variations in con- 

 centration of the cultural 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 

 statement. In an infinitely dilute nu- 

 trient solution, i.e. in pure distilled 

 water, plants make no growth ; the 

 amount of growth increases with the 

 concentration till some limiting factor 

 intervenes, when further increase in 

 concentration produces no increase in 

 crop ; fiually, if the concentration be- 

 come 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 num- 

 ber of culture solutions in use for 

 plant physiological 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, phos- 

 phates and potassium salts are among 

 the commonest and most effective man- 

 ures, whilst non-nutiient salts are not 

 in the use for this purpose, is strong 

 grouudfor allowing nutritive functions 

 to fertilisers ; nor since their complex 



