72 THE SOIi, SOLUTION 



if these are brought to them in the soil solution, and indeed 

 evidence is not lacking that such may sometimes be the case 

 even with the higher plants. Certainly their growth can be 

 much affected by the presence of different organic substances in 

 the nutrient solution. Enough work has been done in this field 

 of investigation to show that the concentration of the soil solution 

 or artificial nutrient solution with respect to the organic com- 

 pounds must generally be low; too high a concentration always 

 inhibits growth or even produces death; and there is probably 

 an optimum concentration, or one at which the plant will grow 

 best; but this optimum concentration varies with the specific 

 nature of the plant, the presence of other dissolved substances, 

 mineral or organic, and possibly with other factors. While a 

 notable amount of work has thus been done in a field of inquiry 

 obviously of practical as well as theoretical interest, almost no 

 definite information has as yet been obtained as to the concen- 

 tration of organic substances in the soil solution, or its effect 

 upon plants under field conditions, excepting in the case of the 

 nitrates, the products of bacterial activities. The concentration 

 with respect to nitrates is known to vary greatly from a few 

 parts to several thousand parts per million, and this sometimes 

 within a few days or even hours. The great changes in con- 

 centration with respect to nitrates, the rapidity of the changes, 

 and the correspondingly large effects on growing plants make 

 this a subject requiring special treatment by itself. This at 

 present seems more easily appreciated from a consideration of 

 the bacteria involved, and will not be discussed more fully here. 1 

 Of the ash constituents of plants, there must be in the soil 

 solution, potassium, magnesium, phosphorus, sulphur and iron 

 for any plant growth, and for the higher crop plants, calcium 

 1 See : The fixation of atmospheric nitrogen by bacteria, by J. G. 

 Lipman, Bull. No. 81, Bureau of Chemistry, U. S. Dept. of Agriculture, 

 1904; A review of investigations in soil bacteriology, by Edward B. 

 Voorhees and Jacob G. Lipman, Bull. No. 194, Office of Experiment Sta- 

 tions, U. S. Dept. of Agriculture, 1907; The physiology of plants, by 

 W. Pfeffer, translated by A. J. Ewart, vol. I, p. 388 et seq., 1900; The 

 effect of partial sterilization of soil on the production of plant food, by 

 Edward John Russell and Henry Brougham Hutchinson, Jour. Agric. 

 Sci., 3, 111-144 (1909). 



