Dec. i, 1920 



Relation of the Soil Solution to the Soil Extract 



389 



between them were calculated by dividing the concentration of each 

 element by the sum of the concentrations of all the elements determined 

 (Table III). The ratios were found to be very similar in the three 

 extracts, the agreement for several elements being especially close in the 

 first two extracts. In the first extract the larger proportion of solutes 

 present were probably derived from the soil solution, while the subsequent 

 extracts represented to a greater degree previously undissolved fractions 

 of the soil. The results would, therefore, seem to indicate that in con- 

 centrated extracts there is a great similarity in composition between the 

 soil solution and the extract containing the substances which immediately 

 go into solution on the addition of a slight excess of water. Even with 

 nitrate, which might be supposed to have such a high degree of solubility 

 that the total quantity present would be contained in the soil solution, 

 it is probable that a certain proportion is held in some adsorbed or 

 undissolved form. If an extract of the soil be made, a readjustment 

 takes place because of the great dilution of the soil solution, and the 

 total quantity of adsorbed nitrate would be greatly diminished, even 

 though the partition ratio between solution and soil remained constant. 

 Thus, it is possible to extract nearly all the nitrate present but difficult 

 or impossible to remove the last traces. 



Table III— Composition of successive teachings of soil 

 [8 parts soil to 1 part water.] 



Solute. 



First leaching. 



Second leaching. 



Third leaching. 



Concentra- individua i 

 tionot flutes to 

 solution. totaL 



^° A Concentra- JS£&£, Concentra- 



Nitrate (NO s ) . . 

 Calcium (Ca) . . . 

 Magnesium (Mg) 

 Potassium (K). . 

 Sulphate (SO,). 



P. p. m. 



42 5 

 90 



Per cent. 



57- ° 

 12. 1 

 11. 8 



3-° 

 16. 2 



tion of 

 solution. 



P. p. m. 

 195 



45 

 43 

 15 

 67 



individual 1 tkm of 

 solutes to s^ution. 

 total. 



Per cent. 



53- ° 

 12.3 



11. 8 



4.1 

 18.4 



P. p. m. 



133 



46 



34 

 13 

 66 



Ratio of 



individual 



solutes to 



total. 



Per cent. 

 46. O 

 IS-8 



11. 6 



4.4 

 22.6 



With regard to phosphate the case is not so clear. Most of the extrac- 

 tion studies described in previous articles have indicated that the various 

 extracts are saturated with respect to phosphate. Thus, if the extract 

 were concentrated without precipitation the concentration of phosphate 

 should be considerably greater than in the soil solution. Since, however, 

 the adsorption or precipitation of phosphate by the soil is a relatively 

 slow process, in the present experiment the time may have been insuffi- 

 cient for readjustment of the equilibrium. From our previous experi- 

 ments we should be inclined to infer that the concentration of phosphate 

 in the soil solution is usually very low, but that immediate replacement 

 occurs as phosphate is absorbed by the plant, thus producing a constant 

 concentration of phosphate over long periods of time. 



