JOWiNAL OF AGKICHTIAL RESEARCH 



DEPARTMENT OF AGRICULTURE 

 Vol.. VI Washington, D. C, Aprii. io, 191 6 No. 2 



SOLUBLE NONPROTEIN NITROGEN OF SOIL 



By R. S. Potter, Assistant Chief in Soil Chemistry, and R. S. Snyder, Assistant in 

 Soil Chemistry, Iowa State College Experiment Station 



INTRODUCTION 



Dilute alkali dissolves a larger proportion of the organic material of soil 

 than any of the other relatively mild reagents. A still larger percentage 

 is extracted from soil previously treated with i per cent of hydrochloric 

 acid (HCl), and this latter reagent dissolves but little of the organic mate- 

 rial. The term "humates" is fast disappearing from current scientific 

 literature, yet one often reads that the reason the preliminary washing 

 with acid renders the organic matter more soluble in the alkali is that the 

 calcium of the calcium humates is dissolved out, making the free humic 

 acids soluble in the alkali. To say that the proteins are rendered more 

 soluble by the removal of the calcium and the heavy metals would 

 explain the solubility just as well and would be more correct scientifically. 



As pointed out by Lipman (4, p. 251), much of the organic nitrogen of 

 the soil must be protein in nature. The chief sources of the nitrogen are 

 crop residues, manures, and bacterial cells, and in these much of the nitro- 

 gen is in the form of protein. Investigations carried out in this laboratory 

 (5) have shown that soils contain a large quantity of the so-called humin 

 compounds. These have a great tendency to be adsorbed by such com- 

 pounds as magnesium oxid and calcium hydroxid, and therefore removal 

 of calcium from the soil by acid would tend to make these more soluble. 



Upon the acidification of the alkali extract a precipitate is obtained which 

 has been called humic acid. This term also is no longer taken seriously. 

 It would seem that the rational explanation of this precipitate would be 

 simply that it was made up of proteins thrown down, as salts of the 

 precipitant, as salts of organic acids, such as nucleic acid (7), or resinous 

 acids (6), both of the latter substances having been isolated from the acid 

 precipitate. It would also contain, no doubt, some free organic acids. 



In analyses of the solution obtained by the prolonged boiling of soils 

 with strong acids and of the hydrolyzed humic acids by the Van Slyke 

 method (8), it was found that the results for the humic acids did not 

 differ markedly from the results for the organic matter of the soils as a 

 whole from which they were derived. Since that time it has occurred to 



Journal of Agrictiltural Research, Vol. VI, No. a 



Dept. of Agriculture, Washington, D. C. Apr. lo, 1916 



cy Iowa — » 



(61) 



