LEAVITT ON SOIL H U M U S . FRoDi THE LABORATORY OF SOIL CHEMISTRY.UNIVERSITY TENNESSEE', AGRICULTURAL EXPERIMENT STATION.]
[CONTRIBUTION OF
SOIL HUMUS AS DETERMINED'BY DIFFERENT METHODS.
By SHERMAN LEAVITT.
Received April 6 , 1910.
From the wide variation in results on samples of several different types of soil when analyzed for percentage of humus by five well-known methods, the writer has been led to believe that these variations may be partly accounted for by the fact that the several methods may individually include or exclude certain organic substances as humus. The Official method for humus,' based on that of Grandeau, is, to my howledge, followed by most of the soil chemists in this country at least to the completion of the extraction of organic matter with four &r cent. ammonia. This method determines the organic matter by evaporation of a clear aliquot free from clay (a difficult result to obtain) and by matter in a dish, burning off the We may therefore assume that soil humus is emperically the total organic matter, combined with a little inorganic matter, soluble in a solution of four per cent. ammonia at ordinary temperature. Thus far, I believe, all methods agree, however they disagree from this point. -4s is well known, the Official method, when followed literally, is very unsatisfactory and gives abnormally high results This is due to the presence of clay which cannot readily be removed by the method as given. Different investigators have accordingly this modified this method or less to difficulty. A~~~~ the modified methods may be mentioned those of Cameron and B r e a ~ e a l e ;Peter ~ and A ~ e r i t t ;Mooers ~ and H a m p t ~ n ;and ~ C. W. Stoddart.i Briefly, the principal modification of each of these four methods is as follows: Cameron and Breazeale filtered off the clay by means of a pasteur-chamberland filter and determined the humus in the filtrate free from clay. peter and Averitt used the Official method in connection with a correction factor of ten per cent. of the residue which is left after burning off organic matter and which must be subtracted from the total loss in weight. This factor seems to apply fairly but, as M~~~~~ and H~~~~~~ well in have shown, it does not apply to all soils. Mooers and Hampton devised the method which l ~ is, partially is in use in this station. ~ ~ i ~thef clay removed by settling for several days, the supernatant liquid being then siphoned off. An aliquot pprtion of this liquid is then evaporated to dryness and baked for some hours on the steam bath. It is re-dissolved Bull. 107 (revised edition) Bureau of Chemistry. 2
J . A m . Ciaern Soc , 26, 2 9 4 5
a Kentucky Station, Bull No. 126, p 63-126 J . Am. Chem. S o c , SO, No. 5 .
'
J . Ind. Eng. C h m . , 1. 72.
269
in four per cent. ammonia, filtered, re-evaporated, baked again, dissolved in four per cent. ammonia, and filtered. The solution should then be absolutely clear. When this method is properly handled, the insoluble residue remaining after burning off the organic matter in a platinum dish is not much over 0 . 2 0 per cent. of the weight Of 'Oil taken C. W. Stoddart's method, as given in the article Of a strong solution referred to, depends upon the Of ammonium to and Precipitate the clay on standing twelve hours. The humus, as humic acid, is then precipitated out by the addition of strong hydrochloric acid to a clear aliquot of the solution free from clay and by whirling in the centrifuge, the precipitate being "ashe', dried a t 105' '*, and weighed On a 'ared gooch crucible. This method would seem to be the ideal one for simplicity and the writer has striven to make this applicable to the soils of Tennessee, but without
success*
these Illethods have been tried on the same soils by two chemists in this laboraOf each tory, working Over a year apart, the Of
method being I take the liberty of quoting the results on two very different types of taken from the paper Of Messrs* and the Hampton and Of placing them obtained b y the writer to better illustrate the point. In the soils which they examined, Mooers and IIarnpton found the Official method unreliable; the method of Peter and Averitt only partially reliable; and the method of Cameron and Breazeale practically t f no value. The writer finds that excellent results can be obtained with the Mooers and Ilampton methtd after a little practice and has little trouble duplicating rcsults on the same samples of s d s which they u ed. As stated before, the writer hoped to make use ot Stoddart's method, which commends itself on account of its simplicity and ease of manipulation, but, as can be easily seen from the last column in the above the Obtained by Stoddart's method are not comparable to those obtained by the modified Official method. This fact was shown in the
following way. Duplicate aliquots of ammonium humate solution from soil No. 602, freed from clay according to the method in use in this laboratory (Mooers and HamPton), Were evaporated to dryness in platinum dishes, being heated in the oven for three hours and weighed. The weight of extract in each case was the Same. One aliquot was burned and the Organic matter as humus determined by loss on ignition, giving 1.22 per cent. humus. The other aliquot was completely dissolved in 5 0 cc. of four per cent, ammonia, the humus precipitated by hydrochloric acid, giving only 0.45 per cent. of humus as humic acid.
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