NOTES ON II I'M US. HUMOGEN. AND ITS ACCESSORY 

 PLANTFOOD SUBSTANCES. 



By Arthur Stead, B.Sc, F.C.S. 



In the carl\- days of Ao^ricultural Chemistry humus was 

 ref^;-arde(l as a simple suhstauce. (1) De Saussure, in [<So4, 

 described it as a " brown coml)ustil)lc powder soluble in alkalies 

 and ammonia." (2) Mulder, in 1S49, showed it to be chemi- 

 cally very complex. Me considered it to consist for tlic most 

 part of ulmin and nlmic acid, humin and humic acid, crenic, 

 apocrenic. and geic acids, to which substances he assigned 

 formulae. Notwithstanding Mulder's work, it is still ([uite 

 common to see humus referred to in text-books of agriculture as 

 if it were a single substance; probably because the agriculturalist 

 looks more to the humus effect on tilth than to its composition. 

 According to modern views, humus is even more complex than 

 ever Mulder imagined; while it only requires a perusal of the 

 literature on the subject to show that chemists are very guarded 

 in their statements concerning its composition. 



(3) Hall describes it as " the black or dark brown material 

 of vegetable origin, which gives to the surface soil its cha^'acter- 

 istic darker colour as compared with the subsoil." 



(4) \\'iley refers to it as "the active principle of vegetable 

 mold." and states that " its com|)osition lias never been definitely 

 determined." 



(5) Hilgard is very non-committal, but states that it is 

 " only the matitre noire that counts," viz., the soluble humus. 

 He also suggests that the relationshi]) between natural and arti- 

 ficial humus is a very close one, pointing out that once artificial 

 humus has absorbed nitrogen it holds it as tenaciously as the 

 natural substance. 



(6) Ingle describes it as "the somewhat indefinite nitro- 

 genous and carbonaceous material resulting from the decay of 

 ])lants." 



These opinions, compared with .Mulder's definite statement, 

 show that the modern chemist regards humus as a very delicate 

 subject to deal with. 



(7) Russell, in referring to organic matter as the distin- 

 guishing cliaracteristic of soil, classifies the organic matter 

 furnished by recent vegetation as — • 



(a) Material that still retains its definite cell structure. 

 {h) Partially decomposed and still decomposing material, 

 (c) Simple decomjjosition products. 



{d) Plant or animal constituents not decomposable in the 



soil. 



Of Group (70 he says: "'["he partially-decomposed material 



forms a particularly vague and indefinite group containing all 



the non-volatile products of bacterial, fungal, enzymic, and other 



