284 EXPERIMENT STATION RECORD. 



FERTILIZERS. 



The assimilation of humus and organic substances by plants, 

 E. Breal {Ann. Afjron., 20 {1894), No. <S', j^P- S53-370). — The preGeuce 

 ill the soil of the bro^n or black substance resultiui>" from tlie slow 

 decomposition of former vegetation, known as humus, has long bet^ii 

 popularly considered an index of fertility. This humus, however, is a 

 substance of very variable composition, undergoing constant change 

 due to oxidation under tiie influence of numerous organisms. Decom- 

 position of humus is more active the higher the temperature. This 

 accounts for the small percentage of humus in soils of hot dry 

 regions. ' Boussingault observed more than 40 years ago that peat 

 bogs do not occur in the tropics excej^t at an altitude of 1,000 meters 

 or more above the sea. 



It has been held by many leading investigators of the subject that 

 humus has little practical significance as a direct food for idants, 

 although it has been shown that certain of its constituents may be 

 absorbed and assimilated by plants,^ and this view seems to be 

 strengthened by the fact that well-developed plants have been grown 

 repeatedly in a soil free from humus or in solutions containing only 

 mineral salts. 



In comparative tests by Deherain of chemical fertilizers and organic 

 manures on a large number of crops carried on for several years the 

 yield has invariably been higher on the plats supplied with humus- 

 forming manures. The results indicate, moreover, that the fertility 

 declined as the proportion of humus in the soil decreased.^ 



In support of the view that plants assimilate organic matter directly, 

 the author cites the cases of the assimilation of humus by the roots of 

 certain trees, such as chestnuts, oaks, and beeches, by means of 

 mycorrhiza as observed by Frank 5 the utilization by fungi of the 

 organic compounds elaborated by algi^e, as in lichens; the insectivorous 

 habits of certain plants, such as Drosera rotundifoJia; the symbiosis 

 of leguminous plants with bacteria of tlieir root tubercles; and the 

 celebrated experiments of Bohm, in which beau plants kept in dark- 

 ness until the starch had disappeared had the starch restored by having 

 the stems and leaves placed in a solution of sugar. 



The author placed recently germinated plantlets of lentils, wheat, 

 and beans in (1) a solution of nitrate and phosphate of potash, (2) a 



'SeeF. W. Dafert and A. B. Cavalcanti's report on the soils of Sao Paulo, Brazil. 

 (Rftlat. Inst. Agron., Sao Paulo, 1893, ])p. 69-72; E. S. R., 6, p. 199) ; and E. W. Hilgard 

 and M. E. .Jaffa on the nitrogen contents of soil humus in arid and humid regions 

 (Agl. Sci., 8 (1894), No. 4, p. 165; E. .S. R., 6, p. 197). 



2See E. 8. R., 5, p. 832. 



^^^ee H. Snyder on the composiHon of native and cultivated soils, and tlie effect of 

 continuous cultivation upon fertility (Minn. Sta. Bui. 30; E. S. R., 5, ji. «57). 



