132 



SOILS. 



of humus that good tilth becomes impossible, the soil becom- 

 ing light-colored and compacted ; while the loss of nitrogen is 

 indicated by the small size of the orchard fruits. Similar losses 

 are of course sustained in the practice of bare summer-fallow, 

 which at one time was almost universal in portions of the arid 

 region. The complete extirpation of weed growth thus 

 brought about, at first considered an unmixed benefit, has ulti- 

 mately had to be made up for by the practice of green-manur- 

 ing; since in the arid region the use of stable manure en- 

 counters many difficulties. 



Estimation of Humus in Soils. It has been usual to determine the 

 amount of humus in soils by means of (dry or wet) combustion, cal-' 

 culating the humus from the carbonic dioxid so formed, while measur- 

 ing the nitrogen gas directly. But in this process the entire organic 

 matter of the soil, humified and unhumified, is indiscriminately in- 

 cluded ; and it is wholly uncertain to what extent the latter will ulti- 

 mately become humus, from the nitrification of which plants are pre- 

 sumed to chiefly derive their nitrogen. 1 In order to obtain definite 

 results, the actual, functional humus must be extracted from the soil 

 mass by some solvent which discriminates between the humified and 

 unhumified organic matter. This cannot be done by direct extraction 

 with caustic soda or potash, which inevitably dissolve unhumified mat- 

 ters and tend to expel ammonia from the humus ; besides themselves 

 acting as humifiers (see this chapter, p. 125.) 



Grantlt-iiu .\fctJioJ : Matiirf Ncir<-. The only method now known 

 which accomplishes this separation, practically excluding the unhumi- 

 fied while fully dissolving the humified matter is that of Gratuit-an : the 

 extraction of the soil, first with dilute acid, in order to set the humic 

 substances free from their combinations with lime and magnesia ; and 

 their subsequent extraction with moderately dilute solutions of am- 

 monia (or other alkali hydrates). Upon the evaporation of the am- 

 monia-solution the humus is left behind in the form of a black lustrous 

 substance (" matiere noire" of (irandeau) much resembling the crust 

 of soot formed in flues from wood fires. As it contains a variable 

 amount of ash, it must be burnt and the ash subtracted from the first 

 weight. 



1 The humus determinations thus made, which include nearly nil those made by 

 German chemists, give the humus-content from 40 to 50 too high. The French 

 determinations are mostly made by the method < f < iramleau. 



