_Estimation of Humus in Soils._ It has been usual to determine the amount of humus in soils by means of (dry or wet) combustion, calculating the humus from the carbonic dioxid so formed, while measuring the nitrogen gas directly. But in this process the entire organic matter of the soil, humified and unhumified, is indiscriminately included; and it is wholly uncertain to what extent the latter will ultimately become humus, from the nitrification of which plants are presumed to chiefly derive their nitrogen.[47] 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 matters and tend to expel ammonia from the humus; besides themselves acting as humifiers (see this chapter, p. 125.) _Grandeau Method: Matière Noire._—The only method now known which accomplishes this separation, practically excluding the unhumified while fully dissolving the humified matter—is that of _Grandeau_: 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 ammonia (or other alkali hydrates). Upon the evaporation of the ammonia solution the humus is left behind in the form of a black lustrous substance (“matière noire” of Grandeau) 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. [47] The humus determinations thus made, which include nearly all those made by German chemists, give the humus-content from 40 to 50% too high. The French determinations are mostly made by the method of Grandeau. _Amounts of Humus in Soils._—While in peat, marsh and muck lands the humus-content may rise above twenty per cent, in ordinary cultivated lands it rarely exceeds about five per cent, and very commonly falls below three per cent, even in the humid regions. In properly arid soils we find a very much lower average, rarely exceeding one per cent, and frequently falling to .30 and even less. This scarcity of humus manifests itself plainly in the prevalently light gray tint of the arid soils.