241 



In regard to the sensitiveness of our cultivated plants to free acids, 

 Ramann cites Maxwell's^ experiments with i-io and 1-50 per cent, solutions 

 of citric acid. He found that all the Cruciferae were quickly destroyed, the 

 Papilionaceae more slowly. Grain suffered greatly, only the pearl millet 

 and Inaize could withstand it. Tolf made discoveries in regard to humic 

 acids, according to which seedlings suffer in acid moor soils. In the acid 

 moor, the diffusion of the salt solutions is sharply arrested. According 

 to Reinitzer and Nikitinsk, pure humic acids are unsuitable for the 

 nutrition of bacteria and fungi. On the other hand, most of the higher 

 plants can endure a moderate amount of these acids. We discover from our 

 cultures of Ericas, Azaleas, Rhododendrons and other Ericaceae in moor soil 

 that a number of plants indeed seem directly adapted to acid soils. 



The dark colored humus parts consist preponderately of Humin and 

 humic acid (Ulmin, according to Mulder). The humus substance must be 

 considered as a mixture of closely related bodies with and without nitrogen, 

 which can be separated into two groups according to their behavior with al- 

 kalis. The brown humin substances, insoluble in the most diverse solvents, 

 swell up in alkaline liquids and pass gradually over Into humic acids. The 

 humic acids (their chemical composition is insufficiently known), containing 

 possibly 59 to 63 per cent. C, 4.4 to 4.6 per cent. H. and 35 to 36 per cent. O, 

 are easily dissolved in alkalis and are re-precipitated from their solutions 

 by stronger mineral acids. If they are withdrawn from acid soils (moor 

 soils) with alkalis or ammonia and precipitated with hydrochloric acid, a 

 voluminous, jelly-like substance is obtain which, in drying, forms a brown 

 or black amorphous mass. The humic acids are separated from their solution, 

 by freezing, in the form of a dark colored powder, which gradually passes 

 over again into solution. Ramann emphasizes the fact that humic acids are 

 somewhat soluble in pure water, but not in water containing salts. The salts 

 of the alkalis and of ammonia with humic acids are soluble in water, but 

 not those of the alkaline earths (calcium and magnesium). Yet the latter 

 also seems to become soluble with an excess of acids. Calcium humate will 

 decompose quickly into calcium carbonate which will combine into new 

 masses of humic acids. 



On an average, the nitrogen content of humus substances is greater in 

 dry regions than in moist ones. By the advancing decomposition, the nitro- 

 gen, which in organic combinations is accessible to plants with difficulty, is 

 carried over into compounds easily absorbed. 



Raw Humus. 

 Humus is beneficial and indispensible only where, in pure deposits or 

 mixed with the mineral skeleton of the soil, it is exposed to constant aeration 

 and to sufficient moisture. Its chief action on plant growth does not lie in 

 its nutrient content or in the carbon dioxid formed by its decomposition of 

 minerals, but in its physical properties. 



1 Journ. Amer. Chem. Soc. 1898, 20, p. 103. 



