280 OTHER ORGANIC COMPOUNDS IN THE SOIL. 



tity in specimens of soil which are submitted to analysis. They are fre- 

 quently, however, met with in springs and in the drainings of the land. 

 They have even been found in minute quantity in rain-water,* it is prO" 

 bable that they ascend into the air in very small proportion with the 

 watery vapour that rises. This exhibits another form, therefore, in 

 which the rains may minister to the growth of plants (see page 36). 



Both acids form insoluble compounds with the peroxide of iron — and 

 hence are found in combination with many of the ochrey deposits from 

 ferruginous springs, and with the oxide of iron by which so many soils 

 are coloured. The apocrenic acid has also a peculiar tendency to com- 

 bine with alumina, with which it forms a compound insoluble in water, 

 and in this state of combination it probably exists not unfrequently, espe- 

 cially in clayey soils. 



When heated with newly slaked quick-lime these acids give off am- 

 monia and carbonic acid. By the action of the air, and of lime in the 

 soil, they are probably decomposed in a similar manner, though with 

 much less rapidity. 



5°. Mudesous acid is another dark brown acid substance, which is also 

 produced naturally in the soil. It resembles the apocrenic, in having 

 a strong tendency to combine with alumina. In union with this acid it 

 is slowly washed out of the soil by the rains, or filters through it when 

 the water can find an outlet beneath. This is seen to be the case in some 

 of the caves on the Cornish coast, where the waters that trickle through 

 from above have gradually deposited on their roof and sides a thick in- 

 crustation of mudesile of alumina.f 



Besides these acids, it is known that the malic and the acetic (vine- 

 gar) are occasionally produced in the soil during the slow decay of vege- 

 table matter of different kinds. It is probable that many other analo- 

 gous compounds are likewise formed — which are more or less soluble in 

 water, and more or less fitted to aid in the nourishment of plants. There 

 is every reason to believe, indeed, that organic substances in the soil pass 

 through many successive stages of decomposition, at each of which they 

 assume new properties, and become more or less capable of aiding in 

 the support of living races. The subject is difficult to investigate, be- 

 cause of the obstacles which lie in the way of exactly separating frorri 

 each other the small quantities of the different organic compounds that 

 occur mixed up together in the soil. But it seems quite clear, that while 

 some agricultural chemists have erred in describing the ulmic and hu- 

 mic acids as the immediate source of a large portion of the carbon of 

 plants, others have no less misstated — as I apprehend — the true course 

 of nature, who deny any direct influence to these and other substances 

 of vegetable origin, and limit their use in the soil to the supply of car- 

 bonic acid only, which, on their ultimate decomposition, they are capa- 

 ble of yielding to the roots. The resources of vegetable life are not so 

 limited ; but as the human stomach can, and does, on occasion, convert 

 into nourishment many different compounds of the same elements, — so, 

 no doubt, many of those organic compounds wH!ch are produced in the 

 soil, or in fermenting manure during the decay of animal and vegetable 



* Fursten zu Salm-Horstmar. Poggend. Annal. liv., p. 254. 

 t Known to mineralogists under the name of Pifotite. 



