34 HUMUS AND HUiMlC Ai^iU. 



chief form in which carbon is obtained from the soilj 

 but there exist in contact with the roots, other som-ces 

 of this important article of food. Every soil contains 

 more or less of organic matter, derived from the decay 

 after death of plants and animals. Where abundant, 

 this gives a black color to the soil, and one containing 

 a large proportion of it is frequently described by 

 farmers as a vegetable mould. While plants, etc. are 

 decaying to form this mould, various compounds con- 

 taining carbon are the result. Quite a number of 

 these have been examined by chemists, but it is not 

 necessary to say much of them here. 



a. Humus is a name often given to the black mould 

 of a rich vegetable soil, and this probably because a 

 great part of the mould consists of a substance called 

 humic acid. This acid may be obtained by boiling 

 some rich mould or peat in a solution of common soda, 

 continuing for an hour or two; filtering through a 

 piece of blotting paper, and then making the liquid 

 quite sour with muriatic acid. Little brown flocks 

 will soon begin to appear, and will fall to the bottom: 

 these are humic acid. 



h. This substance may serve as a specimen of a 

 large class that are contained in the organic part of 

 the soil. They all consist of carbon, oxygen and 

 hydrogen, and in many situations are extremely 

 abundant. They do not decay or dissolve very easily, 

 and it is not supposed that plants get a large pjart of 

 their carbon in this way. It seems certain, however, 

 that they do get some; and it is found that in most 

 cases where soils contain much of this organic matter, 

 they are quite fertile. In all ordinary situations, it is 

 supposed that at least two-thirds of the carbon in plants 

 comes from the air, the remaining third in various 

 forms from the soil. This is shown by the fact that 

 plants cultivated year after year, cause the organic 

 matter of a soil to diminish quite rapidly. 



