44 



AGRICULTURAL CHEMISTRY. 



irace of vegetable matter, and no humic 

 acid ; they are of a glistening white or yel- 

 lowish colour, and in part transparent, like 

 calcareous spar, and may be heated to red- 

 ness without becoming black. 



The subterranean vaults in the old castles 

 near the Rhine, the if Bergstrass," and 

 Wetherau, are constructed of sandstone, 

 granite, or basalt, and present appearances 

 similar to the limestone caverns. The roofs 

 of these vaults or cellars are covered exter- 

 nally to the thickness of several feet with 

 vegetable mould, which has been formed by 

 the decay of plants. The rain falling upon 

 them sinks through the earth, and dissolves 

 the mortar by means of the carbonic acid 

 derived from the mould ; and this solution 

 evaporating in the interior of the vaults, 

 covers them with small thin stalactites, 

 which are quite free from humic acid. 



In such a filtering apparatus, built by the 

 hand of nature, we have placed before us ex- 

 periments which have been continued for a 

 hundred or thousand years. Now, if water 

 possessed the power of dissolving a hun- 

 dredth thousandth part of its own weight of 

 humic acid or humate of lime, and humic 

 acid were present, we should find the inner 

 surface of the roofs of these vaults and cav- 

 erns covered with these substances ; but we 

 cannot detect the smallest trace of them. 

 There could scarcely be found a more clear 

 and convincing proof of the absence of the 

 humic acid of chemists in common vegeta- 

 ble mould. 



The common view, which has been 

 adopted respecting the modus operandi of 

 humic acid, does not afford any explanation 

 of the following phenomenon : A very 

 small quantity of humic acid dissolved in 

 water gives it a yellow or brown colour. 

 Hence it would be supposed that a soil 

 would be more fruitful in proportion as it 

 was capable of giving this colour to water, 

 that is, of yielding it humic acid. But it is 

 very remarkable that plants do not thrive in 

 such a soil, and that all manure must have 

 lost this property before it can exercise a fa- 

 vourable influence upon their vegetation. 

 Water from barren peat soils and marshy 

 meadows, upon which few plants flourish, 

 contains much of this humic acid ; but all 

 agriculturists and gardeners agree that the 

 most suitable and best manure for plants is 

 that which has completely lost the property 

 of giving a colour to water. 



The soluble substance, which gives to 

 water a brown colour, is the product of the 

 putrefaction of all animal and vegetable 

 matter; its formation is an evidence that 

 there is not oxygen sufficient to begin, or at 

 least to complete the decay. The brown so- 

 lutions containing this substance are deco- 

 lourised in the air by absorbing oxygen, and 

 a black coaly matter precipitates the sub- 

 stance named " coal of humus." Now if a 

 soil were impregnated with this matter, the 

 effect on the roots of plants would be the 

 same as that of entirely depriving the soil of 



oxygen ; plants would be as little able to 

 row in such ground as they would if ny- 

 rated protoxide of iron were mixed with the 

 soil. Indeed some barren soils have been 

 found to owe their fertility to this very caus* 

 The sulphate of protoxide of iron (coppe 

 ras,) which forms a constituent of these soils, 

 possesses a powerful affinity for oxygen, 

 and consequently prevents the absorption of 

 that gas by the roots of plants in its vicinity.* 

 All plants die in soils and water which con- 

 tain no oxygen ; absence of air acts exactly 

 in the same manner as an excess of carbonic 

 acid. Stagnant water on a marshy soil ex- 

 cludes air, but a renewal of water has the 

 same effect as a renewal of air, because wa- 

 ter contains it in solution. If the water is 

 withdrawn from a marsh, free access s 

 given to the air, and the marsh is change i 

 into a fruitful meadow. 



In a soil to which the air has no access, 

 or at most but very little, the remains of ani- 

 mals and vegetables do not decay, for they 

 can only do so when freely supplied with 

 oxygen; but they undergo putrefaction, for 

 which air is present in sufficient quantity. 

 Putrefaction is known to be a most powerfu-. 

 deoxidising process, the influence of which 

 extends to all surrounding bodies, even ta 

 the roots and the plants themselves. All 

 substances from which oxygen can be ex- 

 tracted yield it to putrefying bodies ; yellow 

 oxide of iron passes into the state of black 

 oxide, sulphate of iron into sulphuret of 

 iron, &,c. 



The frequent renewal of air by ploughing 

 and the preparation of the soil, especially iu> 

 contact with alkaline metallic oxides, the 

 ashes of brown coal, burnt lime or limestone, 

 change the putrefaction of its organic con 

 stituents into a pure process of oxidation ; 

 and from the moment at which all the or- 

 ganic matter existing in a soil enters into a 

 state of oxidation or decay, its fertility is in- 

 creased. The oxygen is no longer employed 

 for the conversion of the brown soluble mat- 

 ter into the insoluble coal of humus, but 

 serves for the formation of carbonic acid. 

 This change takes place very slowly, and in 

 some instances the oxygen is completely ex- 

 cluded by it; and whenever this happens, 

 the soil loses its fertility. Thus, in the 

 vicinity of Salzhausen (a village in Hesse 

 Darmstadt, famed for its mineral springs, 

 upon a meadow called Griinschwalheimer, 

 unfruitful spots are seen here and there 

 covered with a yellow grass. If a hole be 

 bored from twenty to twenty-five feet deep 

 in one of these spots, carbonic acid is emit- 

 ted from it with such violence that the noise 

 made by the escape of the gas may be dis- 



* The most obvious method of removing this 

 salt from soils in which it may be contained is to 

 manure the land with lime. The lime unites with 

 the sulphuric acid and liberates the protoxide of 

 iron, which absorbs oxygen with much rapidity, 

 and is converted into the peroxide of iron. This 

 conversion is accelerated by giving free access to 

 the air. tiat is, oy loosening tbe soil. 



