THE ART OF CULTURE. 



51 



A thousandth part of loam mixed with 

 the quartz in new red sandstone, or with 

 the lime in the different limestone forma- 

 tions, affords as much potash to a soil only 

 twenty inches in depth as is sufficient to 

 supply a forest of pines growing upon it 

 for a century. A single cubic foot of felspar 

 is sufficient to supply a wood, covering a 

 surface of 26,910 square feet, with the 

 potash required for five years. 



Land of the greatest fertility contains 

 argillaceous earths and other disintegrated 

 minerals with chalk and sand in such a pro- 

 portion as to give free access to air and 

 moisture. The land in the vicinity of Vesu- 

 vius may be considered as the type of a fer- 

 tile soil, and its fertility is greater or less in 

 different parts, according to the proportion 

 of clay or sand which it contains. 



The soil which is formed by the disinte- 

 gration of lava, cannot possibly, on account 

 of its origin, contain the smallest trace of 

 vegetable matter, and ' yet it is well known 

 that when the volcanic ashes have been ex- 

 posed for some time to the influence of air 

 and moisture, a soil is gradually formed in 

 which all kinds of plants grow with the 

 greatest luxuriance. This fertility is owing 

 to the alkalies which are contained in the 

 lava, add which by exposure to the weather 

 are rendered capable of being absorbed by 

 plants. Thousands of years have been ne- 

 cessary to convert stones and rocks into the 

 soil of arable land, and thousands of years 

 more will be requisite for their . perfect re- 

 duction, that is, for the complete exhaustion 

 of their alkalies. 



We see from the composition of the water 

 in rivers, streamlets, and springs, how little 

 rain-water is able to extract alkali from a 

 soil, even after a term of years ; this water 

 is generally soft, and the common salt, 

 which even the softest invariably contains, 

 proves that those alkaline salts, which are 

 carried to the sea by rivers and streams, 

 are returned again to the land by wind and 

 rain. 



Nature itself shows us what plants re- 

 quire at the commencement of the develope- 

 ment of their germs and first radicle fibres. 

 Bequerel has shown that the gramince, 

 leguminosce, cruciferce, cichoracece, umhelli- 

 feraz, com/era, and cucurbitacece emit acetic 

 acid during germination. A plant which 

 has just broken through the soil, and a leaf 

 just burst open from the bud, furnish ashes 

 by incineration, which contain as much, 

 and generally more, of alkaline salts than 

 at any period of their life. (De Saussure.) 

 Now we know also, from the experiments 

 of Bequerel, in what manner these alkaline 

 salts enter young plants; the acetic acid 

 formed during germination is diffused 

 through the wet or moist soil, becomes 

 saturated with lime, magnesia, and alkalies, 

 and is again absorbed by the radicle fibres 

 in the form of neutral salts. After the ces- 

 sation of life, when plants are subjected to 

 decomposition by means of decay and putre- 



faction, the soil receives again that which 

 had been extracted from it. 



Let us suppose that a soil has been formed 

 by the action of the weather on the compo- 

 nent parts of granite, grauwacke, mountain 

 limestone, or porphyry, and that nothing has 

 vegetated on it for thousands of years. 

 Now this soil would become a magazine of 

 alkalies in a condition favourable for their 

 assimilation by the roots of plants. 



The interesting experiments of Struve 

 have proved that water impregnated with 

 carbonic acid decomposes rocks which con- 

 tain alkalies, and then dissolves a part of 

 the alkaline carbonates. It is evident that 

 plants also, by producing carbonic acid 

 during their decay, and by means of the 

 acids which exude from their roots in the 

 living state, contribute no less powerfully to 

 destroy the coherence of rocks. Next to the 

 action of air, water, and change of tempera- 

 ture, plants themselves are the most power- 

 ful agents in effecting the disintegration of 

 rocks. 



Air, water, and the change of temperature 

 prepare the different species of rocks for 

 yielding to plants the alkalies which they 

 contain. A soil which has been exposed 

 for centuries to all the influences which 

 affect the disintegration of rocks, but from 

 which the alkalies have not been removed, 

 will be able to afford the means of nourish 

 ment to those vegetables which require 

 alkalies for its growth during many years 

 but it must gradually become exhausted, 

 unless those alkalies which have been re- 

 moved are again replaced ; a period, there- 

 fore, will arrive when it will be necessary 

 to expose it from time to time to a farther 

 disintegration, in order to obtain a new sup- 

 ply of soluble alkalies. For small as is the 

 quantity of alkali which plants require, it is 

 nevertheless quite indispensable for their 

 perfect developement. But when one or 

 more years have elapsed without any alka- 

 lies having been extracted from the soil, a 

 new harvest may be expected. 



The first colonists of Virginia found a 

 country the soil of which was similar to that 

 mentioned above ; harvests of wheat and 

 tobacco were obtained for a century from 

 one and the same field, without the aid of 

 manure ; but now whole districts are con- 

 verted into unfruitful pasture-land, which 

 without manure produces neither wheat nor 

 tobacco. From every acre of this land there 

 were removed in the space of one hundred 

 years 12,000 Ibs. of alkalies in leaves, grain, 

 and straw ; it became unfruitful, therefore, 

 because it was deprived of every particle of 

 alkali, which had been reduced to a soluble 

 state, and because that which was rendered 

 soluble again in the space of one year was 

 not sufficient to satisfy the demands of the 

 plants. Almost all the cultivated land in 

 Europe is in this condition ; fallow is the 

 term applied to land left at rest for farther 

 disintegration. It is the greatest possible 

 mistake to suppose that the temporary dimi- 



