480 ORGANOLOGY. 



Sahara. After many purposeless attempts to arrest the movement 

 of this sand, by planting wood, the sound plan was hit upon in the 

 year 1789, of planting these sand-hills with coniferous trees. This 

 perfectly succeeded, and in the year 1809 there were alread}' 1.5,000 

 Hessian acres converted into a pine forest, which had grown upon the 

 driest possible soil, and which was entirely destitute of organic substance. 

 The same phenomenon is seen in the pine forests of the Mark and in the 

 oases of the Sahara. In most cases where the inorganic elements of the 

 soil are suitable, and water is found in sufficient quantities, vegetation is 

 possible upon the surface of the earth. 



Lastly, I will refer to a point which unfortunately I cannot illustrate 

 with figures, as no accurate data exist. Our economical arrangements 

 generally, and the way in which' rain water is necessarily got rid of 

 from our cultivated soils, and carries with it into brooks and streams 

 their soluble constituents, make it certain that all our rivers carry 

 annually to the sea a large quantity of organic substance from the land. 

 If we were to calculate this loss for the more considerable rivers, accord- 

 ing to the quantity of organic matter their waters presented on chemical 

 analysis, it would probably greatly exceed all our conceptions. As 

 instances, I would refer not merely to the organic substances, but to the 

 entire plants and animals, which are annually brought down to the sea 

 by the two great rivers of America, the Amazons and the Mississippi. 



In short, regard this matter as we will, the theory which would derive 

 the food of plants from the organic substance of the soil is a remarkable 

 example of the perversities to which a hypothetical natural history may 

 lead without fundamental principles. To show how thoughtlessly the 

 humus-theorists have gone to work, a single example will serve. 

 According to Sprengel, plants derive the principal part of their carbon 

 from humic acid. This they take up as humate of lime, and the 

 advantage of lime to the soil is supposed to lie in its forming this salt 

 with humic acid. It contains, according to Sprengel, 1 Ib. of lime and 

 10*9 Ibs. of humic acid. But the produce of wheat on an acre (after four 

 years' manuring, according to Block) in straw and grain would be 

 1071*24 Ibs. of carbon, which would require 1552-52 Ibs. of humic acid, 

 which would require 142-43 Ibs. of lime to convert it into humate of 

 lime. But this wheat contains, at the highest, in the grain 0-527 Ibs., 

 and in the straw 8*873 Ibs. of lime, which is about T Vth of what it ought 

 to contain. And if we take, for example, the clover, which, for this view, 

 is the most advantageous of plants, we shall find the result the same. 

 According to Block, an acre of clover contains 1020*73 Ibs. of carbon, 

 which is equal to 1479*32 Ibs. of humic acid, which would require 135-7 

 Ibs. of lime in the plant, whilst in, reality, the clover contains only 

 40-29 Ibs., or about one-third the quantity.* 



191. The organic substance of vegetables, so far as the 



* Through the absence of the necessary bases alone, the impossibility of the nutrition 

 of plants through humic acid is proved. At the same time, Liebig's attempt to dis- 

 prove the theory, on account of the insolubility of the humates, must be regarded as a 

 failure. The rain which falls upon the earth supplies only the smallest quantity of 

 moisture which is taken up by the plant. Dew, and especially the absorption of vapour 

 through clay, humus, &c., affords a much larger quantity. It does not appear probable 

 that water would fail. An acre of 40,000 Q feet of meadow land vaporises, according to 

 Schiibler, in 120 days, 6,000,000 Ib. of water, which is twelve times as much as falls, 

 on an average, in Germany (Tubingen) as rain-water in an equal period of time. 



