102 THE SOIL. 



the absorption of food, injures the growth of land- 

 plants. 



If plants really did receive the elements of their food 

 from a solution which could change its place in the soil, 

 then all drainage waters, spring, brook, and river waters, 

 must contain the principal nutritive substances of all 

 plants ; and it must be quite practicable, by continued 

 lixiviation, to extract from every arable soil, without dis- 

 tinction, all the nutritive substances, either entirely, or 

 at least in amount corresponding to the quantity con- 

 tained in a crop. But, in reality, this is not practicable. 

 By the action of water, the field loses none of the prin- 

 cipal conditions of its fertihty, in such a degree as per- 

 ceptibly to impair the growth of plants cultivated on it. 



For thousands of years, all fields have been exposed to 

 the lixiviating action of rain-water, without losing their 

 powers of fertility. In all parts of the earth, where man 

 for the first time draws furrows with the plough, he finds 

 the arable crust, or top layer of the field, richer and more 

 fertile than the subsoil. The fertility of the ground is 

 not diminished by plants growing thereon ; not until the 

 plants are removed from the ground does it gradually 

 lose its fruitfidness. 



The opinion that some cause is at work within the 

 plant itself, which seems to render soluble certain ele- 

 ments of food, and make them available for nutrition, is 

 not contradicted by the experiments of Knop, Sachs, and 

 Stohmann, who have shown that many land-plants, with- 

 out touching a particle of earth, may be brought to 

 flowering and seed-bearing in water, to which the mineral 

 elements of food have been added. These experiments, 

 which have thrown considerable light upon the physiolo- 

 gical importance of the several nutritive substances (see 

 Appendix E.), merely prove how admirably the ground is 



