132 MAM'KES. 



dients, while the ash of potatoes contains more of 

 potash than of anything else. 



In the second section, (on soils,) we learned that 

 some soils contain everything necessary to make the 

 ashes of all plants, and in sufficient quantity to sup- 

 ply what is required, while other soils are either 

 entirely deficient in one or more ingredients, or con- 

 tain so little of them in an available condition, that 

 they are unfertile for certain plants.* 



The different requirements of different plants is 

 the foundation of the theory of special manuring 



* In all cases in which the constituents of the soil are spoken of 

 in this book, it should be understood as applying not so much to 

 its absolute chemical composition as to . the availability of its 

 plant-feeding parts. An atom of potash may be locked up in the 

 inside of a pebble, and be of no more use to the roots of a plant 

 than if it were a hundred miles away, yet a careful chemical 

 analysis would destroy the pebble and weigh its atom of potash. 

 The food of plants in the soil must exist in what Liebig calls " a 

 state of physical combination," that is, coating the outside of its 

 particles ; attached to them by a feeble attraction which is suffi- 

 cient to prevent their being washed away by the water of rains, 

 but which yields to the feeding action of roots. It is his belief, 

 and the opinion seems well founded, that it is only, or chiefly 

 from materials so placed, that plants derive their food ; and that 

 the constituents of the soil, before they are taken up by roots, 

 must be separated from their firmer relations and exposed on 

 the surfaces of particles, as above stated. 



In like manner those elements of manures which are taken up 

 by the plant are first dissolved in water, from which they are ab- 

 sorbed by the particles of the soil, spread over its interior sur- 

 faces, exposed to the action of roots. 



Even the ammonia brought from the atmosphere in falling rain, 

 attaches itself in the same way to the interior surfaces of the 

 soil. 



