On tJic Causes of the Efficacy of Burnt Clay. 507 



Ciays from Diimbclton. 



No. 



I. 



No. 



II. 



No. III. 



\v ater or combination and organic matter . • 



n 

 / 



by 



c 

 0 



M 

 'Oi, 



D • bo 





8 



24 



n 

 / 



66 



o'b<j 





o • 



04 





62 



(1 . o\ 

 y • Z ) 





1 i\ . 



lU* 



K)l 



7 



\n) 



y i)b 





1 * 



t o 

 IZ 



u 



/U 



u • ly 





0- 



44 



0 



54 



0-24 





0- 



62 



0 



12 



0-56 



Magnesia, in a state of insoluble silicate . . , . 



0- 



34 



0 



•39 



0-34 





0- 



73 



1 



04 



1-13 



Potasli and soda, in a state of insoluble silicate . 



0- 



94 





7u 



1-82 





0 



09 



0 



06 



0-08 





61 



71 



62 



82 



61-42 





100 



00 



100 



00 



100-00 



Having given a brief outline of the origin and composition of 

 agricultural clays, let us see, in the next place, on which of their 

 constituents their fertilizing powers depend. 



In a chemical point of view, silicate of alumina (pure clay) 

 does not contribute in itself to the direct nutrition of plants, ns 

 it is not found in the ashes of cultivated plants. We must look, 

 therefore, for the direct fertilizers of clays amongst the accessary 

 or foreign ingredients of agricultural clavs. Of these lime, mag- 

 nesia, sulphuric acid, silica, and chlorine, are, indeed, essential to 

 the growth of plants; but as these substances are found in most 

 soils, or can be supplied at a cheap rate when deficient, the chief 

 value of an agricultural clay depends on the pj'oportion of phos- 

 phoric acid and potash and soda, which it contains. Potash, in 

 particular, is an essential element in all ashes of plants, and acts 

 as most powerful manure. The chief source of potash in ordi- 

 nary soils is the clay, which enters into the composition of almost 

 all soils. Clay, we have seen, is in many cases derived from 

 felspar, a duple silicate of alumina and silicate of potash, and 

 frequently contains some undecomposed fragments of felspar or 

 similar minerals ; from which, under the action of the weather, 

 potash is liberated and rendered available to plants. The large 

 amount of felspar and similarly composed minerals, in some 

 clays, thus furnishes an almost inexhaustible supply, from which 

 plants may derive their potash. Plants, however, can only avail 

 themselves of the soluble potash which exists in clays, and not 

 of the potash which occurs in them in the form of felspar. But 

 as the soluble potash in clay soil sooner or later will be exhausted 

 by the removal of the crops grown upon it, the soil gradually 

 becomes more and more sterile, and at last refuses to grow any 

 remunerative crop. The natural fertilitv of exhausted soil is then 

 restored again by the process of fallowing. By that process a 

 fresh portion of the soil, not hitherto exposed to the action of the 



