Power of Soils to ahsorh Manure. 



139 



The power of the double silicate of alumina and lime to absorb 

 carbonate of ammonia from the air is very important in reference 

 to several practical questions in agriculture. The most important 

 of these are the differences in natural fertility of different soils, 

 and the power of conferring increased fertility on land by abun- 

 dant cultivation. It is not my wish to assert that the dissimi- 

 larity in soils in regard to natural capacity of producing crops can 

 be accounted for by reference to any one circumstance ; on the 

 contrary, it is certain that very many circumstances may combine 

 to give a superiority to one soil over another ; but it is also cer- 

 tain that one soil will be highly enriched by a fallow which will 

 in no degree benefit another. We know that ammonia exists in 

 the air, in small quantity indeed, but when taken as a whole in 

 abundance, materially to affect the growth of plants ; now we 

 have seen that silicate of alumina and lime, which exists in soils 

 and which is an ingredient of clay, has the property of abstracting 

 carbonate of ammonia from the air, and retaining it for the pur- 

 poses of vegetation. As there is good reason to believe that dif- 

 ferent soils may contain unlike quantities of this double silicate, 

 so they will, other things being the same, possess unlike degrees 

 of natural fertility. In this circumstance we may probably find 

 an explanation of the singular fertility of some soils, of which it 

 is recorded that they have been cropped year after year with 

 wheat, for a very extended period, without any apparent diminu- 

 tion in their power of yielding it. Upon examination, nothing 

 extraordinary has been found in the composition of such soils to 

 account for such a degree of fertility ; but it is extremely likely 

 that a further inquiry, with the aid of the light now thrown upon 

 the subject, will show that the superiority of such soils is de- 

 pendent upon their possession of a greater power of acquiring 

 manure from the air by the means now pointed out. The second 

 practical question to which I have referred, namely, the power of 

 conferring increased fertility by abundant cultivation, is one that 

 might profitably engage a very large share of our attention ; but I 

 can only in this place glance very slightly at the connexion be - 

 tween it and the discovery which it is the object of this paper to 

 record. It is now more than a century ago that Jethro TuU pub- 

 lished his work on agriculture — a work which contains doctrines 

 so opposed to all the preconceived ideas and established practices 

 of the farmer, that it is to this day looked upon by many an intel- 

 ligent man as a mere collection of absurd theories : and yet I do 

 not hesitate to say that, making allowances for the imperfect 

 knowledge of the sciences of chemistry and botany of that period, 

 Tull's views were those of a deeply observant and philosophical 

 mind. Tull advocated the constant subdivision of the soil by 

 abundant cultivation at all possible seasons ; and the arguments 



