244 



On the Application of Manures. 



by the decomposition of those rocks into which it enters as an 

 ingredient. 



But the decomposition of a rock proceeds only at a certain and 

 that a very slow rate, and until it is complete the alkali will be 

 retained by an affinity much too powerful to allow of its being 

 extracted by the absorbing surfaces of the roots. 



Hence^ supposing the crop already raised on a field to have 

 absorbed all the alkali which the subsoil at the time contains^ it 

 is evident that no second crop of the same description can succeed 

 until the ground has lain fallow long enough to admit of a fresh 

 supply of alkali being obtained by a continuance of the process of 

 disintegration, or until this necessary ingredient has been added to 

 it from without. 



Hence the advantage in these cases of such manures as wood- 

 ashes, because they contain much potass, and even of the excre- 

 ments of animals, since these also are not destitute of that 

 principle. 



Another ingredient present in all vegetables that yield nourish- 

 ment to man, and especially so in the different kinds of bread- 

 corn, is phosphoric acid. 



This substance is found in many minerals and mineral waters, 

 and even, according to Liebig, in all land anywise capable of 

 cultivation, so that the barren heath of Luneburg contains an 

 appreciable quantity of it. 



T recollect some years ago, when Dr. Buckland was pursuing 

 his researches on coprolites, which consist chiefly of phosphate of 

 lime, that I endeavoured to assist him by searching for this sub- 

 stance in a variety of rock specimens taken from different locali- 

 ties, but that I soon gave up the pursuit, finding traces of its 

 presence so generally as to be led to conclude that no inference as 

 to the presence of coprolitic matter could be satisfactorily deduced 

 from the mere existence of phosphate of lime in the containing 

 rock. 



Under the circumstances stated, we cannot wonder either that 

 bone-dust, which mainly consists of this material, should prove 

 so excellent a manure when appUed for the first time upon land, 

 or that it should lose its efficacy on repetition ; that is, when the 

 soil has become already sufficiently charged with this principle to 

 furnish to the plants all of it which they require. 



The necessity of deriving from the ground whatever fixed in- 

 gredients enter into the constitution of any particular plant, 

 explains in part the advantage of a due admixture in the subsoil 

 of all the more common earths. A certain quantity of silex, of 

 lime, of magnesia, seems to be essential, and hence a soil consist- 

 ing wholly of one of these earths is generally found to be un- 

 productive. 



