M 



MAY MINERAL SUBSTANCES BE APPLIED ALONE? 91 



in such a soil is usually much less than in the case of one that is 

 naturally poor. That a substance produces no sensible effect 

 upon a given soil is therefore no satisfactory proof that it is not 

 propitious to the plant we are growing. It may be that the 

 special circumstances in which we apply it are not fitted to dis- 

 play or to allow of the development of its peculiar action. 



b It is also the result of observation that, in some districts 

 and on some soils, the natural or artificial application of mineral 

 matter alone is productive of most profitable results. Thus, in 

 the neighbourhood of Vesuvius, according to Mohl,* the per- 

 manent richness of the soil is owing in part to the ashes sprinkled 

 over its surface from the mouth of the volcano, which ashes de- 

 stroy the growing crop on an average every eighth year. So, 

 also, the artificial application of inorganic or mineral manures to 

 certain soils in our own country have, without other addition, 

 produced largely increased crops. But these good effects are in 

 every instance dependent upon the natural presence in the soil 

 of a sufficient supply of those organic forms of food required by 

 the plant, and which are not contained in the mineral substance 

 or mixture applied to the land. 



From a consideration of such facts we obtain a key to the cir- 

 cumstances or conditions under which mineral manures may be 

 tried, alone or by themselves, not only with profit, but with 

 positive benefit to the land, as well as an explanation of certain 

 methods of treating the soil, which in practice have, in some 

 localities, been found advantageous. Thus, 



First, If the land is rich, and in good heart, as it is called, 

 saline or mineral manures, without any admixture of nitrogenous 

 matter, may be applied with a fair prospect of advantage. 



* DR JUSTUS LIEBIG'S Verth'dltmiss zur Pflanzenphysiologie, ton DB HUGO 

 MOHL, 1843, p. 47. 



In the passage referred to, Mohl is combating the statement of Liebig that 

 the fertile soil in the neighbourhood of Vesuvius contains no organic matter, 

 and is therefore permanently fertile solely because of the mineral top-dressings 

 it receives from the volcano. But this soil is only sparingly sprinkled with ashes 

 even when the fall is sufficient to destroy the whole vegetation ; and, as Mohl 

 observes, this very destruction^ the means of applying a rich green manuring 

 to the soil, by which the proportion of organic matter contained in it is kept up 

 or increased. 



