l)r Davy's Agricultural Discourse. 299 



to year in artificial culture ; one plant, though not carried off 

 the soil, may be injurious in preparation for a crop of another 

 plant. I make this remark, keeping in mind a fact which 

 was mentioned to me by a gentleman of Barbadoes, one of its 

 ablest practical agriculturists ; it was, that on some estates in 

 this island, the growing of guinea corn, and the ploughing or 

 turning it into the soil in its green state, had a decidedly in- 

 jurious effect. This partial effect, and the effect before al- 

 luded to, are problems which are yet unsolved, doubtlessly 

 admitting of solution, but of solution which can only be at- 

 tained by scientific inquiry. In the instance first mentioned, 

 the soil beyond a certain depth may be exhausted of the 

 inorganic matter required by the plant, and which may be 

 accumulated at the surface in a state unfavourable to the ex- 

 hausting species, and yet favourable to another species. In 

 the last mentioned instance, the guinea corn may so unite 

 the inorganic elements which it extracts and returns to the 

 soil, make such compounds of them, as to be unfavourable 

 either to their solution and entering the sap, or, if soluble, 

 to their affording supporting nourishment to the cane. These 

 are mere conjectures ; but whether true or false, can be de- 

 termined only by exact research. 



Lastly^ Of the fertilizing means derived from mineral or 

 inorganic matter. The most fertile soils appears to be those 

 which are most compounded, which contain the largest num- 

 ber of the inorganic elements of plants, and in a state of 

 minute division, favouring their solution to enter into the 

 composition of the nutritive sap. Lime, magnesia, silica, 

 potash, phosphate of lime, may be mentioned as the most im- 

 portant of these ; and these may exist in the soil in different 

 states, — either as free, uncombined, or in combination, con- 

 stituting mineral species. If the former, they are more 

 readily yielded up to the growing plants, and the soil with- 

 out care is in danger of being sooner exhausted ; if the lat- 

 ter, they are yielded up more slowly as the minerals decom- 

 pose, and, in consequence, exhaustion, even with bad manage- 

 ment, is difficult. Such fertilizing means as these in the 

 soil, depending on its supplying the inorganic elements of 



