*> NORTH-CAKOLINA GEOLOGICAL STJKVEY. 



great abundance. The effects of a fertilizer are the most per- 

 ceptible where these are the most abundant. Hence guano 

 which contains a large amount of phosphoric acid, ammonia 

 and lime, rarely fails to satisfy the wants of the plant and to 

 become the Efficient means of producing a greatly increased 

 crop. Of certain elements it may be said there is never a 

 deficiency. Silica is one, as it is always present in the largest 

 proportion. The same may be said of iron ; but lime, mag- 

 nesia, and especially the alkalies, are frequently wanting, if 

 not altogether, yet not in a sufficient quantity to supply the 

 wants of vegetation. Hence, in fertilizers, the test of their 

 value consists in determining the quantity of lime, potash 

 and phosphoric acid, whiclrthey contain; or, the amount of 

 those special elements which are always in the smallest pro- 

 portion m the soil ; and hence too it is easy to perceive why 

 soils become barren by cultivation, as those elements are early 

 removed in the crops which the soil has borne. 



39. To illustrate this point and make it sufficiently clear 

 to be comprehended by every reader, I propose to state the 

 quantity of nutriment which several of our most important 

 plants consume; and which is derived directly from the 

 soil. 



In order to do this it is necessarry to ascertain what ele- 

 ments exist in the plant, and which must of necessity be taken 

 from the soil in which it grows. These elements are obtained 

 when a plant is burned. The residue of the combustion are 

 earths, intermixed with alkalies, the mass of which is known 

 as ashes ; wheat, oats, potatoe and clover, will furnish striking 

 examples suitable for the illustration of the point in question. 



An ordinary wheat crop, according to Bousingault, when 

 dried, weighs, upon an average, in grain, 1052 Ibs. ; in straw 

 558 Ibs., and the grain furnishes 2.40 per cent of ash, and 

 the straw 7.00. The quantity of ash per acre, in the, grain 

 amounts to 25 Ibs., in the straw per acre is 179 Ibs. 



The proportion ot the elements contained in the 25 Ibs. of 

 ash are : 



