HISTORY OF THE AMERICAN MENHADEN. 233 



minute proportions. Soda does not appear to be an essential ingredient 

 of plant food. lu so far as these latter are essential ingredients of plant 

 food, they are furnished in abundance by every ordinary soil. 



Ingredients most commonly lacking in worn-out soils, and hence most im- 

 j)ortant in fertilisers ; nitrogen, phosphoric acid, and potash. 



304. For our present purposes, then, we have to consider only the 

 potash, lime, magnesia, sulphuric acid, and nitrogen. Of this list the 

 magnesia is commonly, though not always, supplied in sufficient quan- 

 tities in even " worn-out " soils. Sometimes its presence in fertilizers 

 may be of considerable importance to crops. Sulphuric acid and lime 

 are more often deficient, and hence one reason of the good effect so often 

 observed from the application of lime and plaster. 



The remaining substances, the nitrogen, phosphoric acid, and potash, 

 are the most important ingredients of our common commercial fertilizers, 

 because of both their scarcity in the soil and their high cost. It is in 

 supplying these that fish guano, phosphates, and bone manures are 

 chiefly useful. 



In brief, then, in order that crops may grow, they must have at their 

 disposal an adequate supply*, in available forms, of ecch one of a certain 

 list of essential ingredients of their food. Soils differ in respect to their 

 supplies of these food ingredients. The crop cannot rise above the level 

 of the lowest ingredient in the food supply. The chief use of fertilizers 

 is to fill up the gaps. 



Principles to he observed in the manufacture and purchase of fertilizers. 



305. The cardinal principle to be observed by the farmer in the pur- 

 chase of fertilizers is, to — 



Select those which furnish, in the best form and at the lowest cost, the 

 ingredients of plant food thai his crops need a7id his soil fails to supply. 



The principle that should guide the manufacturer should be, to — 



Economize all available materials in his manufacture so as to furnish the 

 valuable ingredients in the best forms in products of high-grade and uniform 

 composition, and at the fairest practicable rates. 



The most important ingredients of our fertilizers, because the most 

 rare and costly, are nitrogen, phosphoric acid, and potash. The two first 

 are the most important. These are supplied in large proportions in fish. 



Composition, character, costs, and uses of fertilizers in general. 



306. It will be to our purpose, then, to note briefly : 



1. The composition of some of our more important commercial ferti- 

 lizing materials, particularly those which, like fish manures, contain nitro- 

 gen and phosphoric acid ; in other words, the analyses of these ferti- 

 lizers. 



2. The comparative costs and values of the active fertilizing ingredi- 

 ents in these articles ; or, in other words, the commercial valuations. 



