VALUATION OF MANURES. 363 



valuable, because all are inclispensible to the healthy growth of 

 plants. We find that the absence of any one constituent of plant 

 food is attended with serious consequences, even though every 

 other one needful be present in abundant quantity. Thus, a 

 deficiency of lime is attended with as much injury as a deficiency 

 of phosphoric acid. In this sense, therefore, lime is as valuable 

 as phosphoric acid. But it is also true that lime is generally 

 present in quantity sufficient for plant food, and when deficient it 

 can be supplied in burnt limestone, or oyster or clam shells, or by 

 means of marl, at small money cost, while to supply phosphoric 

 acid involves a much greater outlay. In an equally true sense, 

 therefore, the different constituents also possess different values. 



The commercial value of any manure, like that of every other 

 article of commerce, is the price which it commands in market. 

 This depends on a variety of circumstances, such as the ease or 

 difficulty, the certainty or uncertainty, with which a supply can 

 be procured ; and the variety of uses to which the article may be 

 put besides that for which the farmer would buy it. In other 

 words, the laws of demand and supply regulate the price. 



It will simplify and facilitate our present undertaking to con- 

 sider that, although many elements go to make up the constituents 

 of plant-food, such as carbon, oxygen, hydrogen, nitrogen, lime, 

 magnesia, phosphorus, silica, etc., and although these occur in 

 numberless combinations in substances found in nature, there are 

 yet only three for which the farmer can afford to pay a price ex- 

 ceeding one cent per pound. This is the fact, no matter what 

 name the manure bears — or what its form, or bulk, or weight — 

 whether barn manure or seaweed, guano or superphosphate, salt or 

 plaster, whether poudrette, or lime, or fish, or whatever be its name. 



If he buys salt, or gypsum, or lime, he takes care to pay less than 

 a cent per pound. If he buys superphosphate, or ashes, or fish 

 refuse, or dung, or guano, he buys them because they contain 

 more or less of one, or two, or all three of the substances to which 

 I have referred. These are, 1st, potash, 2d, phosphoric acid, 3d, 

 soluble nitrogen ; and by soluble nitrogen is meant nitrogen in 

 any combination from which plants can readily take it. Inasmuch 

 as this is most commonly as ammonia it has come to be common 

 usage to employ the terms " ammoniacal " and "nitrogenous" 

 in agricultural parlance as convertable terms, or nearlj^ so. Am- 

 monia consists of 14 parts of nitrogen, by weight, combined with 

 3 of hydrogen. Thus 14-1*7 of the weight of ammonia is nitrogen. 



