AGRICULTURAL CHEMISTRY. 193 



The most abundant, most generally employed, and most permanently 

 useful manures are the excrements and waste of animals. These 

 matters are, in fact, the residue, more or less concentrated, that 

 remains from the oxydation of vegetables which have served as food. 

 By the vital processes, the hydrogen and carbon of the vegetable 

 nutrient principles are chiefly consumed to the gaseous form, while a 

 portion of these, together with nearly all the nitrogen and all the 

 fixed mineral matters, are separated from the animal in the liquid or 

 solid shape, either immediately prepared, or under the agencies of 

 warmth and moisture speedily assuming a suitable condition for 

 nourishing a new vegetation. 



The excrements of domestic animals, containing, as they do, all the 

 ingredients of plants, and those in greatest relative amount which 

 vegetation is obliged to seek for in the soil, constitute the most gen- 

 erally and durably efficient manure in countries like our own, where 

 cattle are largely depended upon as means of supplying food. The 

 dejections of man are a more concentrated and more powerful fertilizer, 

 and . though less adapted for maintaining the fertility of large farms 

 tilled by a few hands, because they are not associated with matters 

 that amend and modify the physical characters of the soil, are a main 

 reliance in countries like China, where the dense population subsists 

 almost exclusively on vegetable food, and under an} - circumstances 

 are an invaluable adjunct to the resources of the farmer. Human 

 excreta should never be suffered to waste so long as the soil is capable 

 of stimulation to higher productiveness. 



Certain animal manures, viz., those very rich in nitrogen, though 

 usually exhibiting great energy of action, are liable to abuse, and often 

 ultimately impoverish the farmer. Peruvian guano, the excrement 

 of piscivorous sea-fowl, yielding sixteen per cent, of ammonia by the 

 decomposition of its uric acid, and the flesh, blood, hair, and wool 

 of animals are manures of this character. Nitrogen is their principal 

 active ingredient; it passes into ammonia or nitric acid, excites a quick 

 growth of vegetation by furnishing abundance of material for cell 

 development, and at the same time rapidly solves the fixed minerals 

 of the soil. The latter, being as rapidly removed by the vigorous 

 vegetation, soon fall into a state of relative deficiency, especially on 

 the poor soils where these applications exhibit their effects most 

 strikingly; and unless restored by some other manure, the absence of 

 them produces the phenomenon of exhaustion. 



It is an objection, indeed, commonly raised against manures con- 

 taining but one or a few nutritive ingredients, that they exhaust the 

 soil. Obviously it is the crops, or what is taken off the soil, that 

 exhaust it; and if a manure assists a crop to rob a field, the abetting 

 farmer cannot rightfully complain, so long as the price of the produce 

 goes into his pocket, although, to be sure, there are various ways of 

 exhausting land, some of which are vastly more profitable than others. 



The great practical lessons taught by experience and confirmed by 



science, relative to the use of manures, are, save all refuse which 



contains any of the elements of vegetation ; apply abundantly the mixed 



ingredients of the dung and compost heaps. As concerns commercial 



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