MAHUBES, THEIR KItfDS AKD USES. 17 



of the soil, and what the probable be nefit to the sanitary 

 condition of the cities? 



Manures are either organic or inorganic, according as 

 they may owe their composition to the animate or in- 

 animate world, or are derived from vegetable and animal 

 matters of all kinds, or from minerals. 



Every kind of matter which, when added to the soil, 

 promotes the growth of a plant, whether by being itself 

 directly taken up by its roots, or by chemically altering 

 any heretofore existing constituent of the soil, so that it 

 may be made available, or by physically altering its tex- 

 ture, may be considered a manure or fertilizer. Every 

 plant consists principally of the gaseous elements, ni- 

 trogen, oxygen, and hydrogen, and of carbon in vary- 

 ing proportions, and smaller quantities of alkalies, earths, 

 silica, sulphur, and phosphorus. Chemical analysis has 

 enabled us to learn the exact proportions of the constit- 

 uents of plants, as well as the composition of soils. If 

 chemical forces were the only forces of nature concerned 

 in the growth of plants, then by supplying the soil with 

 exactly the constituents in definite quantities, which 

 chemical analysis has shown it to lack for the full matu- 

 rity of any given agricultural crop, rules and formulas 

 could be devised, so as to insure regularly the maximum 

 yields of all crops. But the changes, not yet fully un- 

 derstood, constantly going on within the soil and upon 

 its surface in contact with the atmosphere; the mutual 

 chemical reactions of the various soils and fertilizing ma- 

 terials, and the vital actions of plants, with their influence 

 upon the soil in which they grow, perhaps even upon 

 the surrounding air, still more inexplicable and wonder- 

 ful, together with the contingencies of weather, render 

 it impossible to formulate any such rules. Exclusive of 

 the crushing effects of glaciers and other agencies, under 

 the gradual influence of water and the atmosphere, the 

 surface of the rocks have become sufficiently pulverized 



