THE SOIL IS MANURED BY THE SEA AND THE AIR. 497 



degree discharges those functions for wtiich vegetable matter in the soil is 

 specially destined. In stiff clays also, the roots of plants, without actu- 

 ally attaining this inert state, yet decay with extreme slowness in conse- 

 quence of their being so completely sealed up from the access of the air. 

 In both cases the frecjuent and prolonged exposure which a naked fallow 

 occasions, induces a more rapid decay of this vegetable matter, or brings 

 it into a state in which its elements more readily assume those new 

 forms of combination which are capable of ministering to the sustenance 

 and growth of plants. 



Among the other compounds which are produced (p. 161) during this 

 prolonged exposure and more rapid decay of the organic matter of the 

 soil, nitric acid is one which appears to exercise a considerable in- 

 fluence upon the future fertility of the land. The favourable action of 

 the nitrates in promoting vegetable growth is now we-11 known, and 

 the more rapid formation of these compounds, when the land lies na- 

 ked to the action of the sun and air, must not be neglected among the 

 fertilizing influences of the sumnier fallow. 



6°. The soil, besides the clay, (quartz) sand and lime of which it 

 chiefly consists, contains also fragments of mineral substances of a com- 

 pound nature — of felspar, of mica, of hornblende — of those minerals 

 which constitute or which occur in the granitic and trap rocks. These 

 slowly decompose in the soil — more rapidly also the more freely they 

 are exposed to the air — and the substances (potash, soda, lime, magne- 

 sia, silica, &;c.*) which they contain, are by this decomposition difllised 

 more equably and brought within the more easy reach of the roots of 

 plants. When these minerals, therefore, exist in the soil, and when 

 their constituents are of sucii a kind as to favour the growth of any given 

 plant, the effect of a naked fallow being to })roduce an accumulation of 

 their constituent substances in the soil, it Avill be so far favourable in pre- 

 paring the land for an after-crop of that particular species of plant. 

 You are not to be misled, however, by any broad and unguarded state- 

 ments of scientitic men, so as to imagine for a moment that the benefi- 

 cial effects of fallowing in any case are to be solely ascribed to the oper- 

 ation of this one cause. f 



7°. The rains bring down upon every soil periodical supplies of ail 

 those saline substances — common salt, gypsum, salts of lime, of mag- 

 nesia, and of potash in minute quantity — which exist in the sea, and of 

 nitrate of ammonia, produced or present in the air. If any soil be defi- 

 cient in these, then a year's rest from cropping, b3'' allowing them to ac- 

 cumulate, may cause the succeeding herbage to exhibit a more luxuriant 

 growth. 



8°. The same remark applies to soils into which springs frombenealJi 

 bring up variable quantities of lime and other substances which the wa- 

 terSjhold in solution. Such springs are, no doubt, of much benefit in 

 some districts, and when the supply they convey is scanty, a year's 

 accumulation may impart additional fertility to the fallowed land. 



9°. Besides that beneficial action of the air to which I have already 

 adverted (4° and 5°), and which is to be ascribed mainly to the influ- 



» 



• For the constitution of these mineral substances, see pp. 257 to 260. 



t Fallow is the term applied to land left atrest for further disintegration.— lAQhxg^a Organic 

 Chemistry applied to Agriculture, p. 149. 



