AGRICULTURAL CHEMISTRY AND GEOLOGY. 185 



those mineral substances which it is the function of the soil to furnish, but also with those 

 forms of food which it naturally derives from the atmosphere. This supply is an addition 

 to that quantity which the atmosphere contains. 



11. Those forms of vegetable food contained in the soil which are not gaseous or volatile 

 enter the plant through its roots. The vehicle of their transmission is water, by the agency 

 of which they become soluble and transportable. Many of these kinds of food dissolve in 

 pure water, others only in water which contains carbonic acid or a salt of ammonia. 



12. All those substances which exert a solvent action on such ingredients of the soil as 

 are themselves insoluble, cause, by their presence, a given volume of rain water to take up 

 a larger quantity of vegetable food than it otherwise could. 



13. From the progressive decay of the organic matters of manure originate carbonic acid 

 and ammonia salts; they constitute an active source of carbonic acid in the soil, whereby 

 the air and water present in the soil are made richer in carbonic acid than they could be in 

 their absence. 



14. Animal manures not only offer to the plant a certain amount of soil and atmospheric 

 food, but in their decay is supplied, in the form of carbonic acid and ammonia, an indispen- 

 sable means of rendering soluble and available to the plant the insoluble ingredients of the 

 soil in greater quantity and in shorter time than could occur in the absence of decaying 

 organic matter. 



15. Other things being equal, vegetation receives less water through the soil in warm, 

 dry seasons than in wet years ; the harvests in different years stand in relation thereto. A 

 field of given quality yields smaller crops in dry seasons ; by the same average temperature 

 the yield increases to a certain limit with the increase of the quantity of rain. 



16. Of two fields — one richer, one poorer in plant-food — the richer yields in dry seasons 

 more produce than the poorer, other things being equal. 



17. Of two fields alike in character, and containing an equal amount of soil-ingredients, 

 one of which, however, has besides a source of carbonic acid — viz. decomposable vegetable 

 or animal matter — the latter yields more in dry seasons than the former. 



18. The cause of this difference in yield lies in the unequal supply of matters, both as con- 

 cerns quality and quantity, which the plant receives from the soil in a given time. 



19. All obstacles present in the soil, which hinder the solution and absorbability of the 

 plant-food, proportionally destroy its ability to serve as food; they make the plant-food 

 ineffective. A certain physical state of the soil is a needful preliminary condition to the 

 efficacy of the food therein contained. The soil must allow the access of air and moisture, 

 and permit the roots of plants to extend themselves in all directions, and seek out their 

 nutriment. The expression, telluric conditions, comprises every thing necessary to vegetable 

 growth that depends upon the physical qualities and composition of the soil. 



20. All plants need as nouris'kmvntphosphoric acid, sulphuric acid, the alkalies, lime, magnesia, 

 and iron. Certain families of plants require silica ; those that grow on the sea-shore and in the sea 

 itself require common salt, soda, and iodine. In some families of plants the alkalies may 

 be in part replaced by lime and magnesia, and vice versa. All these bodies are collectively 

 designated as mineral food. The atmospheric food of plants is carbonic acid and ammonia. 

 Water serves itself as food and also as a general medium of nutrition. 



21. The bodies that are necessary as food for the plant have an equal value in this respect; 

 i.e. if any one of the entire number be wanting, the plant cannot flourish. 



22. Fields which are adapted to the cultivation of all species of plants contain all the soil- 

 ingredients that are necessary for these plants ; the words poor or unfruitful, and rich or 

 fruitful, express the relations of these soil-ingredients in quantity or quality. 



Among qualitative differences are understood differences in the solubility of the mineral 

 ingredients, or in their capability of entering the vegetable structure through the agency of 

 water. 



Of two soils which contain equal quantities of mineral food, one may be fruitful, (con- 

 sidered as rich,) the other unfruitful, (considered as poor,) when in the latter these nutritive 

 substances are not free, but exist in the state of chemical compound. A body in chemical 

 combination opposes, by its attraction for the bodies it is combined with, an obstacle to 



