"' On the Theory of Manures. 7 



vidual substances forming the food of plants, or assisting in 

 rendering that food available in increasing the produce of 

 plants, the importance of nitrogen to both plants and animals, 

 though undoubtedly sometimes overrated, entitles it to a pro- 

 minent share of attention. It is the basis of fermentation, 

 which cannot be carried on without nitrogen, whether we may- 

 reckon it the fermenting principle itself, or, as some, the food 

 of the fungi which carry on fermentation. It appears that, as 

 in the food of animals the necessary quantity of nitrogen is so 

 mixed up with their ordinary aliment, that in attaining it the 

 other substances, viz. the carbon, hydrogen, oxygen, and saline 

 earthy matters, forming the structure of the body, follow of 

 course; so in plants, when we deposit their food in the soil, 

 the stomach of the plant, it is generally, as in farm-yard 

 manure, a mixture of different substances containing all the ele- 

 ments requisite to build up the structure of the plant, and 

 assist the vital energy of the system in the chemical changes 

 necessary to enable the several organs to perform their func- 

 tions. Nitrogen forming a constituent, less or more, in all 

 plants and all the parts of plants, especially the youngest and 

 most active parts, and being found in much greater quantity 

 in animals, from the carbon given off by respiration, a mixture 

 of these substances, especially when containing a due proportion 

 of the latter, will always be found, along with the nitrogen, to 

 convey the other substances wanted. It is probable, also, that, 

 even in the function of absorption, the most essential elements 

 are intimately united ; the humate and carbonate of ammonia, 

 apparently the greatest source of food to plants, furnishing the 

 carbon and nitrogen, combined with water (or hydrogen and 

 oxygen), the most essential elements of plants. 



In the excellent papers lately published in the Gardener''s 

 Chronicle from Professor Sprengel, whose great eminence in 

 his profession seems properly united to an intimate acquaint- 

 ance with practical cultivation (a most essential requisite in 

 bringing science to bear on practice), the benefits of humus have 

 a very important station. In all the mixtures he recommends 

 as necessary to prepare manures for becoming the food of 

 plants, he gives humus a preference, as the most essential requi- 

 site in preventing the evaporation of ammonia, and retaining it 

 in the compost in the state of humates and carbonates of ammo- 

 nia. Even in the solution of bones, humus is the article he 

 recommends, as both rendering the phosphates soluble by the 

 humic and carbonic acid it furnishes, and at the same time 

 absorbing the ammonia of the cartilage. It appears from his 

 practice, that, where sufficient vegetable remains have been 

 mixed with the animal substances usually employed as manures, 

 the humus has been found sufficient to retain, not only the am- 



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