260 MANUAL OF AGKICULTUEE. 



therein successively treated of, to the leading principles of agri- 

 culture, as they will next be practically adverted to in the re- 

 maining portion of this manual. 



The classification and the cliemical and physical characteristics 

 -of soils will fall to be considered by way of preliminary matter, 

 seeing that the soil is the primary essential requisite for afford- 

 ing the plant a foundation on which .to begin the formation of 

 its structure, and, at the same time, in connection with the atmo- 

 sphere, supplies all the elements necessary for the plant's growth. 

 And before doing so a brief recapitulation is expedient of the 

 plant's mode of absorbing its organic and inorganic constituents. 



The plant is but little dependent on the soil as a source of 

 carbonic acid supply, asiit is the carbon of the atmosphere which 

 it is constantly absorbing and assimilating. Still, a considerable 

 quantity of the atmospheric carbon is being carried into the soil 

 when the form.er is dissolved by rain water. And when it is 

 there freed by chemical action a proportion of it is directly 

 absorbed by roots. Water necessary to the plant, yielding oxy- 

 gen and hydrogen, the roots can absorb ; and likewise, in some 

 measure, the leaves, when the air is damp. Nitrogen, although 

 existing free to so large an extent in the atmosphere, is not 

 available for plant life in equal measure with these other three 

 inorganic elements. It is only to a limited extent that j)lants 

 can absorb the nitrogen of the atmosphere ; and then it is the 

 ammonia and nitric acid it contains and only from the ground as 

 a source, into which, both being soluble in water, they are 

 carried by the rain. On this point Liebig says : — " As regards 

 the quantity of ammonia thus brought down by the rain, ... as 

 1132 cubic feet of air, saturated with aqueous vapour, at 59° 

 Fahrenheit, should yield 1 lb. of rain water, if the pound contain 

 only l-4th of a grain of ammonia, a piece of ground of 26,910 

 square feet — 43,560 square feet being an acre — must receive 

 annually upwards of 80 lbs. of ammonia, or 65 lbs. of nitrogen ; 

 which is much more nitrogen than is contained in the form of 

 vegetable albumen and gluten in 2650 lbs. of wood, 2500 lbs. of 

 hay, or 200 cwt. of beetroot, which are the yearly produce of 

 such a piece of ground ; but it is less than the straw, roots, and 

 grain of corn which might grow on the same surface would con- 

 tain," Other cheaiists, however, calculate the amount of nitro- 

 gen carried by rain to the soil at a much lower figure than the 

 Baron does. Humus, or the organic matter of soils, absorbs 

 ammonia from the atmosphere, and when existing free in the 

 soil ; in like manner does clay. Nitric acid and ammonia are 

 also produced in the soil by the decay of animal and vegetable 

 substances containino- nitroG;en. The former combines with 

 bases, such as potash, soda, or lime, to form nitrates ; and these 

 and the ammonia become absorbed by the clay or humus. It is 



