238 



ries here ; for the question comes : ' What do plants live upon ?' — 

 p. 146. 



The reply to this question involves the consideration of those im- 

 portant agricultural subjects, the properties and mode of action of 

 manures. In the preliminary observations upon the chemical consti- 

 tuents of the vegetable fabric, oxygen, hydrogen and carbon, which 

 form cellulose, the absence of nitrogen from these constituents is 

 mentioned, and the necessity for a supply of that gas adverted to, in 

 order that the plant may be enabled to perform certain chemical pro- 

 cesses necessary for the elaboration of the nutrient matters taken up 

 by the roots. 



" The inquiry into the nutrition of the plant includes, therefore, the 

 inquiry into the sources of carbon and nitrogen ; oxygen and hydro- 

 gen being sufficiently provided by water and atmospheric air. The 

 notion which has hitherto been generally received is, that the plant 

 extracts its carbon and nitrogen from manure, or from the humus of 

 the soil. 



u All animal and vegetable bodies, so soon as they are dead, pass 

 over into a state of decomposition, by means of which they are dissi- 

 pated, sooner or later, in the atmosphere, being changed into carbonic 

 acid, ammonia and water. So long as this process is incomplete, a 

 residue, itself much altered, of a brownish or black colour, remains, 

 which at the commencement of the decomposition is called manure, 

 and towards its close humus, or vegetable mould. It is a complex 

 mixture of very manifold products of decomposition. Now it was 

 argued thus : carbon and nitrogen are abundant in humus ; in a soil 

 that is rich in humus or is well manured, plants thrive better than in 

 one which is poor in humus ; consequently, humus is the source of 

 the carbon and nitrogen of plants. But this reasoning is altogether 

 inconclusive." — p. 148. 



Itis manifestly inconclusive,becauseit does not account for the source 

 of the immense quantities of nitrogen and carbon derived from the soil 

 in cases wherein no, or comparatively very little, organic matter is re- 

 turned to localities supplying organisms in which carbon and nitrogen 

 abound. About 4,000 lbs. per acre of dry wood, containing about 

 1,600 lbs. of carbon, are annually derived from forests which receive 

 no other manure or humus than what they obtain from their own 

 leaves or broken wood : and by carefully conducted experiments in- 

 stituted upon land nearly five acres in extent of area, for twenty-one 

 years, it has been shown that the average annual harvest gained from 

 the soil gave a result of twice as much nitrogen, three times as much 



