HOW THE VEGETABLE MATTER INCREASES. 61 



while each succeeding plant is partly nourished by food from the earth, 

 yet each, when it ceases to live, imparts to the soil all the carbon which 

 during its life it has extracted from the air. Let the quantity which 

 each plant thus returns to the soil, exceed what it has drawn from it by 

 only one ten-thousandth of the whole, and — unless other causes inter- 

 vene — the vegetable matter in the soil must increase. 



Thus while it is strictly true that the carbon contained in all plants 

 has been originally derived from the air, it is not true that the ivhole of 

 what is contained in any one crop we raise, is directly derived from the 

 atmosphere — the proportion it draws from the soil is dependent upon nu- 

 merous and varied circumstances. 



The history of vegetable growth, therefore — in so far at least as the 

 increase of the carbon is concerned — may be thus simply stated : 



1°. A plant grows partly at the expense of the soil, and partly at that 

 of the air. When it reaches maturity, or when winter arrives, it dies. 

 The dead vegetable matter decays, a part of it is resolved into gaseous 

 matter and escapes into the air, a part remains and is incorporated with 

 the soil. If that which remains be greater in quantity than that which 

 the plant in growing derived from the soil, the vegetable matter will in- 

 crease; if less, it will diminish. 



2°. In warm climates the decay of dead vegetable matter is more 

 rapid, and, therefore, the portion left in the soil will be less than in 

 more temperate regions — in other words, the vegetable matter in the 

 soil will increase less rapidly — it may not increase at all. 



3°. As we advance intocolder countries, the decay and disappearance 

 of dead vegetable matter, in the form ofgaseous substances which escape 

 into the atmosphere, become more slow — till at length, between the par- 

 allels of 40° and 45°, it begins to accumulate in vast quantities in favour- 

 able situations, forming peat bogs of greater or less extent. While the 

 living plant here, as in warm climates, derives carbon both from the 

 earth and from the air, the dead plant, during its slow and partial decay, 

 restores little to the atmosphere, and therefore adds rapidly to the vege- 

 table matter of the soil. 



4°. Again, in one and the same climate, the decay of vegetable mat- 

 ter, and its conversion into gaseous substances, is more rapid in propor- 

 tion to the frecjuency with which it is disturbed or exposed to the action 

 of the sun and air. Hence this decay may be comparatively slow in 

 shady woods and in fields covered by a thick sward of grass ; and in such 

 situations organic matter may accumulate, while it rapidly diminishes 

 in an uncovered soil, or in fields repeatedly ploughed and subjected to 

 frequent cropping.* 



Being thus fitted, by nature, to draw their sustenance — now from the 

 earth, now from the air, and now from both, according as they can most 

 readily obtain it — plants are capable of living, — though rarely a robust 

 life, — at the expense of either. The proportion of their food which they 

 actually derive from each source, will depend upon many circumstan- 

 ces — on the nature of the plant itself — on the period of its growth — on 

 the soil in which it is planted — on the abundance of food presented to 



• In removing a crop we take away both what the plants have received from the earth and 

 what they have absorbed from the air— the materials, in short, intended by nature to restore 

 the loss of vegetable matter arising from the natural decay. 



