54 NECESSITY FOR ORGANIC MATTER. 



Having explained the origin of this organic matter, 

 it is only necessary to mention briefly, that 'it is com 

 posed of the same four organic substances previously 

 named, Carbon, Hydrogen, Nitrogen, Oxygen. 



SECTION II. NECESSITY FOR ORGANIC MATTER IN THE SOIL, 

 AND ITS LIABILITY TO EXHAUSTION. 



This part is necessary in the soil for several reasons. 



1. It enables the land, if light and sandy, to retain 

 moisture, and also to retain manures much longer than 

 it otherwise would; to stiif and clayey soils it gives 

 mellowness and lightness. 



2. Another important effect in cold climates, is the 

 darker color which it imparts to the surface. A dark 

 colored soil absorbs more heat than a light one, being 

 consequently warmer and earlier. This is seen in the 

 fact that snow melts sooner from the ploughed field 

 than from the meadow in similar situations; from the 

 dark garden bed, than from the gravelled walk. 



3. Beside these useful purposes, there is no doubt 

 that the organic part of the soil, in a greater or less 

 degree, ministers food directly to the plant through its 

 roots. The supply obtained in this way varies with 

 the situation, but is of much importance to plants, as 

 shown by their increased luxuriance when it is fur 

 nished them in a soil previously deficient. 



This consumption of organic matter by plants to 

 form their own bulk, shows how it is that land long 

 cultivated and scantily manured, at last becomes very 

 poor in this part. Each crop has carried away a 

 portion of it, more than has been returned in the small 

 quantity of manure applied. Another way in which 

 it is exhausted, is by frequent ploughing and stirring, 

 whereby it is exposed to the air, and consequently 

 decomposes rapidly. If you bury straw or other or 

 ganic matter deep under the surface, so as to be ex- 



