ORGANIC SUBSTANCES IN SOILS. 257 



better fitted to absorb moisture and gaseous matter, 

 than either of the earths taken singly ; and in the 

 same way, the addition of organic matters to the soil 

 has the effect of increasing this power still further ; 

 and, therefore, as was before said, the best soils are 

 those containing a quantity of each of the three 

 earths, mixed with a moderate proportion of decaying 

 organic matters. 



669. Some soils contain a very large quantity of 

 organic substances, and indeed not unfrequently con- 

 sist almost wholly of the remains of vegetables : the 

 different varieties of peat are of this character. In 

 such soils we observe the bad effects of the presence 

 of too much organic matter in a soil. It is thus 

 rendered tough, porous, and spongy, so that it is 

 soon saturated with water, or completely dried, by 

 changes of weather. Besides all this, it frequently 

 contains a peculiar acid, produced during the decay 

 of vegetable matter, which is hurtful to the growth 

 of plants. 



670. Another use of the organic constituents of 

 the soil is, that they furnish to the plants growing on 

 it the saline substances which they naturally con- 

 tain ; and which not being subject to decay are gradu- 

 ally washed out by the rains during the decomposition 

 of the organic substances, and are ready to be ab- 

 sorbed by the roots of growing plants. 



671. The decay of the vegetable remains in the 

 soil, never proceeds so far that the whole of the ele- 

 ments which they contain are converted into carbonic 



99* 



