222 HOW CROPS FEED. 



rent of air. Through a mass of this earth 20 inches deep 

 and 3 inches in diameter, pure distilled water (free 

 from carbonic acid) was allowed to filter. It ran through 

 •tcithout taking np any of the gas. Again, Avater contain- 

 in<T its own volume of carbonic acid was filtered through 

 a similar body of the same earth. This v-ater gave xip all 

 its carhonic acid xohlle in contact with the soil. After a 

 certain amount had run off, however, the subsequent por- 

 tions contained it. In other words, the soils experiment- 

 ed with were able to absorb carbonic acid from its aqueous 

 solution, even when their interstices contained the gas in 

 the free state. These extraordinary phenomena deserve 

 further study. 



§ 3. 



NON-NITROGENOUS ORGANIC MATTERS OF THE SOIL.— 



CARBOHYDRATES. VEGETABLE ACIDS. VOLATILE 



ORGANIC ACIDS. HUMUS. 



Carbohydrates, or Bodies of the Cellulose Group.— 



The steps by which organic matters become incorporated 

 with the soil have been recounted on p. 135. When plants 

 ])erish, their proximate principles become mixed with the 

 soil. These organic matters shortly begin to decay or to 

 pass into humus. In most circumstances, however, the 

 soil must contain, temporarily or i^eriodically, unalter- 

 ed carboliydrates. Cellulose, especially, may be often 

 found in an unaltered state in the form of fragments of 

 straw, etc, 



De Saussure {Eecherches, p. 174) found that water dis- 

 solved from a rich garden soil that had been highly ma- 

 nured for a long time, several thousandths of organic 

 matter, giving an extract, which, when concentrated, had 

 an almost syrupy consistence and a sweet taste, was 

 ncather acid nor alkaline in reaction, and comported itself 

 not unlike an impure mixture of glucose and dextriiu 



