178 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



definitely, but at present there is nothing to indicate that the soluble 

 humus plays the controlling part in soil fertility formerly attributed 

 to it. 



A more fruitful method has been to study the fate of the separate 

 plant constituents and ascertain the changes through which they pass 

 when decomposed in the soil. For this purpose the plant substances 

 may conveniently be divided into several groups : the carbohydrate 

 group, including soft celluloses, gums &c. ; the hard resistant fibres 

 made up of resistant cellulose ; the proteins ; the mineral substances ; 

 and the waxes. Of these we may at once dismiss the waxes, as they 

 appear to decompose only slowly in the soil. Nor can we say much 

 about the changes in the mineral matter, as these are not sufficiently 

 investigated. 



Very little is known about the decomposition of the carbohydrate 

 constituents except that it is rapid. Apparently a considerable 

 number of organisms is able to bring it about, including bacteria, 

 moulds, and larger forms ranging up to earthworms. So far as is 

 known the process seems to be analogous to the retting of flax and the 

 formation of skeleton leaves. It is no doubt this material that furnishes 

 most of the energy for the soil population, and we shall see shortly it 

 is indispensable for some of the soil changes that can go on only as long 

 as energy is put into them. 



Its decomposition has another important effect. It forms the 

 bulk of the cell walls, and as soon as it is gone the plant residues 

 lose their structure and definitely begin to mingle with the soil, 

 becoming subjected to the various decomposition agencies at work 

 therein. 



The harder resistant cellulose, making up the leaf skeletons, the 

 fibres, &c., takes longer to decompose in the soil and is not attacked 

 by nearly so many organisms. No single organism is known with 

 certainty to decompose it under the aerobic conditions obtaining in the 

 soil, though an association of organisms breaks it down fairly readily. 

 In absence of air it decomposes with formation of two inflammable 

 gases, marsh gas and hydrogen. This decomposition goes on at the 

 bottom of stagnant ponds and in marshes, and the bubbles of inflam- 

 mable gas can be got by stirring up the mud with a stick. There is, 

 however, no evidence that these gases are formed in the soil, and so far 

 as can be judged the change proceeds in quite a different way. 



The decomposition of protein in the soil is of altogether different 

 significance, because in this case it is the products that are of chief 

 importance. It is difficult to study this change in the soil because of its 

 complexity, and no progress was possible with the early stages until 

 prolonged laboratory experiments had shown the general nature of the 

 decomposition. Even now we do not know for certain that the stages 

 are the same in the soil as in the laboratory, but they seem to be, since 

 typical laboratory products have been found by Schreiner and his 

 co-workers in the soil, including amino- acids, diamino- acids, purin 

 bases, &c. 



