120 SOILS AND MANUKES 



sulphur compounds have a deleterious effect on vegetation, 

 but on exposure to air they are all oxidised ultimately to 

 sulphates, which are harmless. Gas lime should not, there- 

 fore, be used in the fresh state, but should be composted 

 for some time before it is applied to the land. As a source 

 of lime it cannot be considered suitable for general use. 



In the ordinary course of analysis, the next step, after 

 removal of the organic matter, is to extract the substances 

 soluble in concentrated acids. The reagent most commonly 

 employed for this purpose is hydrochloric acid of moderate 

 strength. For most of the substances it is a better solvent 

 than any other, but, for several reasons, extraction with 

 nitric acid is preferred for estimation of phosphoric acid 

 and some other constituents. The process of digestion 

 occupies several days ; when it is complete, the extract 

 containing various substances in solution is filtered off and 

 the insoluble residue is collected and weighed. 



The insoluble mineral residue. The residue which 

 remains after treatment with hydrochloric acid, often 

 called sand or simply insoluble matter, consists of frag- 

 ments of undecomposed rock and minerals and substances 

 which, like quartz, are permanently insoluble. It com- 

 prises a very large proportion usually over 80 per cent., 

 and not infrequently over 90 per cent. of the total dry 

 matter. In soils largely composed of calcareous matter, 

 or vegetable matter, the insoluble mineral residue is, of 

 course, much less. 



As a constituent of the soil its influence is purely 

 mechanical, i.e., it only affects the physical properties. 

 It must be remembered, however, that it is the source from 

 which the substances found in the extract, presently to be 

 described, are derived and constantly, if gradually, replen- 

 ished. It is, in fact, the ultimate source from which all 

 the mineral elements of plant food in the soil originate. 

 Its composition generally resembles that of the parent 



