326 SOIL CONDITIONS AND PLANT GROWTH 



surface being the bared subsoil. This kind of variation is 

 common on clay soils and often leads to differences in agri- 

 cultural value that are fairly marked, but not sufficient to 

 affect the type. Normally the subsoil contains more clay than 

 the surface soil (as in iio, 69, and 70), and any deviation 

 should be carefully investigated. 



The characterisation of soil types is usually effected by 

 mechanical analysis and determinations of calcium carbonate 

 and organic matter. A representative set of soils should, 

 however, be subjected to chemical analysis, the clay fractions 

 being, if possible, broken up by ammonium fluoride and 

 analysed completelv. .f'-iis about which precise information 

 has been obtaine( ■ i . lurial and other trials should be very 

 completely examir 1 order that they may serve as standards 

 in the analysis 01 ..ner soils from the same area. 



The problem set by the farmer is wholly different. He 

 does not want to know to what type his soil belongs, but how 

 he must manure it, etc. If the analyst has an adequate know- 

 ledge of the soil type and the locality he can readily ascertain 

 in what respects the soil differs from the type, and then, from 

 the known results of manurial and other trials on that type, 

 he can give the information wanted with a reasonable degree 

 of probability ; otherwise his report can only be a matter of 

 guesswork. In short, the farmer's problem can be satisfac- 

 torily solved, and the manurial trials fully interpreted, only 

 when a complete soil survey has been made. 



The analyst must consider the soil from three points of 

 view: (l) its physical properties, especially those relating to 

 the ease of movement of the soil water ; (2) its store of plant 

 food, actual and potential ; (3) the rate at which potential, 

 food can be converted into actual food. 



The Interpretation of Mechanical Analyses.^ 



The properties of the various fractions have already been 

 given in Chapter III., but some little practice is necessary 



1 See Appendix for Methods of Analysis and pp. 98 and 102 for details as to 

 dimensions and composition of fractions. 



