174 Clamficatioi) of Soil x 



Then tlie centre S of the triangle pqr, obtained by bisecting the base 

 angles is the requirefl point. This construction avoids the necessity for 

 raising the percentages arithmetically so that they total 100. In general 

 they will be less than 100, for the losses on solution and ignition are not 

 included in the amounts plotted. 



Soils with a high content of organic matter or of calcium carbonate 

 cannot be compared by their position on the triangle alone. By the 

 employment of an arbitrary colour scale, the organic matter content, or 

 the acidity expressed in terms of titration values or of hydrogen ion 

 concentration could be shown simultaneously with its approximate 

 mechanical analysis. Rainfall and other climatic data arc obviously 

 open to a similar method of treatment. 



In order that the position of a point on the diagram may be rapidly 

 interpreted, a diagram showing the proportions of each of the three 

 ingredients present, by steps of 10 per cent, has been prepared. By 

 super-imposing this on any of the soil diagrams the limits of the groups 

 can rapidly be read off. 



The amount of variation permissible in a mechanical analysis for 

 survey purposes has been investigated by Robin.son(3;t). The analyses of 

 the two yields, (a) uiiiforin, (h) too variable, are shown in Fig. 5. Atten- 

 tion is drawn to this in order that an idea may be obtained as to the 

 value that is to be assigned to any amount of scatter in a diagram. 



The general arrangement of the soil types is indicated in Fig. 4. The 

 triangle has been divided up into three main divisions (marked in solid 

 lines) according as one or other of the three ingredients predominates. 

 Each division is subdivided into two classes (by dotted lines) according 

 as one or other of the remaining two constituents is in excess. The inter- 

 relationship between class and class is thus proportional to the length 

 of the dividing line. They may merge into one another or diverge widely. 

 For convenience of reference the classes nuiy be named A^, B^, Bq, 

 Cfj, C^Y and A^,, respectively as shown. 



Fig. 6 shows a number of wheat soils from almost every geological 

 formation. It will be noticed that they tend to be more or less concen- 

 trated around those selected by Hall and Russell as typical. The lighter 

 soils of Norfolk on which wheat is grown, though the soils are not par- 

 ticularly well adapted to the crop, merge into the typical wheat group, 

 which apparently lies near the boundary of B^. and C,, . The soils in 

 Cg are on the whole more typically grass than arable (12, 16, n, 19,20,29, 



31, 36, 40). 



Barley soils are illustrated in Fig. 7. The crop is grown on soils of the 



