68 DRY-FARMING 
two regions must differ greatly in their fertility or 
power to produce and sustain plant life. It cannot 
be believed that the water-washed soils of the East 
retain as much fertility as the dry soils of the West. 
Hilgard has made a long and elaborate study of this 
somewhat difficult question and has constructed a 
table showing the composition of typical soils of 
representative states in the arid and humid regions. 
The following table shows a few of the average results 
obtained by him : — 
Numser PaRTIAL PERCENTAGE COMPOSITION 
OF 
SAMPLES Phos- 
ANALYZED ees Seluble Alumina|Lime|Potash|phoric| Humus 
acid 
Source or 
Som 
Humid region 696 84.17 | 4.04 | 3.66 |0.13) 0.21 | 0.12] 1.22 
Arid region 573 69.16 | 6.71 | 7.61 |1.43) 0.67 | 0.16 | 1.13 
- Soil chemists have generally attempted to arrive 
at a determination of the fertility of soil by treating 
a carefully selected and prepared sample with a 
certain amount of acid of definite strength. The 
portion which dissolves under the influence of acids 
has been looked upon as a rough measure of the pos- 
sible fertility of the soil. 
The column headed ‘‘Insoluble Residue” shows 
the average proportions of arid and humid soils 
which remain undissolved by acids. It is evident 
at once that the humid soils are much less soluble 
