322 SOILS. 



Recognition of the Prominent Chemical Character of Soils. 

 In a former chapter the soils formed from the several minerals 

 and rocks have been discussed in a general manner. We can 

 as a rule obtain some insight into the nature of any soil which 

 we can trace to its parent rock or rocks, if we are acquainted 

 with the composition of the latter. 



Similarly, but in a much more direct manner, we can ob- 

 tain a strong presumption as to the nature of any soil by de- 

 termining the undecomposed minerals present in it. In all 

 ordinary cases the presumption must be that the decomposed 

 portion of the soil has been derived from the minerals still 

 found in it. Of course it may happen in the case of lands de- 

 rived from widely distinct and distant regions that no such 

 characteristic minerals can be found ; this is very commonly 

 true of the soils forming the deltas of large rivers, in which 

 sometimes the only remaining recognizable mineral is quartz 

 in its several forms, with occasional grains of such hardy 

 minerals as tourmaline, garnet, etc. Apart from such cases, 

 the hand lens or the microscope permits us to recognize in 

 most soils the minerals that have mainly contributed to their 

 formation, thus also gaining a clew to their prominent chem- 

 ical nature. 



Such recognition sometimes involves, of course, a somewhat 

 intimate knowledge of mineralogy; yet a little practice will 

 enable almost any one to identify the more important soil- 

 forming minerals, under the lens or microscope, according to 

 the degree of abrasion or decomposition they may have un- 

 dergone. The details of such researches lie outside of the 

 limits of this treatise, but some general directions on the sub- 

 ject are given farther on. 1 



Acidity. Neutrality, .-llknliiiity. A test never to be omitted 



are due wholly to differences in the moisture supply, which he claims is, aside from 

 climate, the only governing factor in plant growth. The tables of analytical 

 results given in Bull. 22 fail to sustain the first contention ; the second is pointedly 

 contradicted both by practical experience, and by thousands of cumulative culture 

 experiments made by scientific observers ; the third fails with the second, except 

 of course in so far as an adequate supply of moisture is known to be an absolute 

 condition both of plant growth, and the utilization of plant-food. It is moreover 

 well known that it is not water alone, but water impregnated more or less with 

 humic and carbonic acids, that is the active solvent surrounding the plant root. 

 1 See Appendix B. 



