AGRICULTURAL CHEMISTRY. 151 



These minerals, while they make up the chief bulk of rocks or of 

 the soil, are always associated with minute quantities of other com- 

 pounds, such as phosphates, chlorids, sulphates, or bodies yielding 

 sulphates, <fcc., upon which the geologist scarcely bestows attention, 

 which are, however, for the scientific agriculturist of great moment. 

 In consequence of this wise provision and of the beneficent inter- 

 mingling of the fragments of rock from widely distant regions, during 

 the drift period and by alluvial agencies, it has resulted that, almost 

 everywhere, there exist in the soil all those mineral bodies which are 

 found in plants. Some one has been, indeed, so impressed with the 

 universality of the distribution of each elementary form of matter as 

 to offer the opinion that all the sixty simple bodies which constitute 

 the globe might be found in every handful of soil or cup of water 

 existing on its surface did we but possess sufficiently delicate tests. 



It sometimes happens, indeed, that where a soil is in place, i. e., 

 has not been transported, but lies covering the rock from which it 

 has been formed, it is very poor and supports only a sparse vegeta- 

 tion, or, perhaps, is totally naked and destitute of all organic life. 

 But these instances are comparatively rare, and their infertility is 

 more often due to want of water, or some external cause, than to the 

 absolute deficiency of those ingredients which are needful in a pro- 

 ductive soil. 



It often happens that a close connexion exists between the rock 

 and the overlying soil; as often, however, the one serves as no indi- 

 cation to the value of the other. 



The mechanical analysis of any soil separates it into portions of 

 different fineness. A coarse sieve removes gravel, consisting of the 

 larger fragments of rock ; a finer one, coarse sand; by washing with 

 water, fine sand is left, while the turbid washings deposit after a time 

 a quantity of impalpable matter which may consist in part of the 

 exceedingly fine particles of rock, and in part of clay, or it may be 

 entirely formed of the latter. 



In most inferior soils the gravel and sand, when abundant, are an- 

 gular fragments of quartz, feldspar, hornblende, augite, and mica, or 

 of rocks consisting of these minerals. It is only these harder and 

 less easily decomposable minerals that can resist the pulverizing 

 agencies through which a large share of our soils have passed. In 

 the more fertile soils, formed from secondary limestones and slates, 

 the fragments of these stratified rocks occur as flattened pebbles and 

 rounded grains. 



The fine joortion of the soil bears, either in quantity or composition, 

 the most direct relation to its fertility. It is this which is capable of 

 yielding to the growing plant the food it requires. The coarser parts 

 of the soil are a vast store of materials in reserve for the distant fu- 

 ture, since, by their slow disintegration, they themselves gradually 

 become so comminuted as to serve the wants of vegetation. 



Clay, which is almost invariably a chief part of the impalpable 

 matter of the soil, has been marked by us as a mineral, and its general 

 composition indicated in the table (p. 150.) It is a product of the 

 action of water and carbonic acid upon such minerals as feldspar, 

 mica, hornblende, and augite. Under the influence of these agents, 



