SOILS AND FERTILIZERS 377 



less stratified or arranged in distinct layers; stones or gravel at the 

 bottom, and nearest the source of movement, finer stones, or finer 

 gravel above and further down in the path of flow, sand and impal- 

 pable matters at the surface and at the point where the stream, be- 

 fore turbid from suspended matter, finally clears itself by a broad 

 level course and slow progress. 



Alluvial deposits have been formed in all periods of the earth's 

 history. Water trickling down a granite slope carries forward the 

 kaolinite (a compound of silica and alumina hydrated, or containing 

 water) arising from the decomposition of feldspar, and the first hol- 

 low gradually fills up with a bed of clay. Thus, in the valleys are 

 deposited the gravel, sand and rock dust brought from the neighbor- 

 ing mountain. Lakes and other bodies of water become filled with 

 silt, carried into them by streams. 



Colluvial soils are those which, while consisting partly of drift 

 or alluvium, also contain sharp, angular fragments of rock, from 

 which they originally came, thus showing that they have been trans- 

 ported to any distance, but are made up of soils in place more or less 

 mingled with alluvium. 



MINERAL SUBSTANCES PRESENT IN SOILS. 



Silica, iron, alumina, lime, magnesia, soda, potash, phos- 

 phoric acid, sulphuric acid, chlorine, and sometimes manganese and 

 flourine, are present in all soils in varying proportions. 



Silica. -Forms the basis of most soils and is by far the most 

 abundant solid constituent of the earth. It is present in soils in 

 proportions varying from 29 per cent to 90 per cent, according to 

 the character of the soil. In its usual insoluble form its action is 

 merely mechanical. In most fertile soils a small amount of it is 

 found in a soluble state, and as such it may enter the sap circulation 

 of plants. 



Iron. Occurs in all soils as a finely diffused ferric hydrate. 

 Its percentage varies from 1.50 per cent to 20 per cent, and it gives 

 the color to red and yellowish soils. Though required in minute 

 quantities, it is absolutely indispensable to plant life. Its function 

 seems to be giving the soil an increased power to absorb and retain 

 heat and moisture, and rendering tillage easier in clay lands. As 

 a plant food it is supposed to aid in producing the chlorophyll, which 

 gives the green color to plants. 



Alumina. The chief constituent of common clay. It does not 

 directly contribute to the growth of plants, and is of but little value 

 as a plant food. As clay, however, it is an essential constituent of 

 soil; it has the property of absorbing and retaining (fixation), the 

 easily soluble compounds of the soil, or those supplied to the soil by 

 fertilization. Alumina varies in amount from one per cent to ten 

 per cent, according to the kind of soil. 



Calcium (lime.) An absolutely indispensable plant food, yet, 

 more important on account of its influence upon the mechanical con- 

 dition of the soil, in the process of nitrification, and in the forma- 

 tion of soluble carbonates. It is one of the prime essentials of an 

 arable soil ; its absence in sufficient proportions is always followed by 



