280 ROCKS AND SOIL 



under all rocks, and often in the cracks of stratified rocks or as over- 

 flows over stratified rocks. 



Metamorphic rocks are altered stratified rocks. 



Weathering is the decay of rock because of the action of the ' ' weath- 

 er." It is due to both physical and chemical action. 



Detritus, or talus, is the weathered rock at the base of a cliff. 



Mantle rock is either residual soil or drift. 



Drift has been brought by air, water, or ice. Alluvial soil was de- 

 posited from water. 



Glacial drift was brought by ice. In this country it is found as far 

 south as the mouth of the Ohio River. 



Erosion is the general cutting down of the land level. The agents 

 of erosion get their energy from gravity. 



Soil consists of loam, humus, and small organisms, such as bacteria. 

 It must also contain water and air. 



According to their general structure, soils are either light or heavy. 



Soil particles are classified as gravel, sand, silt, and clay. 



Water is transferred through soil largely by capillary action. 

 . Tillage is working the soil with implements. Plowing consists in 

 turning the soil over. Cultivating is shallow plowing. 



Irrigation is the artificial watering of land. 



The fertility of a soil is its power of producing crops. 



At least 10 chemical elements are needed by plants; these include 

 the carbon, hydrogen, and oxygen taken from air and water. 



The 3 soil elements that are used up most rapidly are nitrogen, 

 phosphorus, and potassium. 



Fertility is lost by the growing of crops, by improper drainage, by 

 permitting a soil to become sour, by growing one crop too long, by 

 selling the fertility in the wrong form. 



We can restore fertility by allowing land to "lie fallow," by rotating 

 crops, and by using fertilizers. 



304. Exercises. 



1. Are wood ashes as good as lime for making an acid soil sweet? 

 Explain. 



2. A year or two after a fire has " burned over" land having an acid 

 soil there is often a wonderful growth of plants; why? 



