54 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Weathering agencies act upon consolidated and unconsolidated 

 rocks exposed on the surface of the earth, reduce the size of 

 the particles, and change the rocks chemically and physically. 

 The rocks are changed into soil capable of supporting the growth 

 of cultivated plants. 



Broken rocks, however finely pulverized, do not constitute 

 soil. Besides the mere mechanical breaking of the rock, two 

 other processes take part in the conversion of rock into soil. 

 First, a greater or less quantity of organic matter and combined 

 nitrogen are stored up. This process begins with the bare rock. 

 Bacteria first appear. These take up carbon dioxide and nitrogen 

 from the atmosphere and leave organic matter and nitrogen when 

 they die. Then mosses and lichens begin to appear. They also 

 have the power of taking nitrogen from the air. As the weather- 

 ing agencies deepen the soil, the variety of plants increases, but 

 almost always some of the species are present which have the 

 power of causing atmospheric nitrogen to enter into combination. 

 The residues left when the plants die, store the soil with organic 

 matter and nitrogen. A small amount of organic matter and 

 nitrogen are contained in rocks. 1 



The second change in the conversion of rock to soil is due to 

 the fact that plant food becomes more easily taken up by plants 

 than it was in the original rock. This is in part due to chemical 

 changes, and in part to the action of the organic matter which 

 has been added, but perhaps to the greatest extent to the work- 

 ing over of the plant food by the past generation of plants. 



Weathering Agencies. Weathering is the term applied to the 

 natural decomposition or breaking* up of rocks, and weathering 

 agencies are the agencies which do this work. Weathering and 

 weathering agencies are studied by observing the changes which 

 are now going on, and by comparing altered rocks with the 

 original masses from which they were derived. 



Weathering processes are both mechanical and chemical. They 

 are mutually helpful. Mechanical processes reduce the size of 

 the rock fragments, thereby affording more surface for chemical 

 1 Hall and Miller, Jour. Agr. Sci., 2, p. 343. 



