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repellent to the average farm crop, and the farmer must alter its structure 

 to get the best results. As a matter of fact, the chief object of all tillage 

 operations is to change the structure of the soil. A nicely granular, fri- 

 able condition is ideal. Clay soil plowed a little too wet or a little too 

 dry takes on a bad structtire, and in order to get the best results with 

 it, to till at the right time and in the right way, careful knowledge and 

 keen observation are required. 



The moisture supply, aeration, temperature, and the millions of or- 

 ganisms in the soil, are dependent on its texture and structure, and the 

 farmer controls them all largely through tillage, which renders the structure 



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Fig. 12. — Soil moisture is saved by a mulch, or dry layer of soil on the surface. Stones 



may serve this purpose 



more or less favorable to each. Tillage is therefore an indirect rather 

 than a direct operation; and therein rests the necessity of knowledge of 

 the relation of the various factors in productivity, one to the other. For 

 example, a clay soil is generally known as a cold soil, although clay itself 

 is not colder than sand or gravel. But clay generallj^ holds more water 

 than sand, and it is this larger amount of water that renders it colder; 

 consequently, any change in the structure of clay which will cause it to 

 hold less water will give it normally a higher temperature. 



In order to accomplish desired changes in structure, the farmer may work 

 in cooperation with nature's forces — with frost, rain, and the roots of 

 plants ; and without their cooperation his efforts would usually accomplish 

 very little. 



