18 CHAPTER III 



6. To destroy or to prevent weed growth, and in some 

 cases to control insect pests. Weed seeds dormant in the soil 

 are often (through tillage) induced to germinate, after which 

 they can be destroyed by further tillage. 



7. To promote optimum conditions for the development of 

 beneficial soil micro-organisms. 



8. To increase the available plant food. 



9. To keep the soil in good condition during growth. 

 The most important function of tillage is the preparation 



of a good seed-bed, which may be defined as " land free from 

 weeds and surface trash, sufficiently mellow to permit easy 

 penetration of the plant roots, sufficiently compact to hold 

 moisture, and to be free from large air spaces, and sufficiently 

 fine in structure to bring many soil particles in contact with 

 the seed and thus to supply an abundance of moisture to the 

 germinating plant.'" 



Terms Commonly Used. — Even a brief survey of the 

 subject of tillage, such as given herein, necessitates the use of 

 certain terms which are defined below : — 



Texture has reference to the size of the individual particles 

 of which a soil is composed. 



Structure refers to the arrangement of soil particles inde- 

 pendent of size, i.e., the grouping of the particles. 



Tilth has reference to the mechanical condition of the soil 

 and is largely dependent upon the texture and structure of the 

 soil particles. Good tilth implies a thorough, deep and strong 

 granulation resulting in a well-marked crumb structure and, 

 consequently, a friable soil. 



Plasticity. — " Any material which allows a change of 

 form without rupture, and which wall retain this form not only 

 when the pressure is removed, but also when dry, is said to be 

 plastic."^ 



Cohesion. — " The tendency of the particles of a soil to 

 stick together and conserve the mass intact."* The cohesion 

 of a soil in the moist condition is generally known as 

 " tenacity." Cohesion is closely related to plasticity. 



Granulation. — " By this is meant the drawing together 

 of the small particles around a suitable nucleus, so that a 

 crumb structure is produced. It is nothing more or less than 

 a condition brought about by a variable water film and the 

 pulling and binding capacities of colloidal material operating 

 at numberless localised foci."^ In this condition plasticity and 

 cohesion are at a minimum. 



