1754 The Cornell Reading-Courses 



are correspondingly small. Spaces that are too small are likely to be con- 

 tinually full of water, which crowds out air and creates bad ventilation. 

 Poor ventilation, in turn, smothers or prevents the development of roots 

 and bacteria. This interferes with the food supply and is injurious in 

 many other ways. If the spaces at the surface of the soil are too small 

 the water that comes as rain does not find ready admission and is lost, 

 so that the crop may later suffer from lack of water which should have 

 been absorbed. Further, soil spaces that are too small, if clogged with 

 water and consequently poorly ventilated, promote chemical and biological 

 processes which are decidedly harmful. Thus, nitrogen in the form of 

 nitrate fertilizer may be broken down to the condition of a free gas, so 

 that it cannot be used by many kinds of plants. Excessive absorption of 

 water due to very small spaces renders the soil cold, and from this arises 

 a cl - in of injurious results. This brief explanation shows how important 

 is that physical condition of the soil which gives the right size and pro- 

 portion of pore spaces. 



The physical nature of the soil is referred to as tilth. The most advan- 

 tageous character and arrangement of the soil is termed good tilth, while 

 the reverse condition is termed bad tilth. The maintenance of good tilth 

 should be the first object of the farmer. Consequently he should under- 

 stand that physical basis on which good tilth rests and the practical means 

 by which it is regulated. This lesson is devoted to a brief explanation 

 of the physical basis, of the things that most directly affect it, and of 

 the action of the more common implements of tillage in their relation 

 to these properties. 



As has been indicated, good tilth is identified with the right size of 

 pore space throughout the soil mass. These pores depend for their size 

 on the size and arrangement of the particles of soil. The particles of 

 soil are of many sizes. The term used to refer to the size or fineness of 

 the individual particles of soil is texture. A fine-textured soil is one made 

 up mostly of very small particles. A gravelly or a sandy texture is one 

 made up of large particles. 



The arrangement of the separate particles in a soil is termed structure. 

 Soils may have a loose or a compact structure. Clay in a fine, friable 

 condition easy to stir may be said to have a loose structure. If the separate 

 particles arc gathered in groups or kernels such a structure is termed 

 granular. But the same soil when mixed and stirred in a wet condition 

 becomes very dense, and such a soil is said to have a compact, or puddled, 

 structure. 



Texture of soil 



Not only are soils made up of particles of different size, but each 

 soil contains particles of many different sizes. These different sizes have 



