Tilth and Tillage of the Soil 1757 



In a soil of coarse texture the particles of soil are free, one from the other, 

 and rest together as a loose mass. This is well seen in clean sand and in 

 gravel. The size of the pores in the soil is dependent on the size of the 

 particles, and the larger the particles are, the larger the pores will be. For 

 example, the spaces between buckshots are larger than the spaces between 

 bird shots. The total volume of the spaces in a quart of material in either 

 case is approximately the same. If, however, the two kinds of shot are 

 mixed together they will occupy less space than two quarts taken sepa- 

 rately, because the small shot partially fill the spaces between the large 

 shot. If, on the other hand, a few of the large and a few of the small 

 shot were fastened together in bunches and these bunches were brought 

 together, they would be found to occupy more space than either of the 

 other arrangements. 



Structure of soil 



The above illustrates how the arrangement or structure of a soil affects 

 pore space and consequently crop-producing properties. Soils are made up 

 of particles of different sizes, and there are the possibilities of arranging 

 them with granular, open structure or with puddled, compact structure. 

 The finer the soil particles are, the greater possibility there is of changing 

 the nature of the soil by changing the arrangement of the particles. Loam 

 and clay soils are the most difficult to manage because they easily form 

 large, hard lumps or a compact, impervious mass. 



A layer of soil that is particularly dense and difficult to penetrate is 

 termed " hardpan." Many things may give rise to this condition. Streaks 

 of clay sometimes produce such a result. In the hill soils of southern 

 New York, large areas of land have a compact subsoil or hardpan com- 

 posed of shale chips and silt or clay loam closely mixed together. Where 

 the climate is arid, in the western States, the soluble salts sometimes 

 accumulate in layers and cement the soil particles, forming a condition 

 of hardpan. 



In soils of a clayey nature the arrangement that is most desirable is 

 a fine, granular structure. This has been termed crumb structure. (Fig. 

 86.) If the particles are separate and rest together naturally, they form 

 a mass that is too dense for good results. On the other hand, if they are 

 gathered into large chunks, the condition is objectionable because the 

 spaces are too large. Such lumps are called clods. Between these extremes 

 is the happy medium which produces a size and volume of pores that is 

 the most serviceable. It is likely to vary somewhat according to the 

 soil-water conditions, the season, and the kind of crop grown, but these 

 differences can generally be disregarded. 



The object in handling all kinds of heavy soil should be to develop a 

 fine, friable, granular tilth which is easy to manipulate and efficient in ab- 

 sorbing and holding water and in permitting the penetration of roots. 



