THE SOIL STRUCTURE 101 



it may be thousands, of the smaller silt grains; or 

 a regiment of smaller soil grains may themselves 

 be clustered into one large grain by cementing 

 power of the clay. Further, in the presence of lime 

 and similar substances, these complex soil grains are 

 grouped into yet larger and more complex groups. 

 The beneficial effect of lime is usually due to this 

 power of grouping untold numbers of soil particles 

 into larger groups. When by correct soil culture 

 the individual soil grains are thus grouped into large 

 clusters, the soil is said to be in good tilth. Any- 

 thing that tends to destroy these complex soil grains, 

 as, for instance, plowing the soil when it is too wet, 

 weakens the crop-producing power of the soil. This 

 complexity of structure is one of the chief reasons 

 for the difficulty of understanding clearly the physi- 

 cal laws governing soils. 



Pore-space of soils 



It follows from this description of soil structure 

 that the soil grains do not fill the whole of the soil 

 space. The tendency is rather to form clusters of 

 soil grains which, though touching at many points, 

 leave comparatively large empty spaces. This pore- 

 space in soils varies greatly, l)ut with a maximum 

 of about 55 per cent. In soils formed under arid 

 conditions the percentage of pore-space is some- 

 where in the neighborhood of 50 per cent. There 



