452 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



0.002 mm. diameter, the silt, are mainly inert and somewhat indeter- 

 minate m character. The particles below 0.002 mm. diameter are, 

 however, in the main strikingly different from the larger ones. Their 

 parent material was decomposed by weathering agencies and the 

 resulting products, collectively called clay, are chemically reactive, 

 and their small size gives them a very large surface area/weight 

 ratio compared with larger particles. The clay is colloidal and is a 

 powerful absorber of water. A light sandy soil may contam only 

 about 3 to 5 percent and a good, easily workable loam about 7 to 15 

 percent, but a heavy clay may contain 30 to 60 percent, even occa- 

 sionally 70 percent. At these high values the soil becomes almost 

 unworkable. Calcium carbonate varies more in amomit than any other 

 soil constituent and is especially important, as it is slowly leached 

 out till finally the soil becomes acid and its character as a habitat 

 changes completely. The population alters and becomes less 

 diversified. 



The mineral and organic components of the soil become thoroughly 

 intermingled through the activities of the larger soil animals, par- 

 ticularly the various kinds of worms which eat their way through the 

 soil: in particular the clay and organic matter becomes so closely 

 united that no mechanical separation is possible. The particles do not 

 normally remain separate but are built up into fairly stable crumbs 

 which may be 2 or 3 mm. in diameter, honeycombed with cells and pas- 

 sages. It is not known how this is achieved — clay, organic matter, 

 plant roots, earthworms, and other soil organisms are all involved; 

 but it is one of nature's most vitally important processes, for without 

 crmnb formation little plant growth would be possible and indeed 

 most soils would long ago have been blown or washed away. Farm- 

 ers and gardeners have learned empirically how to assist crmnb for- 

 mation, but they have not discovered how it comes about. More 

 investigation is greatly needed : soil erosion is one of the world's most 

 serious problems. 



The soil particles are irregular in shape and do not fit compactly : 

 empty passages and pores remain as they do between soil crumbs. In 

 addition passages are left as roots decay or as soil animals force their 

 way through the soil, and cracks form as the clay shrinks. In an 

 ordinary farm or garden soil the empty spaces are so numerous that 

 in spite of their small size they add up to an almost unbelievably large 

 volume ; at Rothamsted about half of what looks like a solid lump of 

 Eothamsted's clay soil is really empty space. R. J. B. "Williams and 

 G. W. Cooke fomid that many of the pores range in size from about 

 200 to 400 microns, but m the Woburn sandy soil, much poorer in clay 

 and organic matter, they are mostly below 200 microns in equivalent 

 diameter, and the total pore space is only about a third of the volume 

 of the soil. 



