7^8 expeki:ment station hhcord. 



tlio ronniition of aj^yregates. Tliis is espi'cially true of tlic ordinary 

 eartlnvorni, wlio>;(' digging-, burrowing, and excretions are important 

 agencies in the formation of vegetable mold.' 



ClIAXGES IN VOLUME OF TllK SOIL. 



Variations in tlic volume of soils are shown by changes in the space 

 occupied wlicn different quantities of moisture are present. These 

 changes are most mniked in soils whi(;h contain much (-oUoid mate- 

 rial (clay, and espiM-ially hunuis matter), while in sands they are not 

 observed, or only to a slight extent in very tine sands. In the latter 

 case the expansion is due solely to the surroundiirg of particles by 

 water when the soil is very wet.^ 



In clays and clayey soils the changes in volume conseipient on drying 

 cause a vertical and especially a horizontal tension in the mass. Loss 

 of water at the surface of the soil causes a contraction of the upj)er- 

 most layer and a consequent strain from below upward, reaching to a 

 depth dependent uixni the depth of the surfjice drying. Under such 

 circumstances the soil must become more closely compacted and the 

 surface sink to a lower level. These changes in volume often cause 

 cracking of the soil, chietiy due to the horizontal tension already men- 

 tioned. Tliis separation of the soil particles will decrease as the force 

 increases which holds the particles together, that is, as the cohesion 

 of the soil increases (see below). Experience teaidies, however, that 

 even the most tenacious soils have not sufticient resistance to prevent 

 the formation of cracks by latei'al tension when drying proceeds to a 

 considerable extent. It is plain that this tension is not uniform either 

 at the surface or below because the drying and the consequent contrac- 

 tion goes on more quickly in some parts of the soil than in others. The 

 tophiyers dry soonest and most completely and therefore contract more 

 strongly than the lower and moister strata. The cracks formed, then, 

 will be widest at the top and will become narrower on descending.^ 



The cracks formed in the same soil by drying are deeper, narrower, 

 and less numerous the more compactly the particles are arranged, and 

 vice versa .'^ 



Expansion and contraction of the soil are also observed in freezing 

 and thawing, and the greater the quantity of water in tlie soil the more 

 marked are the changes in volume due to this cause, being especially 

 marked in soils rich in humus and clay. 



' C. Darwin, The Formatiou of Vegetable Mold Througli the Action of Earthworms, 

 1882. V. Henseu, Landw. .Jahrb., 1882, p. 661. P. A. Muller, Die uatiirlichea Hunius- 

 forineu, 1887. E. Raraaun, Forsch. Geb. agr. Phj\s., 11, p. 299. E. Wollny, Ibid., 13, 

 p. 382. E. Ebeimayer, Allgeiueiue Forst- and .Tagdztg , 1891, p. 71. 



2 G. Schiibler, Grnndsiitzc der Agrieulturchemie, 1838, pt. II, p. X2. E. Wolff, 1. e., 

 ]). 71. F. Ilabeiiandt, Fiihliug's landw. /tg., 1877, p. 481. ' A. R. von S( Invaiz, Forsch. 

 Geb. agr. Phys., 2, p. 166. 



■'F. Haberlaudt, Wisscnschal'tlich-practische rnter.sueluiugcu aiifdeni Geliiete dcs 

 PHauzenbanes, 187"), vol. 1, p. 22; Foivsch. Geb. agr. Phys., 1, p. 148. 



^E. Wollny, Forscdi. (Jcb. agr. Phys., 5, p. 43. 



