STATE AGRICULTURAL SOCIETY. Ill 



the water remaining apparent; but humus retains it invisibly, its 

 action being nearly like that of a sponge. 



One chief cause of the value of a layer of humus on the sur- 

 lace of the soil doubtless consists in this great retaining power for 

 water, and the success that has attended the practice of green 

 manuring as a means of renovating almost worthless shifting 

 sands, is in great degree to be attributed to this cause. The 

 advantages of mulching are explained in the same way. 



Carbonate of magnesia, it is seen, far surpasses every other 

 material used in Schiibler's trials. It retains two and a half 

 times its weight of water, and loses the same very slowly on 

 evaporation. The opinion has been advanced that this excessive 

 attraction for water is one of the causes of the barrenness of cer- 

 tain soils that abound in this ingredient, and may explain why 

 some soils have been permanently injured by heavy applications 

 of a highly magnesian lime. 



This is the proper place to notice : — 



V. The shrinking of soils on drying. — This shrinking is of course 

 offset by an increase of bulk when the soil becomes wet. In 

 variable weather we have therefore constant changes of volume 

 occurring. Soils, rich in humus, experience these changes to the 

 greatest degree. The surface of moors often rise and fall with 

 the wet or dry season, through a space of several inches. In 

 ordinary light soils, containing but little humus, no change of 

 bulk is evident. Otherwise, it is in clay soils that shrinking is 

 most perceptible; since these soils only dry superficially, they do 

 not appear to settle much, but become full of cracks and rifts. 

 Heavy clays may lose one-tenth or more of their volume on drying, 

 and since at the same time they harden about the rootlets wliicli 

 are imbedded in them, it is plain that these indispensable organs 

 of the plant must thereby be ruptured, during the protracted dry 

 weather. Sand, on the other hand, does not change its bulk by 

 wetting or drying, and when present to a considerable extent in 

 the soil its particles being interposed l>etween those of the clay, 

 prevent the adhesion of the latter, so that, although a sandy loam 

 shrinks not inconsiderably on drying, yet the lines of separation 

 are vastly more numerous and less wide than in purer clays. 

 vSuch a soil does not "cake," but remains friable and powdery. 



