THE SOUTHERN PLANTER. 



G79 



best united the conditions for the regular 

 distribution of the soil-water, urider all 

 circumstances. In them this process is 

 not hindered too much either by wet or 

 dry weather. The retaining power of hu- 

 mus is seen to be more than double that 

 of clay. This result might appear at first 

 sight to be in contradiction to ordinary ob- 

 servations ; for we are accustomed to see 

 water standing on the surface of clay, but 



borne in mind 



It must be 



its imperviousness, holdi 



not on humus 

 that clay, from 



water like a vessel, the water 

 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 surface of the soil doubtless 

 consists in this great retainingpower for wa- 

 ter, 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 mulchino; 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 certain 

 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 in- 

 crease of bulk when the soil becomes wet. 

 In variable weather w^e have therefore con- 

 stant 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, througli a space of several 

 inches. In ordinary light soils, containing 

 but little humus, no change of bulk is evi- 

 dent. 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 dry- 

 ing, and since at the same time they har- 

 den about-thc rootlets which are imbeded 

 in them, it is plain that these indispensable 



organs of the plant must thereby be rup- 

 tured, 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 between 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. Such a soil does not "cake," but 

 remains friable and powdery. 



Marly soils (containing carbonate of lime) 

 are especialy prone to fall to a fine powder 

 j during drying, since the carbonate of lime, 

 ; which like sand, shrinks very little, is 

 I itself in a state of extreme division, and 

 therefore more effectually separates the 

 clayey [)articles. The unequal shrinking 

 of these two intimately mixed ingredients, 

 accomplishes a perfect pulverization of 

 such soils. Prof. Wolff, of the Academy 

 of Agi'iculture, at Hohenheim, Wirtem- 

 berg, states that on the cold heavy soils of 

 Upper Lusatia, in Germany, the application 

 of lime has been attended with excellent 

 results, and he thinks that the lai'ger share 

 of the benefit is to be accounted for, by 

 the improvement in the texture of those 

 soils wiiich follows liming. The carbonate 

 of lime is considerably soluble in water 

 charged with carbonic acid, as is the w^ater 

 of a soil containing vegetable matter, and 

 this agency of distribution in connection 

 with the mechanical operations of tillage, 

 must in a short time effect an intimate 

 mixture of the lime with the whole soil. 

 A tenacious clay is thus by a heavy lim- 

 ing, made to approach the condition of a 

 friable marl. 



VI. The relation of the soil to heat are 

 of the utmost importance in affecting its 

 fertility. The distribution of plants in 

 genei'al is determined by differences of 

 mean temperature. In the same climate 

 and locality, however, we find the farmer 

 distinguishing between cold and warm 

 soils. 



The temperature of the soil varies to a 

 certain depth with that of the air ; yet its 

 changes occur more slowly, are confined 

 to a narrower range of temperature, and 

 diminish downward in rapidity and amount, 

 until at a certain depth a point is reached 

 where the temperature is invariable. 



In summer the temperature of the soil 



