AGRICULTURAL CHEMISTRY. 165 



hand, does not change its bulk by wetting or drying, and when present 

 to a considerable extent in the soil, its particles being interposed be- 

 tween 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 especially prone to 

 fall to a fine powder during drying, since the carbonate of lime, which 

 like sand, shrinks very little, is itself in a state of extreme division, 

 and therefore more effectually separates the clayey particles. The 

 unequal shrinking of these two intimately mixed ingredients accom- 

 plishes a perfect pulverization of such soils. Professor Wolff, of the 

 Academy of Agriculture, at Hohenheim, Wirtemberg, 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 larger share of the benefit is to be accounted for by the improve- 

 ment in the texture of those soils which follows liming. The car- 

 bonate of lime is considerably soluble in water charged with carbonic 

 acid, as is the water 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 liming 

 made to approach the condition of a friable marl. 



We may give a moment's notice to the cohesiveness of the soil. — 

 A soil is said to be heavy or light, not as it weighs more or less, but 

 as it is easy or difficult to work. The state of dryness has great influ- 

 ence on this quality. Sand, lime, and humus have very little cohesion 

 when dry, but considerable when wet. Soils in which they pre- 

 dominate are usually easy to work. But clay has entirely different 

 characters, and upon them almost exclusively depends the tenacity 

 of a soil. Dry clay, when powdered, has hardly more consist- 

 ence than sand, but when thoroughly moistened its particles adhere 

 together to a soft and plastic, but tenacious mass; and in drying 

 away, at a certain point it becomes very hard, and requires a good 

 deal of force to penetrate it. In this condition it offers great resist- 

 ance to the instruments used in tillage, and when thrown up by the 

 plough it forms lumps which require repeated harrowings to break 

 them down. Since the cohesiveness of the soil depends so greatly 

 upon the quantity of water contained in it, it follows that thorough 

 draining, combined with deep tillage, whereby sooner or later the 

 stiffest clays become readily permeable to water, must have the best 

 effects in making such soils easy to work. 



The English practice of burning clays speedily accomplishes the 

 same purpose. When clay is burned and then crushed the particles 

 no longer adhere tenaciously together on moistening, and the mass 

 -does not acquire again the unctuous plasticity peculiar to unburned 

 clay. 



Mixing sand with clay, or incorporating vegetable matter with it, 



