PHYSICAL STATE OF SOILS. 131 



superior state for manure ; and this remark applies to all sub- 

 stances of a similar composition, — to the hydraulic limestones, 

 for example. By these the plants are furnished, not only with 

 alkalies, but also with silica, in a state fit for reception. Many 

 of the hydraulic limestones, or the natural cements, as they are 

 called, after being mixed in their burnt state with water, yield to 

 it, in a few hours, so much caustic alkali, that the water may be 

 employed as a weak ley for the purposes of washing. 



The ashes of brown coal and of mineral coal are used in many 

 'listricts as excellent, means of improving certain soils. Those 

 ashes are to be preferred that gelatinize on the addition of an acid, 

 or that become stony and hard after some time, like hydraulic 

 cement, when mixed with lime and water. 



The mechanical operations of the farm, fallow, the applications 

 of lime, and the burning of clay, unite in elucidating the same 

 scientific principle. They are the means of accelerating the 

 disintegration of the alkaline silicates of alumina, and of sup- 

 plying to plants their necessary constituents at the commence- 

 ment of a new vegetation. 



It must be distinctly understood, that the previous remarks 

 apply only to those fields which are in a favorable mechanical 

 state for the development of plants ; for this, in conjunction with 

 the other necessary conditions, has the greatest influence on fer- 

 tility. A stiff, heavy clayey soil offers too much resistance to 

 the spreading out and increase of the roots of a quick-growing 

 summer plant. It is obvious that such a soil will be rendered 

 more accessible to the roots, as well as to air and moisture, by a 

 simple mixture with quarz or with sand, and this may often prove 

 more effectual in improving it than the most diligent ploughing. 

 When we supply to a soil easily penetrable by the roots of plants, 

 as well as by air and moisture, in the form of ashes, the consti- 

 tuents that we re 1 loved in the crops, the soil will retain all its 

 original favorable physical state. In like manner, we can restore 

 the original chemical composition to stiff, heavy clay soils ; 

 but it is better for such soils to restore the necessary ingredients 

 in the form of stable vard manure, than to do so, as in the 

 former case, by means of ashes. By the improvement of the 

 physical condition of the soil, its fertility is increased- In this 

 respect excrements are of very various values, although they may 



