92 THE SOIL. 



require a much larger supply of lime than others, to 

 effect their restoration to a healthy state. 



It is certain, that in all such cases, the beneficial 

 action of the lime is not attributable to an original 

 deficiency of that body in the soil for plants growing on 

 it ; for in that case, considering the rapidity with which 

 ft is diffused through the soil, the effect would manifest 

 itself very soon, and even in the course of the first year. 

 But it takes several years before the favourable change 

 in the condition of the soil is effected ; proving that the 

 lime operates, not simply as food, but by producing an 

 alteration in the soil, which requires time, that is, a 

 succession of operations. 



Qn a drained marshy soil, in which lime has dimin- 

 ished the excess of hydrated silicic acid, a second appli- 

 cation will not produce the same result, because the 

 offensive substances, once removed, -will not return; 

 while on a heavy, stiff clay or .loam, the application 

 may be repeatedly successful. These kinds of soil are 

 thereby made more friable and richer in available 

 potash. The nature of the change produced is most 

 clearly shown in the hydraulic lime obtained by cal- 

 cining native cement stones (a hard marl). These 

 cement-stones consist of a mixture of lime and clay, the 

 former being in larger proportion than in calcareous 

 clay soil. After burning, if it is stirred up with a large 

 quantity of water, the separated potash imparts to the 

 fluid all the properties of a weak lye. Clay which 

 before calcination with lime refused to dissolve in 

 acids, is, after calcination, soluble in acids to the whole 

 extent of the silicic acid present. 



A calcareous clay soil withdraws from a solution of 

 silicate of potash much less potash after calcination than 

 before, but a much larger quantity of silicic acid.* 



Besides the chemical agents mentioned here, which 



* At Bogenhausen, near Munich, loam was calcined in the air, and 

 brought into contact with a solution of silicate of potash ; before calcina- 

 tion, a litre of this earth took up 1148 milligrammes of potash, and 2007 

 milligrammes of silicic acid; after calcination, no potash, and 3230 milli- 

 grammes of silicic acid. 



