84 THE SOIL. 



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. 



On a drained marshy soil, in which lime has diminished 

 the excess of hydrated sihcic acid, a second apphcation 

 will not produce the same result, because the offensive 

 substances, once removed, will not return ; while on a 

 lieavy, stiff clay or loam, the apphcation may be repeatedly 

 successful. These kmds of soil are thereby made more 

 friable and richer in available potash. The nature of the 

 change produced is most clearly shown in the hydrauhc 

 lime obtamed by calcining native cement-stones (a hard 

 marl). These cement-stones consist of a mixture of hme 

 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 im- 

 parts to the fluid all the properties of a weak lye. Clay 

 which before calcination wdth 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 sihcic acid.* 



Besides the chemical agents mentioned here, which the 

 farmer may employ to effect the proper distribution of the 

 nutritive substances stored up in his field, and to make 



* At Bogenliaiisen, near Munich, loam was calcined in tlie air, and 

 brought into contact Avith a solution of silicate of potash : before 

 calcination, a litre of this earth took up 1148 milligi-ammes of potash, 

 and 2007 milligrammes of silicic acid ; after calcination, no potash, 

 and 3230 milligrammes of silicic acid. 



