I906.J How LONG DOES LlME LAST IN THE SOIL ? 323 



excess of acid, which demands more lime from the soil to 

 neutralize it. Quick lime and slaked lime when applied to the 

 soil quickly go back to the state of carbonate of lime or chalk in 

 which they existed before they were " burnt " in the kiln, hence 

 it is really this substance, carbonate of lime, that we denote 

 when we speak of " lime " in the soil. The superiority of burnt 

 lime over chalk or limestone for application to the soil lies 

 simply in the fact that it falls naturally into a fine state of 

 division, some of it also passing into solution, so that it is more 

 easily disseminated throughout the soil and acts with greater 

 rapidity and in smaller quantities. But to return to the point 

 at issue, only freshly burnt (quick) or slaked lime, chalk, 

 limestone, and marl contain " lime " in the farmer's sense, 

 a base capable of neutralizing acids ; in bones, in super- 

 phosphate, and in gypsum the lime is combined with acids,, 

 and is no longer capable of acting as a base.* 



Owing to the former prevalence of the practice of liming,, 

 many of our soils, originally deficient in this important con- 

 stituent, have been supplied with it in quantities sufficient for 

 the health of the crops they carry ; it is well known, however, 

 that there are various natural agencies at work removing lime 

 from the soil, and the question naturally arises how long a 

 given dressing of lime or chalk may be expected to remain in 

 the soil. The Rothamsted soils afford exceptional opportu- 

 nities for determining this point, since all the carbonate of 

 lime they contain has been applied artificially by the pro- 

 cesses above described and remains in the top nine inches of 

 soil, 7'.^., in the layer which alone gets moved during the ordi- 



* The compounds of lime of any agricultural importance may be tabulated as 

 follows : — 



1. Bases, capable of neutralizing acids 



Quick lime, burnt lime, stone lime = lime. 

 Slaked lime = lime and water. 



Chalk, limestone, marl, old mortar, &c. = lime and carbonic acid. 

 Basic slag = lime and phosphoric acid (lime in excess). 



2. Neutral salts, in which the lime is already neutralized by a strong acid. 



(jypsum = lime and sulphuric acid. 



Bones and mineral phosphates = lime and phosphoric acid. 



3. Acid salts which contain more acid than the lime can neutralize. 



Superphosphate V^j^^g phosphoric acid in excess. 



Dissolved bones J r r 



"Gas lime" consists of slaked lime more or less saturated with compounds of 

 sulphur ; it has but little basic property left in it and so cannot take the place of lime 

 or chalk. 



D D 2 



