WHY LIMING MUST BE REPEATED. 399 



There can be no question, therefore, that the lime gradually disappears 



or is removed from the soil. 



^ The agencies by which this removal is effected are of several kinds. 



1°. In some cases it sinks, as we have already seen, and escapes into 

 the subsoil beyond the reach of the plough or of the roots of our culti- 

 vated crops. 



2°. A considerable quantity of lime is annually removed from the 

 soil by the crops which are reaped fr#m it. We have already seen 

 (Lee. X., § 4,) that in a four years' rotation of alternate green and corn 

 crops the quantity of lime contained in the average produce of good 

 land amounts to 248 lbs. This is equal to 60 lbs. of quick-lime or 

 107 lbs. of carbonate of lime every year. The whole of this, however, 

 is not usually lost to the land. Part at least is restored to it in the ma- 

 nure into which a large proportion of the produce is usually converted. 

 Yet a considerable quantity is always lost — escaping chiefly in the 

 liquid manure and in the drainings of the dung-heaps— and this loss 

 must be repaired by the renewed addition of lime to the land. 



3°. Bat the rains'^ and natural springs of water percolating through 

 the soil remove, in general, a still greater proportion. While in the 

 quick or caustic state, lime is soluble in pure water. Seven hundred 

 and fifty pounds of water will dissolve about one pound of lime. The 

 rains that fall, therefore, cannot fail, as they sink through the soil, to 

 dissolve and carry away a portion of the hme so long as it remains in 

 the caustic state. 



Again, quick-lime, when mixed with the soil, speedily attracts car- 

 bonic acid, and becomes, after a time, converted into carbonate, which 

 is nearly insoluble in pure water. But this carbonate, as we have 

 already seen (Lee. III., § 1), is soluble in water impregnated with car- 

 bonic acid — and as the drops of rain in falling absorb this acid from the 

 air, they become capable, when they reach the soil, of dissolving an 

 appreciable quantity of the finely divided carbonate which they meet 

 with upon our cultivated lands. Hence the water that flows from 

 the drains upon such lands is always impregnated with lime, and 

 sometimes to so great a degree as to form calcareous deposits in the in- 

 terior of the drains themselves, where the fall is so gentle as to allow the 

 water to linger a sufficient length of time in the soil. 



It is impossible to estimate the quantity of lime which this dissolving 

 action of the rains must gradually remove. It will vary with the 

 amount of rain which falls in each locality, and with the slope or inchna- 

 tion of the land ; but the cause is at once universal and constantly oper- 

 ating, and would alone, therefore, render necessary, after the lapse of 

 years, the application of new doses of lime both to our pastures and to 

 our n.rable fields. 



4°. During the decay of vegetable matter, and the decomposition of 

 mineral compounds, which take place in the soil where lime is present, 

 new combinations are formed in variable quantities which are more so- 

 luble than the carbonate, and which therefore hasten and facilitate this 

 washing out of the lime by the action of the rains. Thus chloride of 

 calcium, nitrate of lime, and gypsum, are all produced— of which the 

 two former are eminently soluble in water — while organic acids also re- 

 sult from the decay of the organic matter, with some of which the lime 

 tbrms readily soluble compounds (salts) easily removed by water. 



