46 THE MECHANICAL ANAL YSIS OF SOILS [chap. 



has a deflocculating effect. However, as soon as lime 

 is applied to the soil it becomes converted into carbonate, 

 and some of it will be always going into solution as 

 bicarbonate, a salt which possesses great flocculating 

 power. 



In practice, the application of such small quantities 

 of lime as a ton or even half a ton to the acre have the 

 greatest value in ameliorating the working of clay land ; 

 not only does it move more readily and fall more easily 

 into a good tilth, but by becoming coarser grained it 

 allows the rain to percolate more freely and thus dries 

 earlier in the season, so that the limed land can often be 

 worked several days before the unlimed land can be 

 touched. Though the Rothamsted soil is by no means 

 of the heaviest, it is only because of the repeated 

 additions of carbonate of lime in former years that 

 it can be retained under arable cultivation ; portions of 

 the same land without carbonate of lime lie so wet 

 in the spring that they were laid down to grass in 

 consequence of the repeated failures to secure a good 

 seed bed. 



Chalk, or carbonate of lime, is present in all soils, 

 with the exception of a few extremely open sands and 

 peaty soils that are practically of vegetable origin. 

 The proportion varies enormously, according to the 

 origin of the soil ; on some of the thin loams derived 

 directly from the great calcareous formations like the 

 chalk or the oolite, the calcium carbonate in the soil 

 may rise to as high a proportion as 60 per cent., but in 

 the majority of the loams under cultivation the pro- 

 portion is below 1 per cent., and it often is barely 

 detectable in clays and sands. Chalk in the soil is 

 essentially a transitory substance, as it is constantly 

 removed by the action of percolating water charged 

 with carbonic acid, arising from the decay of vegetable 



