THE SOIL 331 



Peat, like clay, holds much water ; it is therefore badly 

 aerated, and root-respiration is difficult. Further, it con- 

 tains little mineral food and a great excess of organic 

 matter ; and in addition it is acid or sour. No wonder, 

 then, that it is a poor soil for plants, or that so few species 

 grow on it. 



Calcareous soils. Chalk and Limestone soils. In a 

 soil derived from chalk or limestone, carbonate of lime is 

 always present. These soils support a vegetation differing 

 in many respects from that growing on soils deficient in 

 lime. The presence of lime in soils may be determined as 

 follows : 



Take 5 grammes of chalk soil, boil it in water, and pour 

 oh the liquid. Repeat this two or three times and so 

 remove the soluble calcium salts (sulphate, chloride, and 

 nitrate). Drain off the water or press between filter paper, 

 and to the residue add dilute hydrochloric acid (1 in 10 

 parts of water). Note if there is any effervescence ; if so, 

 it denotes the presence of a carbonate, e. g. of magnesium, 

 lime, or iron. If the gas is copious it may be led into lime- 

 water and tested. What gas will it be ? 



Filter off the insoluble matter and add ammonium 

 hydrate in slight excess to the filtrate. Filter off any pre- 

 cipitate which may result (e. g. iron or alumina) and add 

 ammonium oxalate. Is a white precipitate formed ? 

 If so, it is calcium oxalate. Thus it is shown, not only that 

 the soil contains a carbonate, but which specific one it is, 

 namely, calcium carbonate. 



Rain-water carries with it into the soil carbon dioxide, 

 which has acid properties, and which, acting on the calcium 

 carbonate, dissolves it, so that a soil over limestone may 

 become relatively poor in lime, except for fragments of lime- 

 stone rock in the soil. This denudation of mineral salts 

 from a soil, and its consequent impoverishment, is called 

 ' leaching '. 



