28 An Account of the Soil [pt. i 



some of it under a microscope. The chalk has often been 

 distributed to other soils, sometimes by flowing water 

 and sometimes by glaciers as in the chalky boulder clay 

 of the eastern counties. A second mode of origin of 

 calcium carbonate is from the weathering of rocks, and 

 a third from the decomposition of plant and animal re- 

 mains. A good deal of chalk, however, has been added 

 to the soil by farmers in the past : some of the fields in 

 Hertfordshire still contain as much as 1 or 2 per cent, 

 put on as top dressings 50 years or more ago. 



The constituents dealt with in the preceding para- 

 graphs the various sands, silts, clay and the chalk 

 compose almost the whole of the mineral part of the 

 soil. But although the balance is only small in amount 

 it is of great importance to the plant, for it contains an 

 essential article of plant food calcium phosphate. This 

 substance arose in the first instance from the rocks, but 

 often the material in our soils has already done duty in 

 past ages, and has helped to build up the skeleton of 

 some organism, on the death of which it has again re- 

 turned to the soil to do duty once more. It is readily 

 detected by heating 20 grams of soil with concentrated 

 hydrochloric acid on a water-bath for an hour, filtering, 

 and adding to the filtrate a solution of ammonium molyb- 

 date^. A yellow precipitate comes down containing the 

 phosphoric acid extracted by the hydrochloric acid. 



The red or yellow colour of the solution is due in part 

 to the iron present. On neutralising with ammonia a 

 dense red precipitate containing iron and aluminium 

 oxides comes down and can be filtered off : the presence 

 of iron can then be confirmed by the beautiful blue pre- 

 cipitate obtained when the red material is dissolved in 



^ 8ee Appendix, p. 233. 



