64 PROXIMATE CONSTITUENTS OF SOILS 



id) Iron. — A few grams of the soil should be ex- 

 tracted by boiling for ten minutes with strong hydro- 

 chloric acid. An equal bulk of distilled water is added, 

 the insoluble material filtered off, and a portion of the 

 filtrate tested direct for iron by the addition of potassium 

 ferrocyanide solution. 



Most soils contain quite sufficient iron for the 

 requirements of the crops grown on them. 



ie) Lime and Magnesia. — The quantity and value 

 of lime in a soil is usually estimated by the amount 

 of carbonate present, and this may be approximately 

 estimated by the method described above (36 b, p. 57). ' 



The magnesia may be detected by taking a portion of 

 the hydrochloric acid extract from {d)^ adding ammonia 

 and ammonium chloride to remove iron, alumina, etc., 

 and then ammonium oxalate to precipitate the calcium. 

 The filtrate from these operations is evaporated to 

 small bulk, mixed with strong ammonia and sodium- 

 phosphate solution and allowed to stand. Magnesia is 

 thus precipitated as magnesium ammonium phosphate, 

 MgNH.PO,. 



CHAPTER VI 

 Chemical Properties of Soil 



Nitrification. 



Through the agency of bacteria, nitrogenous compounds in the 

 soil are eventually oxidised to nitrates, in which form they are 

 capable of being assimilated by growing plants. The process of 

 nitrification goes on in two stages : one set of bacteria produce 

 nitrites^ which are then oxidised further to nitrates by micro- 

 organisms of the type of nitrobacter. A base, such as calcium or 

 magnesium carbonate, must be present in the soil, because 

 nitrification ceases as soon as the medium becomes at all acid. 



