62 



PROXIMATE CONSTITUENTS OF SOILS 



igniting the residue from the hygroscopic moisture 

 to dull red heat, and observing the loss in weight. 



{c) A small quantity of grey, green, or blue clay, 

 from the deep-seated formations, may be observed, 

 on ignition, to turn red. This is caused by the iron, 

 present generally in ferrous silicates like glauconite, 

 becoming oxidised to the ferric condition. 



Chemically speaking, clay is mainly kaolinite, Al2O3.2SiO2.2H2O, 

 but most clays on closer examination will be found to contain 

 very fine grains of quartz, felspar, calcium carbonate, etc. 



Classification of the Soils ^ with Characteristic Composition. 



40. 



Plant Nutrients in the Soil. 



A fertile soil must contain reasonable quantities 

 of nitrogen, phosphoric acid, and potash, besides lime, 

 and small quantities of iron and magnesia. Each may 

 be tested for qualitatively : — 



(a) Phosphates. — A few grams of soil are ignited 

 in a basin or crucible, allowed to cool, and boiled in 

 a small flask with about 10 c.c. of strong nitric acid. 

 After cooling, a little water is added, together with 

 some ammonia, the latter in insufficient quantity to 

 neutralise all the acid. The liquid is then tested for 

 PgOg with ammonium molybdate solution. 



{b) Potash. — About 5 grams of the soil are boiled 

 with 30 C.C. of dilute HCl for five to ten minutes. 



