PRACTICE XVII 



DETERMINATION OF HUMUS IN SOILS 



Weigh out 5-g. or 10-g. samples of the air-dry soils used in 

 Practices XIII and XYI. Place on a filter in. a funnel and 

 leach out the lime and magnesia with dilute hydrochloric acid.^ 

 When the lime and magnesia are all leached out as shown by 

 testing the filtrate by collecting a few cubic centimeters of the 

 acid filtrate in a test tube direct from the funnel and adding dilute 

 ammonia drop by drop until the solution is slightly alkalme, the 

 addition of a few drops of ammonium oxalate will produce no pre- 

 cipitate. If lime is present, a white opaque crystalline precipitate 

 is formed. Wash out the hydrochloric acid with distilled water. 



Dry the soil and filter at room temperature for a day, then in 

 the oven at 100° C. for 8 hours. 



Place soil and filter in a shaker bottle. Add 150 cc. of dilute 

 ammonia^ for light soils and 250 cc. for dark soils, the amount 

 depending on the quantity of humus in the soil. Shake for 3 

 hours. Filter. 



If evaporation, which would concentrate the filtrate, can be 

 prevented, evaporate 100 cc. of the humus filtrate to dryness 

 and calculate total humus ; otherwise evaporate all of the fil- 

 trate. In this case care should be taken to wash out with am- 

 monia any humus that may be held mechanically in the soil or 

 filter paper. 



Dry the evaporated humus at 100° C. for 5 hours. Weigh, 

 ignite, and weigh again. The loss in weight is the humus. Cal- 

 culate from the weight of air-dry soil the weight of water-free 

 soil used, and express the humus in per cent of water-free soil. 



Of what benefit is a large amount of humus in a soil ? 



If an acre of soil, 6| inches deep, weighs 2,000,000 pounds, 

 how many tons of humus in the surface of the above soils ? 



References. 



" Soils," Lyon and Fippin, p. 127. 



"The Soil," Hall, pp. 42-47. 



" The Soil," King, pp. 94-96. 



" Soils," Hilgard, pp. 125-140, especially p. 132. 



Circular No. 82, Illinois Agricultural Experiment Station. 



^ For dilute hydrochloric acid use 25 cc. hydrochloric acid, sp. gr. 1.19, with 808 cc. 

 of distilled water. 



2 For the ammonia 178 cc. saturated ammonia with 422 cc. of distilled water. 

 These solutions will be prepared by the instructor. 



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