LaMotte STH Series - Manual (Page 27)

Soil Test Reagents
This information is provided to enable the soil analyst to correlate
LaMotte soil test results with results obtained using other soil test
procedures.
Soil Extraction Procedure - The *Universal Extracting
Solution (5173) is composed of sodium acetate which has been
adjusted to a pH of 4.8 with acetic acid. This solution extracts the
soluble nutrients, such as phosphorus and nitrogen, as well as
soluble and exchangeable potassium and other cations, such as
calcium and magnesium.
Potassium Test - The Potassium test is based on the fact that
potassium salts give a yellow crystalline precipitate with sodium
cobaltinitrite, *Potassium Reagent B Tablets (5161). *Potassium
Reagent C (5162) is denatured ethyl alcohol which facilitates the
formation of the precipitate in more or less colloidal form.
Phosphorous Test - Phosphates react with ammonium
molybdate, *Phosphorous Reagent 2 (5156), to produce salts of a
complex ammonium phosphomolybdate, which produce the blue
molybdenum oxide color when reduced. *Phosphorous Reagent 3
Tablets (5157), the reducing agent, are stannous chloride.
Nitrate Test - The Nitrate test is based on the so-called
Denige’s test in which two dye intermediates are coupled in the
di-azo reaction to form a color dyestuff. The nitrite required in
this reaction is supplied by reduction of the nitrate in the soil
extract. *Nitrate Reagent #1 (5146) is a solution of potassium
acid sulfate. *Nitrate Reagent #2 Powder (5147) is a mixture of
the two intermediates, N(1-naphthyl)-ethylene diamine
dihydrochloride and sulfanilamide; the reducing agent, which is
zinc dust; and the filler, which is barium sulfate.
Humus Test - In the humus test the disodium salt of
ethylene-diamine-tetra acetic acid, *Humus Screening Reagent
Powder (5119), extracts the dark colored humus fraction from the
soil. This is measured against a special color comparator which has
been calibrated to give a general relationship between this color
and the content of organic matter.
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