LaMotte STH Outfit - Manual (Page 22)

22
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 (5161A). *Potassium Reagent C (5162) is
denatured ethyl alcohol which facilitates the formation of the precipitate
in more or less colloidal form.
Phosphorus Test - Phosphates react with ammonium molybdate,
*Phosphorus Reagent 2 (5156), to produce salts of a complex
ammonium phosphomolybdate, which produce the blue molybdenum
oxide color when reduced. *Phosphorus Test Tablets (5706A),
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 fi ller, which is barium sulfate.
Humus Test - In the humus test the disodium salt of ethylene diamine
tetraacetic 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.
Soil pH Test - Sensitive short range pH indicators are used to measure
the pH of a distilled water soil extract. Barium Sulfate is used to
accelerate settling of the soil water mixture.
Calcium Test - *Calcium Test Solution (5108PS) is sodium oxalate,
which reacts with calcium to form a precipitate of Calcium Oxalate.
Ammonia Nitrogen Test - *Ammonia Nitrogen Test Solution
(5103PS) is Nessler’s Reagent.