Soil Tests

Several methods have been developed to extract the exchangeable magnesium fraction from soils. When preparing soils for extractions, the drying temperatures of 40 to 105°C do not affect the extractability of magnesium (210). In most soils, magnesium can be extracted with a solution containing ammonium acetate (211-213), CaCl₂ (210) or with water (214). However, for soils with a low cation-exchange capacity, acidic extractions are recommended (215). For alkaline soils, a water extraction is utilized (214). Another extraction method (AB-DTPA, ammonium bicarbonate-diethyleneaminepentaacetate) is utilized for alkaline soils; however, this method is suitable only for the extraction of sodium and potassium, since magnesium as well as calcium will react and precipitate with the bicarbonate in the extraction reagent (216). In Sweden, soils are extracted with ammonium lactate at pH 3.75 (10), and in Turkey, chemical extractions methods include various concentration of hydrochloric acid in addition to the ammonium acetate procedure (212).

After proper extractions are performed, the magnesium concentration of solutions can be quantified by ion-selective electrodes, flame-plasma emission spectroscopy, or atomic absorption spectroscopy (217). The wavelength used in atomic absorption is 285.2 nm. In the United States, the Environmental Protection Agency (EPA) (218,219) guidelines indicate that magnesium concentrations of samples have to be determined by inductively coupled plasma (ICP) spectrophotometry according to methods described in EPA Method 200.7, by ICP-mass spectrometry in EPA Method 200.8 (218), or by atomic absorption method 7450 EPA 7-series (219).