Test Methods

The soil sample analysis is performed using industry-accepted analytical methods. The primary mineral extractant used is the Mehlich 3
extractant. In addition, 5% of all samples tested are "check standards", which helps us to verify overall consistent test results.

testing soil samples

Soil Parameter

Test Method

Potassium

Potassium is extracted from soil by combining 1.5g soil with 15ml Mehlich 3 extractant, which is then shaken for 5 minutes. The centrifuged extract is tested using an AA spectrometer at 699.9nm.

Phosphorus

The Bray P1 extractant is used for neutral and acidic soils and Olsen extractant for alkaline soils. One gram of soil is shaken with 10ml of extractant for 5 minutes. The centrifuged extract is tested using a moybdate-sulfuric acid reagent which develops a blue color upon reacting with phosphate. The solution is tested using a spectrophotometer set to 880nm and results are presented in ppm.

Calcium and Magnesium

Same as for potassium, except that absorption is tested at 422.7nm and 285.2nm for calcium and magnesium respectively.

Organic Matter

Organic matter is tested using the Walkley-Black method. A weighed sample of soil is digested with an acidic dichromate solution and the residual dichromate is determined spectrophotometrically. Walkley-Black is considered the gold standard for organic matter testing. "Loss on ignition" testing is available by request - just make a note on the submission form.

Soil pH Test

We test soil pH by combining 12g soil and 18ml water and testing the mixture with a calibrated ph meter.

Lime Requirement

Lime requirements are determined using direct titration with lime. To three 1:1 mixtures of soil and water are added different amounts of lime. The mixtures are shaken for three hours and the final pH of each sample is measured. A curve is generated relating lime added to final pH, and the lime required to achieve a desire pH is derived from the curve.

Soluble Salts

Conductivity is using a 1:1 soil paste with distilled water. Results are presented as mS/cm.

Metallic Micronutrients

The metallic micronutrients are extracted by combining 10g of soil with 20ml of DTPA-AB extractant, and then shaken for 2 hours. The metal concentrations are tested using an AA spectrometer which is set for the wavelength required for each metal.

Sulfur

A 1:2.5 mixture of soil and a solution of calcium phosphate monobasic are shaken for 30 minutes. The filtered solution is treated with BaCl2 to precipitate the sulfate sulfur. The precipitated barium sulfate is separated and tested using AAS to determine barium concentration, which is then used to calculate the sulfur concentration in the soil.

Boron

Boron is tested using the hot water extraction method. Ten grams soil is mixed with 20 ml of a 0.1% solution of CaCl2 and boiled gently for 5 minutes. The supernatant is treated with a curcumin reagent. The solution is dried, and the residue is redissolved in ethanol. The solution is tested spectrophotometrically at 540nm together with standards.

Texture

Texture is measured using the hydrometer method. Briefly, 50 grams of soil is dried, ground and sifted through a 2 mm sieve to separate the gravel fraction. Then the finer fraction is thoroughly mixed with 1000 ml of a dilute sodium hexametaphosphate solution in a sedimentation cylinder. The soil particles gradually settle to the bottom of the cylinder. The density of the solution is measured using a Bouyoucos hydrometer at 40 seconds and 2 hours to determine the silt and clay content of the soil. The sand content is determined by subtraction.

Heavy Metal Screening

The metals are extracted from the soil using a mixture of dilute mineral acids heated to 80'C for three hours. The extract is centrifuged and tested using an AA spectrometer at to determine lead and cadmium concentrations. The arsenic concentration is determined using the Hach Company colorimetric arsenic test.

Please note that we do not use the official EPA methods to test for heavy metals. The EPA methods are laborious and expensive to perform and impractical for ordinary screening purposes. We believe that our methods give results that are sufficiently accurate to provide a good indication of whether heavy metals are present in the soil at above normal levels.