Technology & Analysis
The following methods have been selected by our soil experts due to their wide scale use and popularity in the soil testing industry. This allows farmers and ranchers the ability to confidently compare soil test results provided by SPI with those done by a previously used soil lab. The selected methods listed below will be rigorously employed by our laboratory technicians to ensure a high quality analysis of your soil that can be used for years to come to accurately monitor your soil health journey!
pH
We determine pH by using a 1:2 soil to water slurry that is mixed for 10 minutes before settling for 30 minutes. Once settled, we take a measurement of pH using an Accumet AB150 pH probe. The probe is calibrated before analysis using pH 4 and 7 standard solutions.
Following the determination of soil pH, 10 mL of Sikora-2 buffer is added to the soil slurry and shaken for an additional time on the orbital shaker for 10 minutes. The solution will sit for 30 minutes before measuring pH again using an Accumet AB150 pH probe. The probe is calibrated before analysis using pH 4, 7, and 10 standard solutions.
Buffer pH
Electrical conductivity is determined by measuring 10 grams of soil and 10 ml of DI water in a 1:1 soil to water slurry. The mixture is shaken for 10 minutes on an orbital shaker before resting for 30 minutes. Measurements are made using an Accumet AB150 conductance probe.
Electrical Conductivity (EC) AKA Soluble Salts
Nitrogen
Inorganic Nitrogen: Nitrate (NO3) bound Nitrogen
We determine NO3-N using colorimetric chemistry on a SEAL AA-500 segmented flow autoanalyzer (spectrophotometer) that reduces NO3-N to NO2-N by use of a Cadmium reduction coil measured at 540 nm wavelength. Depending on the goals of your soil testing, we can extract NO3-N using 2M KCl (the traditionally used extract) or using H3A Haney soil extract.
Inorganic Nitrogen: Ammonium (NH4+) bound Nitrogen
We determine NH4+-N using colorimetric chemistry on a SEAL AA-500 segmented flow autoanalyzer (spectrophotometer) that reacts extracted ammonia from your soil with an acid reagent (dichloroisocyanuric acid) that combines to produce a vibrant color change. The intensity of the resulting color change is directly proportional to the concentration of ammonia in the soil and we measure this at the 660 nm wavelength.
We determine organic nitrogen by first determining total nitrogen (TNb) on a Vario TOC Select total organic carbon and total nitrogen analyzer by Elementar Americas, Inc. This machine combusts soil samples extracted with either water or H3A at 850°C to determine the total nitrogen present in the soil sample. With this number determined, we then subtract the inorganic nitrogen (NO3-N+NH4+-N) from total nitrogen to arrive at total organic nitrogen.
Organic Nitrogen (Included with the Haney Test)
Organic Matter
Organic matter is determined by loss on ignition (%LOI) which uses a two stage process of removing moisture content from a given mass of soil before exposing the soil sample to extremely high temperatures which burns off organic matter. The difference between the moisture free measurement and the combusted measurement is considered to be the amount of organic matter that was in the soil expressed as a percentage.
Phosphorus
Inorganic forms of phosphorus found in soil are called orthophosphates (H2PO4- and HPO4-2) and we determine the concentration of these forms of phosphorus by using colorimetric chemistry on a SEAL AA500 segmented flow autoanalyzer measuring at 660 nm. We use either Olsen's bicarbonate or H3A soil extract solutions for this procedure.
Inorganic Phosphorous
We determine total phosphorus in the soil using water or H3A extractants. These extracts are ran through an Agilent 5800 ICP-OES.
Total Phosphorous
Texture
We determine soil texture by using the Bouyoucos hydrometer method. This method involves mixing 50g of dried and < 2mm sieved soil into a beaker with 100 ml of dispersant which sits overnight for 8 hours. After sitting overnight, the mixture is added to ~900 ml of water in a graduated cylinder, shaken again, and allowed to settle for 40 seconds before taking our first hydrometer and temperature measurements. Measurements are repeated again after 2 hours of sitting and final calculations are done to arrive at the percent sand, silt, and clay in your soil.