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Soil organic matter is typically determined using a dry combustion method or loss on ignition. This simply means the sample is exposed to a very high temperature to oxidize the carbon, which is lost as a gas. If dry combustion is the method being used, the carbon produced is measured with a detector to determine the content. If loss on ignition is used, the difference in weight before and after exposing the soil to high heat is used to determine the organic matter content.
Organic matter is a contributor to soil CEC, meaning it influences the ability of a soil to hold nutrients and water. It is usually reported on a percentage basis. Occasionally, organic matter will be reported on grams per kilogram (g/kg) basis. To convert from percent basis to g/kg, simply multiply the percent value by 10 (2.3% organic matter equals 23g/kg).
The majority of phosphorus held in soil is precipitated in compounds of various solubilities or sorbed to clay surfaces or minerals. In acid soils, acid extractants are used to release some of the labile (potentially available – soluble and desorbable) phosphorus which provides an estimate of phosphorus availability. In alkaline soils, neutral or alkaline extractants are used to a similar effect.
While available phosphorus is usually labeled as “available” and reported in parts per million units, it is an indexed value. The parts per million unit is based upon the amount of released phosphorus into solution from the extractant used. The higher the amount of phosphorus released, the higher the predicted phosphorus availability to a crop grown in that soil. Soils with high extracted/available phosphorus levels have lower nutrient recommendations (if nutrient is recommended at all).
Potassium (at least that which is assumed to be plant available) resides primarily on the soil cation exchange complex (clay surfaces and organic matter). To liberate potassium from the cation exchange complex, most extracting solutions use ammonium (NH4+) as a cation to displace potassium releasing it (and other cations) into solution. The concentration of potassium in solution is measured, and it is typically reported as exchangeable.
The higher the amount of exchangeable potassium determined for a soil, the lower the need for nutrient supplementation. Do realize that since the extraction method primarily focuses on the potassium residing on the CEC, soils with higher CEC (more clay/organic matter) have a greater capacity to hold more potassium than lower CEC soils (more sand/less organic matter).
Soil pH is typically measured using a 1:1 soil:water method. Some labs may use weak salt solutions (typically calcium chloride) as well, but the differences between these two approaches are minor. On the soil test report, it is typically reported as soil pH, but it may be reported as active acidity. Soil pH/active acidity provides a measurement of hydrogen ions in solution.
As important as soil pH is, it only provides you one piece of information – is the pH appropriate for the crop you are trying to grow? For most agronomic crops, our greatest concern is – is pH high enough? Neutral (7) or slightly acidic soil conditions are preferred by most crops. Anything below seven is considered acid, and anything above seven is considered alkaline. If pH is to acidic, a lime rate will need to be determined to raise pH to a desired level. If pH is too alkaline (more of an issue for specialty/hort crops), then elemental sulfur rate will need to be determined to lower pH.
Soil pH is a measure of active acidity, and buffer pH is a measure of potential acidity. The actual relationship between hydrogens in solution actively contributing to solution pH is quite low compared to the hydrogens residing on clay surfaces or held by organic matter (together these two constitute a soil’s CEC). We must account for hydrogens on the cation exchange complex and their ability to “replace” neutralized active acidity. If we did not account for potential acidity (especially for finer-textured soils), we would dramatically underapply lime and not affect the desired change in soil pH.
Soil pH indicates if lime is required, and buffer pH determines how much lime is required (based upon a desired pH level). Finer-textured soils (higher CEC) have higher lime requirement than coarse-textured soils (lower CEC). Conversely, coarse-textured soils will require more frequent application of lime than fine-textured soils.
A soil’s cation exchange complex (CEC) is a measure of the ability of the soil to hold positively charged ions (cations). Remember, most soils in North America have a net negative charge, meaning they have the capacity to hold cations (calcium (Ca2+), magnesium (Mg2+), potassium (K+), aluminum (Al3+), hydrogen (H+)). The CEC of a particular soil is determined by how much clay a soil has (as well as the nature of that clay) and the amount of soil organic matter. Soil CEC is typically measured by displacing all the cations on the CEC with a molecule like ammonium (NH4+) by mass action. The soil is essentially flooded by ammonium which knocks the other cations off the CEC. Soil CEC is then determined by measuring the concentration of calcium, magnesium, potassium, and sodium as well as considering potential acidity (see Buffer pH section).
A soil’s CEC provides a lot of information about that soil. High CEC mineral soils will have higher clay content meaning more nutrient holding capacity, greater potential to store water (sometimes to a detrimental level because of poor drainage), and lower likelihood of nutrient leaching. Low CEC mineral soils will have lower clay content meaning less nutrient holding capacity and lower water holding potential/better drainage, and greater likelihood of nutrient leaching. Extremely high CEC soils (>50 meq/100g) are likely to be classified as organic soils (meaning >20% of the soil is organic matter).
Soil CEC is reported on an amount of charge per unit of soil. The two units typically utilized are centimoles of charge per kg of soil (cmol+/kg) or milliequivalents per 100g of soil (meq/100g). These two units are equivalent, so there is no need to make any conversion between them.
The information provided in this infographic is only discussing the general concepts of soil chemical/nutrient analysis. Realize that different labs utilize different methods of extraction (and possibly different analytical instruments) to provide results to the agronomist/farmer. The use of different extractants/methods can alter soil test information, so exercise caution when switching/comparing lab information from multiple labs. If similar extractants/methods are used, there should be good agreement between different labs.