Test Values Explained
pH is the measure of acidity of a substance. pH is expressed as a number, not a percentage or other quantity. A value of 7.0 on the pH scale is considered neutral, that is, the substance is neither acidic nor basic. Lower numbers on the pH scale denote increasing H+ ions and acidity. The acidity of the soil solution affects many physical, chemical, and biological reactions necessary for plants to survive and thrive. The optimum pH value varies by crop and region, but is generally between 6.0 and 7.0 pH (slightly acidic), though alfalfa thrives between pH 6.8 to 7.0.
Figure 3. Relative availability of plant nutrients by soil pH.
Buffer pH - Once a need for adjusting pH higher is identified, the buffer pH value indicates the amount of liming material necessary to make the adjustment. Low buffer pH values indicate that more lime materials are necessary to raise pH than higher buffer pH values.
"Buffering" in this case refers to the ability of the soil to "recharge" acidity, and is a function of aluminum (Al3+) minerals and H+ in the soil (and is related to CEC). If a soil is poorly buffered (sandy), less liming material are needed to raise pH a certain level than a soil that is highly buffered (clayey soil). Some soil tests may report buffer pH as lime test index, or LTI, which is merely the buffer pH value multiplied by 10.
Organic Matter - Expressed as a percentage, OM is the non-mineral content of the soil sample and is usually determined by combustion. Organic matter has many functions, including water-holding capacity, nutrient cycling, and contributing to soil structure and CEC.
CEC - Cation Exchange Capacity is measured in milliequivalents per 100 grams of soil. CEC is determined primarily by soil clay mineralogy and OM level in the soil. Cation exchange capacity represents a measure of electrostatic charge sites in the soil that can hold cations, or positively charged ions, like Ca2+, Mg2+, Mn2+, Zn2+, K+, H+, and Al3+.
For each "equivalent" charged site on the soil particles, a single positive charge can be held (denoted by the superscript number on these ions). In the case of K+, a K+ ion has a single positive charge, and is held by one CEC site. For a Ca2+ ion, two CEC sites are necessary to hold one Ca2+ ion, three CEC sites for Al3+, etc.
Higher CEC values in soil represent more capacity for nutrient cations to attach. For example, a high CEC soil can theoretically hold many K+ ions, but would require more K+ to be applied as fertilizer to fill the sites located on the soil particle surfaces before it is easily released to the soil solution and taken up by plants.
P - Phosphorus is reported in parts per million (ppm) or pounds per acre (1 pound per acre = 2 parts per million). This is not a measurement of total P in the soil, however, it is an estimate of P that is available to plants. The testing procedure utilizes an extractant that is correlated with P uptake that might occur by plant roots. This value is reported on the soil test as "extractable P." Not all labs use the same process; some may use the Bray-Kurtz P1, Mehlich III, Olsen, or other procedure. Reported P values will vary according to the test procedure and are not directly comparable to each other.
P is an important element in plants, as it is part of the DNA, RNA, the energy transfer molecule ATP, as well as amino acids and proteins.
K+ - Potassium is reported as ppm or lb/acre of "exchangeable K." Like P, all of the K+ in a soil sample is not available to the plant. Most of the K is locked in mineral structures, some is available slowly from the clay edges, and other K+ ions may attach to the CEC (exchangeable) and flow easily to the soil solution. The K+ extraction solution measures K+ that can be readily moved from the CEC and into soil solution, simulating the processes that might exist with a root system in the soil.
Potassium is vital in water regulation and enzyme activation in plants. Plant stomata, which are openings in the leaf used for gas exchange, open and close by movement of K+ in and out of the cells surrounding the opening.
Potassium (and other nutrients) in plant stover or residue on the soil surface can cycle back to the soil with rainfall events and residue breakdown (this sometimes includes up to 80% of K in residue). If little to no rainfall occurs after harvest prior to soil sampling, the reported K+ value may be lower than expected because of this lack of cycling. Dry conditions also limit the movement of slowly available K+ to the CEC.
Ca2+ - Calcium content of the soil is reported in ppm or lb/acre, sometimes listed as "exchangeable calcium." Calcium is a common element in a lot of mineral soils, and though they may occur in some regions, deficiencies are rare in most soil environments above pH 5.5. The easiest correction to a suspected Ca2+ deficiency is raising soil pH with a liming compound, and Ca2+ is considered sufficient at ≥200-300 ppm. Calcium is essential to plant cell wall structure, cell division, and many enzymatic processes.
Mg2+ - Magnesium levels are measured in ppm or lb/acre, sometimes listed as "exchangeable magnesium." Magnesium is a key element in chlorophyll and many enzymes and enzyme activation. Deficiencies can occur in crops, and can be remedied with Mg-containing fertilizers or dolomitic limestone. Mg2+ is sufficient for most crops at ≥50-100 ppm.
Base Saturation - This is a description of the major cations held on the CEC on a percentage basis. Typically, Ca2+, Mg2+, K+, and H+ are listed (and sometimes Na+ and Al3+) as they are of the highest concentration. Other cations may attach, such as the metal micronutrients, but they are often measured in ppm, not a percentage (1 ppm = 0.0001%)
The percentage of H+ is related to the buffer pH value discussed earlier. When H+ moves off of the CEC and into soil solution, the solution becomes more acidic. The portion of H+ on the CEC is sometimes referred to as "reserve acidity."
Base Saturation Ratios
There is a long-standing debate on the usefulness of base saturation for determining lime and fertilizer rates, particularly the Ca:Mg ratio. There is a lack of evidence that a certain ratio of bases is necessary for optimum crop performance. A large body of research shows that if nutrients are in the soil at high enough levels and pH is in the proper range, optimum yields in agronomic crops can be achieved across a wide range of Ca:Mg ratios.
There are a few ratios of note for specific crops and soils, however. The ratio of Mg:K is of importance to forage growers. If excessive K+ is taken up by the plants and Mg2+ is low, the forage may contain low Mg2+ content and cause a metabolic condition in livestock called grass tetany (hypomagnesemia). Dolomitic limestone and Mg-containing fertilizers are options, but supplying feed additives with Mg to livestock may be more economical.
When Mg2+ exceeds Ca2+, or when Na+ becomes high (>15%), this may be indicative of possible soil structural issues, such as clay dispersion or flocculation, surface sealing, and decreased water infiltration. This is most likely to occur in areas of naturally occurring Na+ and Mg2+, but these are not widespread in North America.
Soluble Salts - The soluble salt content (or conductivity) is expressed in dS/m or mmho/cm (which are equivalent). If soluble salts are too high in the soil, plants cannot take up sufficient water, wilt, and in severe cases, die. In most crops, performance and yield decline rapidly at 3 to 4 dS/m and higher, and may be affected as low as 2 dS/m.
Salt problems are a concern primarily in the Plains and western regions of North America where various salts are native to the soil in high levels. Some effluent irrigation waters and fertilizers have the potential to cause salt problems as well, most notably muriate of potash (KCl; 0-0-60) and urea (46-0-0). Use extreme caution when applying in-furrow or banding.