Soil Test Note – No. ST001

Dr. Stanley L. Chapman, Extension Soils Specialist

COMPARING SOIL TEST DATA AND RECOMMENDATIONS

Questions often arise about interpretations of soil test results and recommendations.  Such interpretations can be particularly confusing when different labs report test results in different terms.

Parts Per Million and meq/100g

Soil tests for plant nutrients are usually reported in parts per million (ppm) or pounds per acre (lbs/A).  Until the 1980s, two common extractants for soil test phosphorus (P) were the Weak Bray (P₁) and the Strong Bray (P₂).  The University of Arkansas used the Weak Bray P test until 1987 when they converted to Mehlich III.  Ammonium acetate was formerly used to extract the cations.  Mehlich III is more universal extractant for all of the soil test elements except nitrate-N.

The University of Arkansas Soil Testing Lab reports all routine elements in pounds per acre.  Soil test results may also be reported in milli-equivalents per 100 grams (meq/100g).  Soil test research results are usually reported in this manner.  A milli-equivalent is 1/1000 of the gram-equivalent weight of the element.  For example, the gram-equivalent weight of potassium is 39 grams; the milli-equivalent weight is .039 gram.

Parts per million (ppm) and meq/100g can be readily converted to pounds per acre as shown below.  “Lbs per acre” refers to the pounds in the upper 6 to 7 inches of surface soil weighing 2,000,000 pounds.

Pounds per acre test results may be converted to ppm or meq/100g by reversing the calculations.

Cation Exchange Capacity

Cation exchange capacity (CEC) is generally measured as the relative amount of cations (positively charged ions) displaced by ammonium ions or other known cation.  The CEC is roughly equivalent to the sum of the exchangeable hydrogen, potassium, sodium, magnesium and calcium ions and may be expressed as “total cations” or CEC in meq/100g.

The more humus and clay, the higher is the CEC.  The CECs of most Arkansas soils vary from 5 or less in light, low organic matter sandy soils to over 50 in heavy clay soils.

Percent Saturation

Percent saturation is calculated by dividing the CEC or total cations (in meq/100g) into the meq of the particular cation in question.  For example:  Assume the CEC of a particular soil is 15 meq/100g and it contains 3 meq/100g of magnesium.  Divide 15 into 3 and multiply by 100 to get 20 percent magnesium saturation.  Percent saturation with the other cations is similarly calculated.  Normally, percent K saturation will be greater than 1.0; percent Mg saturation will be greater than 5.0; and percent Mg saturation will be greater than percent K saturation.  Percent sodium (Na) saturation is commonly expressed as exchangeable sodium percentage (ESP).  Values greater than 15 percent in the subsoil indicate a sodic soil.  Percent sodium saturation values greater than 10 – percent in the topsoil should be reason for concern.

The ideal base saturation concept is based on the principle that availability of a given cation is influenced by the concentration of other cations present.  We do not disagree with this principle.  However, we know that plants remove soil nutrients efficiently over a reasonably wide range – not just at the “ideal” point; therefore, we rely more on response data that show specific nutrient needs by similar soils.

The following is an example of how University of Arkansas soil test results relate to the milli-equivalent base saturation concept.

In this example, the percent saturation of the CEC is 55.2 for calcium, 6.9 for magnesium, 3.4 for potassium, 6.9 for sodium and 72.4 for total base saturation.  Hydrogen is the only cation shown that is not a base; but since it is a positive ion (cation), it participates in this cation exchange process.  Hydrogen is the main acidic ion.

While University of Arkansas agronomists consider CEC in a broad sense in making fertilizer and lime recommendations, some commercial labs and some state labs use it more specifically and make recommendations for lime and potassium in higher amounts with increasing CEC.  Some soil scientists believe that the cation exchange capacity should be saturated by specific percentages of the base cations – Ca, Mg and K.  Hence the terms “percent base saturation.”  For example, there is some agreement that for general crop needs, the soil CEC should be saturated 65 to 85 percent by Ca, 6 to 12 percent by Mg, and 2 to 5 percent by K.  This concept leans more toward “fortifying the soil” than our philosophy of fertilizing for specific plant needs.

Some labs request and recommend for desired yield goals.  The University of Arkansas considers the soil profile, yield response data and weather limitations; recommendations aim at the highest economical yield attainable with good management.

Revised, January, 1999

The Arkansas cooperative Extension Service offers its programs to all eligible persons regardless of race, color, national origin, religion, gender, age, disability, marital or veteran status, or any other legally protected status, and is an Equal Opportunity Employer.

[Home]            [Table of Contents]