Web Special
Types of Soil Tests
By The American Gardener’s Contributing Editor Rita Pelczar

The types of soil tests vary from lab to lab, but the following are some of the most frequently offered—although not necessarily standard—tests and an explanation of what they indicate.

Texture (Particle Size Distribution): estimates the percentages of sand, silt, and clay.

Soil reaction (pH): estimates the relative acidity or alkalinity of a soil as determined by the hydrogen ions in the soil solution. A soil is considered neutral if its pH is between 6.5 and 7.5; a pH lower than 6.5 indicates an acidic soil, a pH above 7.5 is an alkaline soil. This is sometimes called the "active acidity."

Buffer Index (Lime Index): determined by calculating both the active acidity and the reserve or potential acidity of a soil as determined by the number of hydrogen and aluminum ions attached to cation exchange sites in the soil. Both active and reserve acidity measurements are required to establish the lime index, or the amount of lime needed (if any) to adjust the soil acidity to the desired level.

Base Saturation Percent (BS%): represents the percent of the cation exchange capacity that is occupied by basic nutrient elements, such as calcium (Ca++), magnesium (Mg++) and potassium (K+). BS% indicates pH and lime status of soils.

Cation Exchange Capacity (CEC): an estimate of the total quantity of positively charged nutrient ions (cations) a soil contains and can hold against leaching. As the CEC value increases, the soil's capacity to hold nutrients increases.

Soil Organic Matter (SOM): an estimate of the decayed or humified organic material in the soil. It is expressed as a percentage of the total soil.

Estimated Nitrogen Release (ENR): this estimate of the amount of nitrogen that will be released over a year and is based on the soil organic matter content and the texture of the soil.

Nitrate Nitrogen: the measure of readily available nitrogen in the soil, but it can fluctuate significantly during the season. It is often used in combination with the percentage of organic matter to make a nitrogen fertilizer recommendation.

Humic Matter (HM%): a measurement of humic and fulvic acid in the soil and is used to help estimate lime requirements.

Phosphorus and Potassium Indicies: estimates of the levels of these macronutrients.

Calcium Percent and Magnesium Percent: levels of these macronutrients are based on the percentage of the cation exchange capacity.

Micronutrients or Trace Element Indices: estimates of essential plant nutrients that are required in minute quantities; they include boron, chlorine, copper, iron, manganese, molybdenum, and zinc.

Soluble Salts: a measurement of the inorganic soil constituents (ions) that are dissolved in the soil water. Although sensitivity to high levels of soluble salts differs from species to species, it can cause serious problems for plants.

Sodium adsorption ratio (SAR): evaluates the sodium content of soil. Excess sodium can cause soil to crust and become hard when dry, making water absorption very slow.

Lead Scan: is a test for possible lead contamination, which can be a problem around some older buildings or building sites. Lead is not a plant nutrient, but is tested to determine possible health problems from elevated lead levels in soil.