What is meant by pH, CEC, Soil Texture, OM, PPM, ME, etc.?
(Understanding the Terminology of the Soil Test)
It is important to have a basic understanding of the language and symbols used on a soil test. Even when using the same type of test, on the same parameter, labs may report findings in different measurement units or label categories slightly differently. We will give short definitions of some of the more important and sometimes confusing terms.
pH is a very important characteristic of soil that affects the availability of nutrients to the plant root. It is a measure of the acidity or alkalinity on a 1-14 scale with 7 being neutral. The H stands for the Hydrogen ion and the scale measures the ability of the soil to donate H to the soil solution (acid soil) or absorb H from the soil solution (alkaline soil).
CEC, or Cation Exchange Capacity, is a measure of the ability of the soil to hold and release positive ions (cations). Most plant nutrients are in the form of cations, so the ability of the soil to hold and release cations (nutrients) is important in deciding on a fertilizer program. Sandy soils tend to have a lower CEC than clay soils.
Base Saturation refers to the relative concentrations of ions on the soil colloid. Percent saturation of Hydrogen, Potassium, Calcium, and Magnesium are most often listed on soil tests. Base Saturation is closely related to CEC.
Soil Texture is a description of the particle sizes present in the soil. Clay has the smallest particle size, then silt, with sand being the largest particle size category. Smaller particle size soils (clayey soils) will have greater surface area per volume of soil and may pack more closely. They also tend to have higher CEC levels vs sandy soils.
OM, or Organic Matter, is a measure of the solid form of carbon in the soil. Organic matter can exist in the soil in a number of forms:
dead vegetation (very reactive)
fine humus (slowly reactive)
glomalin (very slowly reactive)
Each of these forms is important and has a usefulness in the soil, but their effects differ. Tests for OM range from total ignition of the soil sample (total carbon), to various extractions that attempt to measure the potential of the carbon to interact with the surrounding soil.
PPM (parts per million) vs ME (milli-equivalents)
PPM is a direct measure of weight (or mass). If a soil test reports 500 PPM Calcium (Ca), that means that in every 1,000,000 lbs of soil tested there are 500 lbs of Ca “available”. But many of the properties of the soil that affect plants are electrical moreso than physical. The measure of nutrients by use of milliequivalents is a way of showing that the same amount (mass, weight) of two different nutrient ions may have a very different electrical effects in the soil. For instance a sodium (Na) ion has a charge of +1 while a calcium ion has a charge of +2. So it takes 2Na to replace 1Ca on the soil colloid . That relationship has some important effects when it comes to counteracting sodium buildup in the soil. See here, here, and here for more on that subject.
The relationship between ME and PPM
|Number to divide PPM by to get ME
|Divide by 200
|Divide by 120
|Divide by 390
|Divide by 230
|Divide by 90
above table from NSW Australia Extension
For an example calculation derived from table above see below: