Cation Exchange Capacity
The cation exchange capacity (CEC) listed on a soil test report refers to the number of negatively charged sites on the soil particles (mostly clay and organic matter) that can retain plant nutrients. It is expressed in units of meq/100g or cmolc/kg (1 meq/100g = 1 cmolc/kg). Positively charged nutrients, such as potassium, calcium and magnesium, are electrically attracted to clay particles in soil. Other positively charged elements, such as sodium and hydrogen, are also adsorbed on soil particles. Sodium can negatively affect soil structure and hydrogen ions determine the soil pH.
Soils that have higher CECs are considered to be more fertile than soils with lower CECs, as they can potentially hold more nutrients for longer periods of time. These soils also have a greater water-holding capacity. Knowing the CEC of your soil can help you decide on a fertilizer application frequency, as high CEC soils require less frequent applications.
A soil’s organic matter reading represents the organic constituents of the soil. Most of it is comprised of plant and animal residues. Organic matter in the soil contributes to the soil structure, the soil fertility and the water-holding capacity of the soil. Soils rich in organic matter (4-5%) will be more fertile. The organic matter in a soil can contribute nitrogen, phosphorus and sulfur to the crop.
Sodium Absorption Ratio
The acronym SAR on a soil test report stands for sodium absorption ratio. It is used to predict water infiltration problems in the soil and soil-structure problems. SAR is the ratio of sodium to calcium plus magnesium in the soil solution. Soils with an SAR greater than 10 are considered to be sodic. Sodic soils have structural problems, which result in poor water infiltration. The soil tends to swell when wet and crack when dry.
Soil pH is one of the most important parameters on your soil test report. The pH level of the soil can tell you a lot about the potential availability of plant nutrients and about the possible toxicities of other elements, such as aluminum. Soils with a pH greater than 7.0 are considered to be alkaline soils. Micronutrient deficiencies, such as an iron deficiency, are common in these soils.
Crops grown in soils with a pH level lower than 5.5 might show toxicity symptoms of metals (e.g. iron and manganese) and deficiencies of other nutrients like magnesium. Liming the soil is mostly recommended in such soils. The ideal soil pH range for most crops is between 5.8 and 6.5, a range in which most nutrients are available for the crops.
The letters E and C on a soil report stands for electrical conductivity. It is a measure of the salinity of the soil. The EC is most commonly measured in the soil solution. Units of expression are usually ds/m, mmho/cm or microsimens/cm, where 1 ds/m = 1 mmho/cm = 1000 µs/cm. The EC is one of the easiest ways to assess fertilizer levels in the soil, as well as the yield potential and soil salinity status and suitability for what’s being grown. Different crops have different tolerance levels to salinity. Above a certain threshold, yields will decrease. The yield reduction is proportional to the increase in the EC level.
Soil health is a vast field of study. Getting your soil tested for common problems is the first step towards correcting poor soil quality.