Soils and Media - Understanding CEC and AEC

Have you seen the term CEC on soil test or media analysis reports and have wondered what it meant? The following is a short article on this subject.

Clay minerals and humus compounds have net negative charges in the soil. Positively charged ions, or cations, in the soil solution will be attracted to these negative charges and will be drawn to soil colloids. These cations include the plant nutrients Potassium K+, Magnesium Mg2+, Calcium, Ca2+, and Nitrogen in the ammonium form NH4+ (ammonium N) as well as some micronutrient metals Manganese Mn2+, Copper Cu2+, Zinc Zn2+, and Iron Fe2+. Soils therefore hold these nutrients which otherwise could move downward out of the rooting zone of plants with draining water after precipitation events or in dissolved surface flow. Plant roots release hydrogen ions (H+) which can exchange with these positively charged nutrients (cations) being held on clay or organic matter, moving the cations to the soil solution and making them available for plant root uptake. This process is called Cation Exchange and the amount of exchange sites in a soil is called the Cation Exchange Capacity (CEC). In soilless media, the cation exchange commonly comes from vermiculite, peat, or composted organic materials. The higher the CEC, the more positively charged nutrients the soil or media will hold.

The cations in a soil solution are balanced with an equal number of negatively charged ions, anions. Plant roots release bicarbonate (HCO3-) as an anion into the soil solution. A number of plant nutrients exist in the soil solution in anion form. These include NO3- (nitrate Nitrogen), SO42-(sulfate S sulfur), H2PO4-, HPO42- (both phosphate anions), and MoO2- (molybdate). Boron is in the form of boric acid H3BO3 [B(OH)3] and will interact with water to form the B(OH)4− anion.

Soils and soilless media have a limited amount of anion exchange. Soils do contain some hydroxyoxides of Al and Fe that have net positive charges. This is particularly true of highly weathered soils of warm humid temperate regions and the tropics. Some volcanic derived soils have relatively high anion exchange capacity. Clay minerals also have some positive edge charges and thus some anion exchange capacity. Humus has an overall net negative charge but does contain positive charge sites. In Delaware, we have higher anion exchange in subsoils with higher clay content where we often see accumulations of SO42-. Of the anions, nitrate is the most weakly held by anion exchange and nitrate in the soil solution is subject to losses with water movement in or across soils (leaching, runoff). Sulfate is also subject to loss but to a lesser degree as it held more tightly with the anion exchange sites on clays. Phosphates behave differently and can complex with metals to form insoluble compounds that do not leach. Boron and molybdate are subject to leaching losses as anions.

From a crop nutrition aspect, a soil or soilless medium with significant cation exchange capacity can hold cation plant nutrients and exchange them for plant uptake. This provides a reservoir of nutrients for the plant and allows for fertilization at broad intervals. Nitrogen of course is a “wildcard” because the cationic form, ammonium, is rapidly converted to the anionic form, nitrate, by bacteria in warm moist conditions (nitrification). Ammonium is retained at significant levels only under cold conditions that slow bacterial activity. Because soils and soilless media have limited anion exchange capacity and nitrate is weakly held on any anion exchange, a large portion of nitrate remains in the soil solution and is subject to leaching (and runoff) losses. This is why nitrogen is lost to subsurface and surface waters from agricultural lands and horticultural use areas at a higher rate than in natural areas generally. Nitrate from applied nitrogen fertilizers and organic nitrogen sources will be subject to these losses due to the limited capacity of soils to hold that nitrate. Excess nitrates in our waters can lead to eutrophication of water bodies. Elevated nitrate levels are also a human health concern for drinking water. Other anions such as sulfate, molybdate, and borate can also be subject to leaching losses but are generally less of a concern than nitrate and do not contribute to environmental problems in our area.

The anionic plant nutrients subject to leaching losses (nitrate in particular) must be applied regularly and in amounts that will satisfy plant growth needs but not result in large leaching losses. This includes nitrogen from ammonium or urea sources because it is quickly converted into nitrate by nitrification. Controlled released fertilizers have been developed to help achieve this and are commonly used in horticultural applications. They are less common in agriculture due to the relatively high cost.

Written by Gordon Johnson, Extension Horticulture Agent, UD