Cation exchange capacity analyzed for nursery pine bark substrates

Cation exchange capacity is a commonly used soil chemical property that describes the maximum quantity of cations a soil or substrate can hold while being exchangeable with the soil solution. Cation exchange capacity is associated with a soil or substrate's ability to hold added mineral nutrients, with higher CEC soils providing more consistent cation supply. James Altland, James Locke, and Charles Krause from the U.S. Department of Agriculture, Agricultural Research Service in Wooster, Ohio, said their study, published in HortTechnology, was done to develop a better understanding of pine bark CEC as it is used in northeastern U.S. container nursery substrates. The analyses were performed to determine if CEC varies by pine bark batch, and to show the influence of pine bark particle size, substrate pH, and combinations of pine bark and sphagnum moss on cation exchange capacity.

The scientists analyzed four batches of nursery-grade pine bark from two nurseries. Sphagnum moss from a single source was separated into several particle size classes and measured for CEC. The pine bark was also amended with varying rates of elemental sulfur and dolomitic limestone to generate varying levels of substrate pH. Results showed that the CEC varied with pine bark batch. "Part of this variation is attributed to differences in particle size of the bark batches," the authors said. "Pine bark and peatmoss CEC increased with decreasing particle size, although the change in CEC from coarse to fine particles was greater with pine bark than peatmoss." Substrate pH from 4.02 to 6.37 had no effect on pine bark CEC. The pine bark batch with the highest CEC had similar CEC to sphagnum peat; amending this batch of pine bark with sphagnum peat had no effect on composite CEC.

The researchers said that the data demonstrate several key points about CEC of nursery substrates comprised primarily of pine bark. "First, cation exchange capacity varies by pine bark batch, and variation can be explained, in part, by differences in particle size distribution (CEC increased with decreasing particle size for all bark batches). Second, substrate pH over the range of 4.02 to 6.37 did not affect CEC. Finally, pine bark substrate CEC does not necessarily increase when amended with sphagnum peat."

The authors recommended that growers who want to improve nutrient retention and pH stability should have their substrates, as well as the parent components, analyzed for CEC so they can make informed decisions about the need for amendments and ideal amendment rates. They added that that the methods used in their study could be easily adopted by most commercial or research-based substrate analysis laboratories.