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 often associated with a soil or substrate’s ability to hold added mineral nutrients, with higher CEC soils providing more consistent cation supply (Manning and Tripepi, 1995). Broschat (2011) attributed increased growth of downy jasmine (Jasminum multiflorum) and areca palm (Dypsis lutescens) to greater absorption of ammonium and potassium (K), respectively, from higher CEC in substrates amended with clinoptilolitic zeolite. Bigelow et al. (2001) evaluated various amendments to a sand-based medium used for putting greens and found that ammonium leaching decreased proportionally to increasing CEC of the amendment.
Cation exchange capacity is also related to pH buffering, as many of the cation exchange sites are pH dependent (Helling et al., 1964). Argo and Biernbaum (1997) reported that CEC influenced buffering of pH, calcium (Ca), and magnesium (Mg) in six greenhouse substrates. Rippy and Nelson (2007) reported that peatmoss samples with higher CEC had a greater pH buffering capacity than those with lower CEC, resulting in less pH drift.
Despite the importance of CEC in container nutrition and pH buffering, little has been documented on factors affecting CEC of conventional bark-based substrates. Nursery substrates vary by region of the country. In the northeastern United States, most nursery substrates are comprised primarily of pine bark (60% to 80% by volume) and sphagnum moss (10% to 30% by volume), with minor additions of other components such as compost, sand, gravel, and humus (personal observation). Pine bark CEC has been studied sparingly in the scientific literature. Nash and Pokorny (1990) reported a value of 96.6 meq/L for a milled pine bark, and Rideout and Tripepi (2011) reported a CEC of 81.9 meq/L for 90% pine bark amended with 10% sand. The most thorough analysis of pine bark CEC to date is work by Daniels and Wright (1988); however, they only provided CEC on a weight basis, which is less informative for container substrates than CEC on a volumetric basis (Biernbaum, 1992). Furthermore, the CEC values provided by Daniels and Wright (1988) are the weighted sums of CEC for several pine bark particle size fractions, which may provide inaccurate estimates of composite CEC since nesting and settling of particles was not taken into account (Nash and Pokorny, 1990).
Comparatively speaking, sphagnum moss CEC has been studied more extensively. Levesque and Dinel (1977) compared four peats of varying biological composition and chemical properties (including CEC) and reported that CEC increased with decreasing particle size. Rippy and Nelson (2007) evaluated CEC variation in 64 peatmoss samples selected from three mires across Alberta, Canada, and found CEC was positively correlated to the amount of rusty peatmoss (Sphagnum fuscum) present in the sample. The effects of amending sphagnum peat have been evaluated for substrate CEC and subsequent plant growth. Li et al. (2009) determined that CEC of a 60 peat:20 perlite:20 vermiculite substrate did not change by replacing portions of the peat with composted dairy manure. Others have noted correlations between CEC in peat-based substrates (with numerous amendments) and plant growth (Broschat, 2011; Johnson et al., 1981; Li et al., 2009), nutrient retention (Biernbaum, 1992; Bigelow et al., 2001), and substrate buffering (Argo and Biernbaum, 1997). The objective of our research was to develop a better understanding of pine bark CEC as it is used in northeastern U.S. container nursery substrates to improve nursery fertilization management. Specifically, our goals were to determine if CEC varies by pine bark batch, and the influence of pine bark particle size, substrate pH, and combinations of pine bark and sphagnum moss on CEC.
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