Pinebark.indd

NCDA&CS Agronomic Division www.ncagr.gov/agronomi (919) 733-2655 Jan/Feb 1995 Chemical Characteristics of Pine Bark
by M. Ray Tucker, Agronomist
The chemical properties of pine-bark media differ When dolomitic lime is applied to acid organic from those of mineral soils. These differences infl uence substrates, hydrogen (H+) is displaced from the the amount of lime and fertilizer required for optimum organic functional groups (R-COO-H) and replaced growth. There are several reasons for their differences with calcium and magnesium. When organic acids in optimal pH and nutrient-holding capabilities. (H+) are neutralized, plants benefi t from increased pH, calcium and magnesium. Lime also enhances The pH of native pine bark is generally 4.5 or microbial activity within the media.
lower (Table 1). The pH of mineral soils ranges from 5.0 to 6.0, depending on past production history. Although adding mineral soil to bark media is not Although the optimum pH for mineral soils ranges a widely accepted practice in the nursery industry, it from 5.5 to 6.0, nursery crops grow quite well in pine- provides an effective means of reducing phosphorus bark media with a pH of 5.0 to 5.5. The difference leaching without any adverse effect on plant growth. in optimum pH of the two media is due to the toxic The amount of mineral soil required to hold effect of aluminum in mineral soils, whereas pine-bark phosphorus is quite small relative to the other media substrates generally contain little or no aluminum.
components. To overcome concerns about drainage, use a sandy loam soil. These soils are generally well Pine-bark media and mineral soils differ in their drained and are composed of sand particles coated nutrient-holding capability because of differences with clay. The clay coating provides reactive sites in their chemical makeup. Nutrient retention sites in required for retention of phosphorus. The chemical pine bark are associated with the organic fractions characteristics of native pine bark are shown in (R-COO-H). These sites hold positively charged ions Table 1. These data were generated from analysis of such as calcium, magnesium and potassium quite pine-bark samples over several years from individual well. However, the organic fraction repels negatively growers and commercial bark suppliers.
charged phosphate ions (H PO - and HPO -2) causing them to be lost from the media by leaching. Research has shown that phosphorus leaches quite readily from pine-bark substrate.
Table 1. Chemical characteristics of native bark
before amendment.
Leaching of phosphate from pine-bark media poses several problems. Optimum growth is hindered; Soil Test Parameter
Ranges
fertilizer costs increase; and phosphorus runs off into surface waters, posing a threat to the environment.
In contrast, nutrients in mineral soils are held by negatively charged nutrient retention sites that originate from the clay fraction. Clays also contain iron and aluminum oxides, which have a strong affi nity for the negatively charged phosphate ions. Reaction of phosphate with iron and aluminum removes it from the soil solution, which prevents it from leaching. However, phosphorus retained by the oxides is still readily

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