Comparison of Pine Bark from Multiple Suppliers: Variation in Available and Unavailable Water

Bark has been an important component in horticultural substrates for years (and is rapidly increasing in greenhouse professional mixes and in retail potting soil mixes), and in the nursery industry bark is the most common substrate component used in the United States. Bark is obtained as a byproduct of the timber industry (pulp mills, saw mills, pole peeling operations, etc.) when it is striped off logs after harvest. The objective of this work was to evaluate the differences in available and unavailable water in pine bark substrates from different sources/suppliers.  Two sources of aged pine bark at three different moisture contents (55%, 45%, and 35%) were tested to determine available and unavailable water (by vol). Source 1 was aged 1 month and Source 2, was aged nine months. Each pine bark had an initial moisture content (by weight) around 60%. This was adjusted to the desired testing moisture content (by weight) by air-drying samples then allowed to equilibrate for 24 hours. Once at the desired moisture content, small metal containers were filled with enough pine bark to cover the bottom of the container, approximately half the total volume or 25mls (total volume of the container was 50mls). The samples in the metal containers were then placed in a WP4C Dewpoint Potentiometer to measure the water potential (MPa). After obtaining the water potential (MPa), each sample was removed from the metal container and put onto a metal plate, weighed, and then inserted into a moisture balance to determine the dry weight, and confirm the sample was at the desired moisture content. Unavailable water was calculated as follows: subtracting wet weight and dry weight, multiplying by the density (g/cc), and then converting to a percentage. Container capacity and density were determined by using the NCSU porometer method. Available water was then calculated by subtracting the percent unavailable water from container capacity. Available water increased (and conversely unavailable water decreased) with decreasing moisture contents in Source 1 bark. Source 1 showed higher percentages of unavailable water than Source 2. This could be a result of Source 2 being aged more, different handling practices, milling practices, etc.  Previous research on fresh and aged pine bark for growing nursery crops suggested that it is the available water in different pine barks (and bark ages) that cause plant growth differences and irrigation management differences.