by Dava Hayden
Extension Associate for Nursery Crops

It is an essential management practice to test and document each load of substrate prior to planting.  Each load received from your supplier may vary and will need to be tested.    Some substrate suppliers manage their product closely to ensure consistency from shipment to shipment.  Other suppliers simply provide a product that allows their customers to adjust the substrate to meet their specific needs.  Testing substrate prior to planting will allow amendments to be properly incorporated in to the mix for greater plant benefit.  Failure to test each delivery of substrate may lead to plant toxicities or deficiencies if pH substrate problems are detected later in production.  Worse yet, expense to correct the problem or the loss of growth can costs far more than a preventative approach to substrate management.

Kentucky nursery growers of woody ornamentals generally use aged pine for its desirable characteristics.  Provided below is an exert from “Something to Grow On- Nutrient Management, key to growing healthy nursery crops” Department of Floriculture and Horticulture Department, Cornell University.  For more information go to this link:

Bark is often used as a media component to increase the air porosity within a mix. Some bark fragments contain up to 43% internal porosity, from which roots can absorb water if penetration of the particle occurs (Pokorny, 1987). Pine bark, which is acidic in nature, also has a low initial fertility-- an important characteristic of growing media. Composted bark has a higher cation exchange capacity than raw bark and represses pathogenic fungi (Hoitink, 1980).

Several bark particle sizes have been recommended for media composition. Suggested formulations for container-grown crops include:

  • a mix characterized by 25-33% of the pine bark particles less than 0.5mm in size;
  • peat moss based media containing 25-50% pine bark; or
  • media containing various bark particle sizes attained by using a hammermill with a screen size of 2 - 2.5 cm.

The use of bark in container media offers both advantages and disadvantages. Bark which has not been composted properly induces nitrogen deficiency problems; however, composted bark with sufficient nitrogen fertilizer added during the process should not pose this problem. Bark from alder, poplar, maple, and oak are prone to decay as a result of a high cellulose content; plants grown in media containing these barks may experience nitrogen deficiency as the constituents rapidly decompose. Because of this, it is necessary to add more supplemental nitrogen to hardwood rather than softwood bark before or during composting to preclude nitrogen deficiency (Bilderback 1982). Hardwood bark breaks down three times more quickly than softwood bark. Continued decomposition of composted hardwood bark media during the growing season increases the water-holding capacity and decreases the air porosity of the mix. In addition, hardwood bark seems to repress nematodes and root pathogens more effectively than softwood bark; fungicidal inhibitors and antagonistic organisms present in composted hardwood bark contribute to this repression. Some barks contain organic or inorganic toxins, including high levels of monoterpenes, phenols, or manganese that may prove harmful to plants. Phenolic compounds in fresh barks are especially toxic to young nursery crops. Tree species, age, time of harvest, soil type, and geographical region are factors that affect phytotoxicity. Bark derived from older trees, lower portions of the tree, or removed during winter months tends to be more phytotoxic than bark removed from younger trees, upper portions of the tree, or during spring months. In addition, obtaining bark of uniform quality and particle size is often difficult.

The characteristics of softwood and hardwood bark are quite different. Some softwood bark can be used without composting; hardwood bark must be composted before use or phytotoxicity may ensue. Aging and composting bark is usually an effective way to eradicate toxins. Fresh pine bark repels water to a greater extent than aged pine bark or composted hardwood bark; to increase the moisture content of pine bark, soak it under a sprinkler system. Although pine bark has a lower water-holding capacity than peat moss, it holds a greater amount of available water for the plant (Pokorny 1979). Avoid water stress in newly planted nursery crops by watering regularly, particularly during the 30 days after planting.

Many plant materials appear to grow well in fresh pine bark (Self and Pounders 1974). Fresh pine or softwood bark usually has an initial pH range of 4.0 - 5.0; as pine bark ages, the pH does not increase appreciably. To increase the pH of pine bark, add 4 - 15 lbs. of dolomitic limestone per cubic yard; within a few weeks the pH of the media should equilibrate to a suitable planting pH (Bilderback 1982). Aged pine bark is often favored over fresh pine bark by growers; this may be attributed to a more desirable particle size distribution in the former (Pokorny, 1975).

Recently harvested hardwood bark is usually characterized by a pH of 5.2 - 5.5. Lime should not be added to hardwood bark mixes; as the bark ages or is composted, the pH may exceed 7.0 as a result of the natural calcium content of the bark. To avoid magnesium deficiency in hardwood bark mixes, incorporate one pound of magnesium sulfate into each cubic yard of mix. If a bark-sand mix is desired, add a low pH sand to decrease the pH of composted hardwood bark media (Bilderback, 1982).

As a general guideline, most woody ornamentals crops produced in soilless mix,  desire a pH of 5.5 to 6.2 for optimum plant growth.  This generality is not true for all woody ornamental nursery crops, some prefer higher or lower pH. Solubility of mineral nutrients are affected by substrate pH.  Iron, manganes, boron, copper, and zinc are most soluble when pH is above 5.0 and below 5.5.  On the contrary, as pH increases greater than 6.5,  the availability of iron, manganese, boron, copper, and zinc decrease and micronutrient deficiencies symptoms begin to appear. Chlorosis is an indication that the pH is too high.    

When testing substrate pH, you should actually be testing your substrate solution, or the water within your substrate.  Use the water source from which your plants will be irrigated to determine your substrate solution.  If you receive soilless substrate that does not meet your needs, then soil amendments should be added.

If your pH is too low applications of either dolomitic limestone or a liquid basic fertilizer (one that contains nitrate) should be made.  Below is an exert from, the July 2004 edition of Digger Magazine.  “Changing Container Substrate pH -What are the affects of peat moss, lime source and lime rate?”, by James Altland.  The article in its entirety can be viewed from the following link:

Influence of lime rate: With no lime added, Douglas fir bark (with or without peat moss) has an initial pH of 4.2. In our experiments, 5 pounds of pulverized lime raised substrate pH to 6.4, and 10 pounds raised pH over 7. One might assume that doubling the rate of lime would raise pH twice as high. Keep in mind that pH is measured on a logarithmic scale, meaning for each unit increase in pH, H+ concentration decreases 10-fold. Conversely, a unit decrease in pH means the H+ concentration increases 10-fold. Adding 5 pounds of lime resulted in an increase of 2.2 units of pH, which translates to about a 158-fold (102.2) decrease in H+ concentration. Adding 10 pounds of lime caused pH to rise just 0.7 units higher than adding 5 pounds. That's still a 794-fold (102.9) increase in H+ concentration over containers receiving no lime.

If your pH is higher than 6.4, elemental sulfur or a liquid acidic fertilizer (one containing NH4) can be used to reduce the pH of your substrate. Often, when using a pine bark substrate, high pH is not a problem unless too much lime was added or if you mix contains hard wood decomposition.  For further information on reducing pH, link to

More information will be included at a later date.  Additional information will include;

  • Over-coming delayed nitrogen mineralization, or high N:C, from fresh bark substrates
  • Wetting agents to control moisture loss