Managing substrate pH of soilless substrates

Heidi Lindberg<wollaege@msu.edu>, and W. Garrett Owen, Michigan State University Extension - January 08, 2018

The symptoms of nutrient deficiencies and toxicities are often confused with other issues. To learn more, attend a session for greenhouse and nursery growers in Grand Rapids on Jan. 24, 2018.

Figure 1. The optimal substrate pH for most greenhouse crops is between 5.4 and 6.4. Figure by Garrett Owen, MSU Extension.

Did you know different substrate components in soilless mixes have different pHs? Peat moss and aged pine bark only have a natural pH between 3.0 and 4.0. The optimal substrate pH for most greenhouse and nursery crops, however, is between 5.4 and 6.4 (Figure 1). Therefore, soilless substrate manufacturers add calcitic, dolomitic or hydrated limestone or calcium sulfate to raise the pH into that optimal pH zone.

Every year, Michigan State University Extension floriculture and nursery crops educators encounter situations where the pH of the substrate is either too high or too low for the crop being grown. With over 1,500 species of ornamental plants being grown by Michigan growers, it can be complicated to not only know the pH and electrical conductively (EC) requirements for each crop, but how to grow them effectively using similar media components.

Nutritional disorders can exhibit very different symptoms on different species of plants. For example, one sample of New Guinea impatiens (Figure 2) from last season had lower leaf chlorosis, marginal black spotting, and the dying of leaf tissue. These symptoms are often misdiagnosed and the growers subsequently do not correctly address the problem. In this case, the substrate pH of the New Guinea impatiens plants were 4.6 (recommended 6.0 to 6.5). The low pH caused the iron (Fe) and manganese (Mn) to be overly available to the plant causing a Fe and Mn toxicity. Without testing, this problem may have been misdiagnosed because we often see nutrient deficiencies related to high substrate pH often caused by the very high water alkalinity.

Fe and Mn toxicity on New Guinea impatiens

Figure 2. Symptoms of iron (Fe) and manganese (Mn) toxicity because of low substrate pH on New Guinea impatiens. Photos by Garrett Owen, MSU Extension.

There are a few options to correct low pH:

Apply flowable lime at the rate of 1 to 2 quarts per 100 gallons of water. Be sure to rinse foliage.
Apply a 1:15 solution of hydrated lime. To do this, mix 1 pound per 3 to 5 gallons of warm water and stir thoroughly.
Apply potassium bicarbonate (KHCO₃) at the rate of 2 pounds per 100 gallons of water. Be sure to rinse the foliage of the plant immediately, leach the substrate and do not over apply.

MSU Extension emphasizes that regularly testing the pH and EC of your crops is essential to preventing nutrient toxicities and deficiencies of crops. Testing is also essential so that accurate diagnosis will lead to appropriate corrective procedures.

For more information, check out these previous MSU Extension articles:

To learn more about how to manage pH for ornamental crops and how alternative substrates effect your pH management, consider attending “Managing Substrate pH and EC for Optimal Plant Nutrition” at the Kent County MSU Extension Office (775 Ball Ave. N.E., Grand Rapids, MI, 49503) on Jan. 24, 2018, from 10 – 11:30 a.m. Register now!

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).