Alert: Roots Infected with Pythium
Posted: May 7, 2012
Growers in all these cases are combining the use of slow release fertilizer AND soluble fertilizer, resulting in significant over-fertilization. While the conductivity of the saturated paste extract of the potting mix should be between 1 and 2 mS, these samples are running in the range of 6-10 mS. These high concentrations of soluble salts damage the roots directly by slowing the net flow of water into the roots, and indirectly by predisposing the plants to root rot and damping off diseases.
Usually, leaching pots that have high conductivity with ample water can help the situation. The problem with pots that have been treated with both insoluble and soluble fertilizer is that leaching the pots will only release more salts. To save the plants, they need to be repotted into fertilizer-free mix and then fertilized only with more soluble fertilizer. If the slow release is just on the surface, it can be scraped off and then leaching will effectively reduce salts.
Recommendations for avoiding over-fertilization include the following:
- Purchase and routinely use a conductivity meter
- Record which readings are consistently associated with high quality plants vs. those with poor quality plants
- Do not combine soluble and slow release fertilizer
- Always add enough liquid fertilizer so that 10% of what is added leaches out the bottom of the container
- Leach excess fertilizer from plants prior to moving them to environments where growth is slower
- When salts are excessive due to a soluble fertilizer, leach by applying plenty of tap water so that it pours out of the bottom of the container. Repeat 2-3 hours later or the next day.
More detailed information can be found in the Penn State Plant Disease Fact Sheet, Over-fertilization of Potted Plants, by Gary Moorman.
- Professor of Plant Pathology
- Extension Educator, Horticulture