Fertilizer accumulation in growing media

We’ve received reports of high soluble salts in growing media in greenhouses. Read about the potential causes and solutions.

In the last week, we have received multiple media test results with very high electrical conductivity (EC) values, with readings as high as 6.1. Many bedding plants grow best when the EC is around 2.0 to 2.5 using the saturated media extract method. Lower values (1.0 to 1.5) are suitable for salt-sensitive crops such as New Guinea impatiens, begonia, and gerbera. A high media EC can reduce crop growth, causing stunting and sometimes leaf burn. It also limits plants’ ability to use water in the root zone.

A high media EC can be caused by a number of factors. Some likely ones include:

  • Insufficient leaching. It’s common for growers to only water the top portion of a pot or flat. Over time, the fertilizer accumulates and the EC progressively increases.
  • Fertilizer rate is too high. Most bedding plants grow well at 125 to 150 ppm nitrogen at each watering, although crops such as petunia and chrysanthemum have higher requirements. Be sure you’re not applying more fertilizer than you need.
  • The injector is not working or the mixing rate is not correct. Be sure to check your math and that the injector is working properly and the settings are correct.

Once the media EC is high (above 3.5), the best thing to do is to begin watering without fertilizer, making sure that enough water is applied so that about 20 percent goes through the container. This may need to be repeated once or twice. Then, try lowering the fertilizer rate. Keep an eye on the pH values, as leaching and changes to your fertilizer regimen can impact the root zone pH, too.

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