LSU AgCenter's Weekly Message

Troubleshooting Raised Bed Problems: Part 1

Many of us grow vegetables and other plants in raised beds. They’re helpful where drainage is poor. Even where it isn’t, they can make weed management easier and reduce the amount of bending required to grow a garden.

However, raised beds are not always without problems. I’ve had a couple of calls recently from people who had, I suspect, used too much high-nutrient material and ended up with beds that had excessively high salt contents. When we speak of salt in this context, we’re not talking about table salt but about many types of ions, including plant nutrients, that can contribute to salt stress for plants.

The materials used in a raised bed also affect pH. Most vegetables do well between pH 6.0 and 6.5, so gardeners should be cautious about materials that are much more acidic or alkaline.

When building a raised bed, it’s helpful to include some organic materials that have high levels of plant nutrients, but consider the amount that you’re using. Composts vary widely in their contents. Those made from animal manures sometimes have high nutrient and salt contents, while those made from yard waste often have relatively low nutrient levels. (Manure-based composts also bring to mind microbe-related food safety, but I won’t try to address that here.) Mushroom composts tend to have high salt contents and high pHs. These types of materials can be good in moderation, but they usually need to be blended with other, low-nutrient materials.

When people run into problems with their raised beds, they sometimes send the substrate for testing as a routine soil sample. The process used to test soil was developed for field soil (mixtures of sand, silt, and/or clay, with a small amount of organic matter). When these procedures are used on mixtures made up primarily of organic materials, as most raised bed substrates are, the results do not accurately reflect what the plants experience. The nutrient contents shown in these soil reports from raised beds are often much higher than what the plants will be exposed to in the short term.

The LSU Soil Testing and Plant Analysis Laboratory suggests sending substrates that are less than 20% field soil as potting mix samples instead of soil samples. Routine potting mix samples cost $15 each instead of the $11 it costs for a routine soil sample, and the test used requires a greater volume of material.

Potting mix sample reports include results for pH and for concentrations of nitrate, phosphorus, potassium, calcium, magnesium, and soluble salts. The reports provide interpretations of the results, such as “low,” “very high,” and “optimum,” which are helpful when troubleshooting. However, they don’t provide recommendations about how to fertilize, so you may need to contact us for guidance.

In the next article, I’ll write about a couple of other issues people encounter.

Let me know if you have questions.

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Dr. Mary Helen Ferguson is an Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu or 985-277-1850 (Hammond) or 985-839-7855 (Franklinton).