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Plants stop growing, roots gone #880811

Asked August 09, 2024, 10:27 AM EDT

I have two plots, both with full sun on a slight slope. Both have lots of clay that I'm conditioned with gypsum and worked, so they both drain very well. Both are very fertile. I've havdsoil tests done and they are both high, even excessive, in phosphorous, potassium and magnesium. The pH is 7.5, a little high. My problem is that plants struggle on one half of one of the plots. Whether I plant plants or from seed, the plants will start out fine, but about halfway through the summer, they will wilt and die, or remain green but just stop growing. When I pull them at the end of the season, I'll find that the roots are mostly gone. Even weeds are less robust in that area. I'm attaching a photo from a previous year (drier). You can see the problem in the foreground. In the background you can see the better half of that plot where the weeds are doing better. Interestingly, I planted garlic there last October and harvested by mid-Summer. They were fine. It's just the plants that I start in the Spring and plan to harvest throughout Summer or later that have the problem. Any ideas? Bugs? Fungus? I don't notice anything in the soil. I have lots of composted cow manure that I've been giving both plots every year, but maybe I'll back off next year due to the high levels of nutrients I've got now.

Addison County Vermont

Expert Response

Tom:  Thank you for contacting the UVM Extension Master Gardener Helpline and describing the problems in your vegetable garden.

Based on this information, I would advise you to stop all applications of composted cow manure, and also any application of fertilizer, other than fertilizer containing nitrogen only, until the excessive levels naturally come down. 

The use of too much manure-based compost can cause the excessive concentrations of Potassium, Phosphorus, and Magnesium that are showing up in your soil test, as well as the higher than optimal PH reading. 

You may also want to plant a cover crop this fall, so the plants will start to take up the excess nutrients.  In the spring the cover crop can be mixed into the soil, increasing the organic material content of the soil. (The addition of organic material into clay soil is the preferred method of treatment.)

When looking for a nitrogen only fertilizer (such as blood meal) the first of the three numbers listed on the package is Nitrogen, the second and third numbers are the amount of Phosphorus and Potassium.  The second and third numbers should be zeros, such as 13-0-0, etc., meaning that there is no phosphorus or potassium, only nitrogen.

Soil compaction may also affect plant roots.  Try not to walk in the garden when the soil is wet.  Also, never till wet soil. I couldn't tell from your photo if soil compaction is a problem, but just something to keep in mind.

The optimum PH for garden vegetable plants is around 6.5.  Normally, to lower soil PH, the addition of a sulfur-based soil acidifier is recommended.  However, it is not recommended for clay soils, due to the danger of excessive salt build-up.  This is further discussed in the fact sheet from the University of Minnesota Extension (link attached below).
 

How to correct problems caused by using too much compost and manure | UMN Extension

I am also attaching links to various University Extension fact sheets discussing clay soils, and soil fertility.

Gardening in Clay Soils | USU

Managing Garden Soil Fertility.pdf (uvm.edu)

Soil and Plant Nutrition: A Gardener’s Perspective - Cooperative Extension: Garden and Yard - University of Maine Cooperative Extension (umaine.edu)

I hope you find this information helpful. 

Susan, UVM Extension Master Gardener Volunteer Replied August 11, 2024, 4:18 PM EDT

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