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How to reduce Phosphorus and Magnesium levels in garden soil #870782

Asked May 30, 2024, 2:18 PM EDT

My soil test shows 'excessive' levels of both Phosphorus and Magnesium, and I'm wondering how to reduce these to an acceptable level. I see there are methods of increasing both of these, but I'm not seeing how to reduce them! Thank you for your help - Beth

Chittenden County Vermont

Expert Response

Hi Beth,

Thank you for submitting your question to the UVM Master Gardener Helpline. You have quite a bit of organic matter in your garden and perhaps the high phosphorus is from use of manure? High phosphorus is most concerning with large farms that have runoff into a waterway. Excessive phosphorus in surface water can cause explosive growth of aquatic plants and algae, decreasing dissolved oxygen concentrations, which can harm aquatic life. In the home garden away from waterways it is less of a concern, but it is still reasonable to try to reduce this. To reduce phosphorus avoid using manure composts and fertilizers that contain phosphorus.
Growing beans or peas or fall legume cover crops helps increase nitrogen in the soil and decreases phosphorus levels.

The high Mg is generally less of an issue. Do you add Epsom Salts (Mg) to your crops (some use this for peppers)? If so, don't use this. Are you noticing stunted growth or browning on the leaf edges of your plants (a sign of excess Mg which can deplete micronutrients)? If not, I would not be concerned. Plants are tolerant of a fairly wide range of Ca, Mg, and K in the soil. Gypsum decreases Mg levels, decreases compaction, and enhances soil
permeability without affecting pH. In vegetable gardens, the recommendation is 20-30lb granular gypsum per thousand sq feet of soil. Spread this on top of the garden bed and water. The addition is supposed to be good for 3 years and results improve each of the three years from this one addition.The best time to use gypsum is when a bed is being prepared before planting.

Also leaves used as mulch will add phosphorus and magnesium. Pine bark would be a better mulch and could also lower the pH of the soil.

I am enclosing some references that should be helpful. One is a question very similar to yours.
https://ask2.extension.org/kb/faq.php?id=749014
https://www.uvm.edu/sites/default/files/Extension-Community-Horticulture/InterpretingSoilTests.pdf

Happy gardening!

Betsy, UVM Extension Master Gardener Volunteerwww.uvm.edu/extension/mastergardenerWe do not intend to endorse any products we mention or criticize products we do not mention.When we include a link to a resource that recommends a chemical, it is for informational purposesonly; the chemicals may not be registered for use in Vermont. Replied May 31, 2024, 5:24 PM EDT

Thank you, Betsy! This is very helpful.

Before sending off my soil sample, I'd already chosen not to use manure this year (I usually get it from the neighbors' horses, but it always gives me an annoyingly abundant crop of sedge to have to weed out...), so that was a good move apparently!

I'm growing peas and beans, and will do a cover crop at the end of the season (neglected to do that last year).

Re: magnesium - I don't use Epsom salts/grow peppers, so I'll look in to applying gypsum.

One factor that maybe is affecting the levels of nutrients is the presence of Snake/Jumping Worms for the past 3 years. I treated my vegetable beds with Tea Seed meal last year, and so far I'm only seeing a few small, sluggish (i.e. not wildly thrashing!) worms, so maybe that helped a bit. I'll know later in the season when I usually notice the big ones in abundance. Ugh.

Again - thank you for this guidance - Beth

On 05/31/2024 5:24 PM EDT Ask Extension <<personal data hidden>> wrote:

The Question Asker Replied June 01, 2024, 8:07 AM EDT

My pleasure, Beth!

How to reduce Phosphorus and Magnesium levels in garden soil #870782

Expert Response

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