Excessive Phosphorus in the Soil

When I attended a class on growing tomatoes earlier this month, registration included a free soil sample. Not one to turn down free gardening stuff, I took soil from seven of my fourteen beds and mixed it together for my sample. I just received my test results and discovered that I have excessive phosphorus (and calcium) in my soil! What happened?? And what do I do about it?

I have not added any phosphorus in my soil since I first began gardening – other than the tiny bit that is in the fish emulsion I use. After speaking to a few local farmers, I found that the soil in the Charleston, SC, area is notorious for its phosphorus levels. I also learned that the cow manure compost I’ve been adding to my soil could be increasing the phosphorous, as well. And that may explain some of the symptoms in my plants lately.

Here are some of the symptoms of excessive phosphorus in soil:

  • Increased weed growth (I surely have that!)
  • Stunted plant growth (got that, too)
  • Harms beneficial root fungi, which help the plant absorb water and nutrients
  • Decreases the plant’s ability to uptake zinc (deficiency shows as bleaching of plant tissue)
  • Decreases the plant’s ability to uptake iron (deficiency shows as yellowing between leaf veins)

The last two items are especially applicable when acid-loving plants are growing in neutral to alkaline soils. My soil pH is 6.9, so I’m right there.

Soil Test Results

My Soil Test Results

Phosphorus does not move in the soil as nitrogen does, so its staying power is higher. Because of this, it can stay in the soil in excessive amounts three to five years. So, what do I do??

Here are some suggestions I found on various university websites:

  • Avoid using fertilizers with phosphorus. (My test results suggested I use 15-0-15 fertilizer)
  • Avoid using manure composts
  • Add more carbon (“brown”) items to the compost you make yourself; this will balance the compost
  • Plant nitrogen-fixing plants in the garden (like beans & peas)
  • If seeing symptoms of zinc & iron uptake deficiency (see above), apply a foliar spray of 0.5%-1.0% zinc/iron solution every one to four weeks, depending on severity of symptoms. (Because phosphorus affects the uptake of zinc and iron through the roots, foliar application is the only means of getting the micronutrients to the plant.)

My test results also stated that I have an exorbitant amount of calcium in my soil! I’m thinking my soil sample contained a piece of eggshell from my compost pile. Regardless, I guess I won’t be adding gypsum when I transplant my tomatoes!

I’m glad I had the professional soil test done. It showed me what I need to address immediately (the phosphorus & calcium), as well as confirmed that my other nutrients are within proper parameters.

A gardener’s work is never done. But isn’t that the joy of it?



13 comments on “Excessive Phosphorus in the Soil

      • I wonder where it came from. This soil garden is only one year old. We lost a huge oak tree and with all the new sun I have been planting vegies. Never have brought in manure. Even the raised beds, which cover the old trunk, have excessive phosphorous. They were filled with my soil plus purchased soil.

        Do you water with city water or well water?

  1. After speaking with someone from Clemson about my soil test results, it seems the excessive phosphorus in my soil has nothing to do with me or what I’ve done/not done in my garden. The soil in the Charleston area is known for its excessive phosphorus. Go figure.

  2. Living in Eastern NC, across the river, from PCS Phosphate, it is obvious that the area is rich with phosphorus. Soil test in my raised bed garden are off the chart in phosphate. Who know? We have created our beds with the native soil, compost (yearly) and composted cotton gin trash (what is left after everything else is extracted from the raw crop (this we now just added between the mounds on a 2 year rotation).
    Leaves, then straw are also added between rows and around base of the plants as a weed mulch and for water retention.
    Going to try the recommended foliar spray (zinc and iron) treatment and the use of only 20-0-0 fertilizer.

  3. I live in Aiken SC and also have excessive phosphorus and calcium in all 4 of my soil samples measured by the Clemson Extension. I found your post while searching online about the issue. It’s been a while since your original post; have you had any success in any changes you’ve done in caring for your garden? What’s worked and not worked for you?

    • Hi, Ashley. Knowing I have lots of phosphorous and calcium in my soil has made me more conscious of the amendments I use. First, I go easy on the cow manure compost, which can increase the phosphorous of the soil. I use a compost that has added cow manure, rather than a 100% cow manure. I also use only organic fertilizers, which have a low NPK number (including that phosphorous) and are slow-release.

      For the calcium….that has been great for my tomatoes. No blossom end rot…ever! However, I’ve learned that the large amount of calcium in relation to magnesium in the soil can make it difficult for the plants to absorb the magnesium. So, I use an epsom salt foliar spray (bypassing the root absorption system) every two weeks. If you look up “foliar sprays” in the search button of my site, you’ll find all my recipes, including the epsom salt one.

      I’m still learning myself how to deal with soil issues, so this may not be my final take on the subject. But, so far, plants have been growing. Maybe not as quickly or as healthy as I would like, but again…I’m still learning.

      Five weeks til spring and counting! YEAH!

      Thanks for stopping by.

  4. THANK YOU! I have been trying to find a good list of what to do/not do. Yours is the first that truly made sense that was also from someone who is knowledgeable. (I live in NC.) If you see this…does adding nitrogen actually help lower phosphorus, or is it just to help the nitrogen level? I do have LOTS of baptisia seeds, so I am happy to sow those everywhere. 🙂 Thank you so much for this post!!

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