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.
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?