One of the known benefits of applying gypsum is displacing sodium (Na) in the soil with calcium so that it can leach below the root zone with irrigation or rainfall.
Why is sodium bad for plants?
Most people are aware of the damage and corrosive effects of salt on automobiles. On heavily traveled highways, 40 to 80 tons of salt may be applied per mile of highway per year. We know the effect of that salt on vegetation along highways – or lack thereof. Salt contains both sodium and chloride and plants can accumulate toxic amounts of both.
When excess sodium (Na+) ions are available in the soil, a plant will absorb them instead of more beneficial potassium (K+) ions, which is a macronutrient for the plants. It can also replace other nutrients (cations that are positively charged), making it problematic when in high concentrations and ultimately limits the growth of plants.
Curious about sodium levels in my soil, I pulled several soil samples (8 inch depth) in March from a couple fields and from my vegetable garden and sent them to a laboratory that does soil analysis. As part of their standard soil test, they measure phosphate (an anion) as well as cations (+ charge) potassium, calcium, magnesium, sodium and hydrogen (H is a measure of pH).
Over the years looking at soil tests we can measure salts but generally in field situations in the Midwest there is no measurable sodium and salts levels are generally not high enough to impact crop production. Most growers don’t irrigate and those that do, don’t use irrigation water that is high in sodium so we don’t accumulate sodium in our soils.
But when I got the analysis back from the laboratory I was surprised to see that while the field samples had no measurable sodium (that is what I expected), the garden had sodium levels of 163 ppm, or over 300 lbs per acre of sodium – very high.
The high test results on the garden soil shocked me and I began to wonder what was applied to this area over the last 35 years (house was built in 1980) that would accumulate that much sodium when generally sodium is not very measurable in Nebraska. I’m guessing it’s in the water. The house has an underground sprinkler irrigation system that has been broke for as long as we lived the house for 7 or 8 years. The source of the irrigation water is our well, so perhaps the water has high levels of sodium. We do have a water softener but I have never tested the water, so I guess it is time to see if the water is the source of sodium.
The laboratory recommended applying 0.5 tons per acre or 2 pounds per 100 square feet of gypsum. Fortunately I have about 500 lbs of pelletized gypsum sitting in bags in my garage and will spread it. Now I wonder about the lawn which I did not test and intend to do.
The sodium measured was exchangeable sodium and held on the soils exchanged complex. The only way to displace the sodium is to exchange it with calcium. The sodium will pop off the exchange sites and can then be washed deeper in the soil will water.
So how many of you have tested your lawns, gardens or flower beds for sodium? Pull a soil sample, send it to a reputable laboratory and ask them to make sure they run exchangeable sodium.
Dr. Dan Davidson posts articles on soil health and management related subjects. If you have suggestions for topics or questions, feel free to contact him at firstname.lastname@example.org or call 402-649-5919.