We all know that it’s a bad idea to pour salt on the ground in our gardens. After all, that’s what invading armies did—they salted the ground, effectively sterilizing it and therefore starving the population. Even the ubiquitous recipe for “Homemade All-Natural Weed Killer”—you know, the one with salt, vinegar, and Dawn detergent (and since when is Dawn “all natural”?)—warn against using the concoction where you want other plants to grow. Salt in the soil is bad news for gardeners.
Yes, you wouldn’t intentionally spread rock salt on your dirt, but that’s only one way to end up with soil too salty to support plants. There are other, more insidious ways to salt your soil. Natural Salinity.
Unfortunately for us gardeners, some soils are just naturally salty. The salts may have come from weathered rocks, or have been left behind by evaporating lakes and seas. Areas of low rainfall are more likely to have soils high in salt.
Manures are often used as soil amendments. They’re typically inexpensive and readily available. But if those manures come from feed lots (which is frequently the case), you could be buying a whole lot of salt with your cow poop. That’s because the grain fed to cattle in feedlots is very salty. (If you want to know why, check out this article in Feedlot Magazine.) That salt passes through the animals and ends up in their manure.
In one study[i] by the horticulture department at Colorado State University, Fort Collins, packaged dehydrated manures were tested for salt content. The results for the steer manure were shocking—if you added just the recommended amount—four inches deep over the soil—and till it in, you have just added enough salt to raise the soil salinity to the highest safe level. If you do that the next year as well, far from improving the health of your plants, you may doom them. The bottom line here is to avoid buying dehydrated steer manure. (Cow manure from dairy operations is much lower in salt but may have high copper levels, as copper baths are used to treat hoof diseases.)
Fertilizers. We add fertilizers to our soil in order to provide essential nutrients for our plants. But did you know that in addition to that nitrogen, phosphorus, and potassium, you’re also adding salt? Nitrogen sources such as ammonium sulfate, ammonium nitrate, and urea are particularly high in salts, and repeated applications can increase the salinity of your soil. Before adding any fertilizer, a soil test is highly recommended, as it can tell you exactly what your soil needs for maximum fertility. Too little or too much can hurt your plants.
If you’re growing—and fertilizing—plants in containers, it’s a good idea to occasionally flush the potting mix to wash away accumulated salts.
Even making your own compost has potential hazards. Be sure to avoid adding salty ingredients (such as leftover cooked and salted veggies) to your pile.
No one wants icy sidewalks or driveways, and there are a number of products on the market to melt that slippery surface. However, many of them will not only melt the ice, they’ll also kill your plants. Rock salt has long been used on roadways, but it not only rusts cars and bridges, it kills anything growing along the roadside.
Now, other products have begun to replace it, such as calcium chloride and magnesium chloride. Of these two, the latter is more garden-friendly. While magnesium is an essential micro-nutrient, too much magnesium chloride quickly becomes toxic.
If you have paved surfaces next to valued plants, consider using sand for traction instead of a de-icer. Or choose plants more tolerant to salt for those areas.
To be continued…
[i] As yet unpublished when it was discussed in a class for continuing Colorado Master Gardeners, 2010.