Salts are naturally present in our soils, and in small amounts, do no harm. It’s when the percentage of salt increases that plant growth is affected. Soils containing enough salt to retard plant growth or kill plants are called “saline.” Such soil is often found in low-lying areas with poor drainage. It occurs in areas where water collects, bringing in the salt either as surface runoff or groundwater. But the water can only leave via evaporation, leaving salts behind that build up over time. When these soils dry out, they are characterized by a white crust. In the same way that our skin will dry out when we swim in the ocean, plant tissue will dry out in the vicinity of salt.
It’s notoriously difficult to grow trees and shrubs in the vicinity of a septic tank mound because of the accumulation of salts. They grow slowly, are of a smaller size, and wilt quickly when under stress. Leaf burn is common. It took me ages to figure out why bur oaks and Siberian crabapples languished and died near my septic mound, while sea buckthorn never even blinked and continued to sucker rampantly.
Applying too much fertilizer or irrigating with water with a high salt content also contributes to the accumulation of salts in soil. Chemical fertilizers are, after all, salts, and should be used in moderation. If using well water, it pays to check its salinity before applying it to your garden. Adding manures with a high salt content should also be avoided.
If you have areas of saline soil, there is no “magic cure” or chemical that will make them better. It is difficult to remedy soils that are naturally high in salt. But there are some practices you can follow that will lessen their effect on your plants.
Begin by growing plants that are salt tolerant. If they have the word “sea” in their common name, or maritima in their botanical name, that generally indicates that they are natives to areas close to the sea. And if you happen to be a plant living on a cliff overlooking the North Sea or the Atlantic Ocean, chances are that you’re being continuously sprayed with salt water, and have developed a greater tolerance to salt than a member of the same genus further inland. Think of sea holly, sea lavender and sea thrift.
Vegetables with a fairly high tolerance to salt include beets, asparagus, kale and spinach. Among the trees and shrubs with salt tolerance are false indigo bush, bearberry, sagebrush, barberry, broom, smooth sumac, Ural false spirea, sea buckthorn, Siberian salt tree, silver buffaloberry, hawthorn, Russian olive, elms, and villosa lilac. Trees and shrubs with moderate salt tolerance include Manitoba maple, caragana, wolfwillow, green ash, Tartarian honeysuckle, and chokecherry.
Leaching salts out by improving drainage is possible but often difficult and expensive and may involve the use of drain tiles. Not always an option! But preventing plants from drying out by keeping the soil moist is an option. Avoid frequent light waterings. Instead, irrigate deeply but infrequently. The object is to dissolve the salts and then leach them down into the soil profile, out of reach of plant roots.
Incorporate organic soil amendments that are low in salt such as peat moss, compost, or grass clippings. These materials act in the opposite manner of salts, helping to increase the soil’s moisture holding ability. In effect, by increasing the amount of water held by the soil, they are diluting the salt concentration within the soil.
This can be tricky. One needs to ensure adequate water is available, as the salts make it harder for plant roots to extract water. But too much water will prevent good soil aeration and increase the loss of nitrogen.
Increasing the height of the planting beds also allows better drainage. In vegetable gardens this can be done through traditional raised beds. In flower and shrub borders, try “berming”, adding additional soil (well amended with organic matter) to a height of one to two feet. Remember, adding height also increases the exposure of the soil and plants to wind and drying out, so monitor the soil moisture carefully and irrigate when needed. Mulching the soil surface with an organic mulch reduces surface evaporation considerably.
Sara Williams is the author and coauthor of many books including Creating the Prairie Xeriscape, Gardening Naturally with Hugh Skinner and, with Bob Bors, the recently published Growing Fruit in Northern Gardens. She continues to give workshops on a wide range of gardening topics throughout the prairies.
This column is provided courtesy of the Saskatchewan Perennial Society (SPS; firstname.lastname@example.org ). Check our website (www.saskperennial.ca) or Facebook page (www.facebook.com/saskperennial) for a list of upcoming gardening events.