Are there salinity problems for urban soils?

Salt. Some of us cannot get enough of it on our food. Think of a potato chip without salt, it’s a very bland thing! Others avoid salty foods due to health issues and are a bit more disciplined keeping their intake low.

In a similar way, plants have varying salt tolerances. There are desert plants that have special adaptations allowing them to live soils with high salt content. These soils may have the visual appearance of salt on the soil surface and lack soil structure (i.e. small aggregates of particles) that is crucial for water and air flow. On the flip side, many of our common home and garden plants like Japanese maples are very sensitive and will be affected at even low salt values. However, for these conditions the soil physical properties are not affected.

shovel with soil that has white substance - salt - on it
Salt (white substance) contaminated soils from a pine-oak woodland in California. Credit: Robert Zasoski

Salt levels in soil are referred to as “salinity.” For humans, we think of salt as table salt (aka sodium chloride), but salts are made up of many more compounds than just those. You can have salts made up of calcium, magnesium, or potassium that are paired with chloride or other ions like sulfate. Some salts are nutrients that plants need. We apply these salts as fertilizers in our yards to help plants grow. Addition of these nutrients can be very helpful, but only to a certain level.

High salinity in soils can negatively impact plants. It’s the same as with humans. Some people can age well and still tolerate eating salty potato chips, others cannot. The Japanese maples cannot tolerate high salinity, but other plants like oleander can.

The signs that plants are affected by high salinity in soils show when they become stunted in size. This is due to the decrease in energy the plant has for growth. The plant is stressed. Instead of growing new leaves, it is allocating more of its resources dealing with the excessive salt intake.

With new plants in your yard, you may not notice this decrease in plant size until several years, when additional symptoms begin to show. Other common symptoms include blue-green tint to the leaves and eventually the leaf tips will appear burned. Alternatively, some plants may have younger leaves that are yellow or appear to be wilting despite adequate watering. If the soil condition isn’t corrected, the plant will eventually throw in the towel and you’ll be heading to your local garden store for a new plant.1

Where does salt in the soil come from? Well, soil comes from rocks and organic materials (e.g. decomposed plants). As these materials form soils through weathering, they will create “salts” found in the soil. The soil can replenish its salt content by these natural processes over long periods of time. In urban areas with gardens and lawns, we typically have plants that were not adapted to this slower nutrient “resupply” and we have to replenish the soil with fertilizers to keep the plants happy. Both synthetic and organic fertilizers have the capability to add too many salts to the soil, so it is important that you apply appropriate amounts to your soil.

In addition to fertilizers, there are other sources of salts that are found in urban areas.

Yellow tips of cholla cactus with mountains in background
Choosing plants – such as cholla cactus – that fit into your landscape naturally, can reduce the need for watering and avoid salinity problems. Credit: SV Fisk

Recycled irrigation water is a great way to reuse non-potable water in our cities. However, unlike typical drinking water it contains higher amounts of salts. In the San Francisco Bay Area of California, planted redwoods that had been irrigated by municipal water in the past are now receiving recycled water. The evidence of tip burn on these trees is apparent as you drive down the freeway. With the continued use of recycled irrigation water, eventually these plants will need to be replaced with those that are more tolerant of salty soil conditions. Don’t lose hope though! Soils with moderate amounts of salts often can be corrected.

In northern climates, homeowners may inadvertently be adding to salinity problems in theirs and neighbors’ yards. In order to keep ice off sidewalks and driveways – and keep residents safe, they often turn to salt. If you do need to use salt in your yard, only spread enough to safely deal with the ice. In addition, you can use different types of salts on your ice problems. Learn more about why you should be careful with driveway salts by reading this blog.

The first step to correcting these saline soils is to diagnose the problem. If the symptoms discussed above appear on your plants and/or you know you get a little too enthusiastic with fertilizer application or driveway salt, you may have high salt content in your soil. A local soil lab test can verify this by running a salinity test. Typically, you can increase the amount of irrigation you apply to your plants to leach the salts below the root zone. Note: this will only work if you have soils that drain well. If you are in a drought-stricken area, you may have to wait for precipitation to accomplish this, which could take multiple seasons. If the issue is primarily sodium (also verified with a lab test) you may need to incorporate gypsum into the soil prior to leaching. Gypsum is calcium sulfate, and helps with managing high sodium levels.

If all else fails, choosing plants for your yard that can endure higher salt content may help with an area that is beyond correcting. Adapting the plants to fit the soil may be ultimately the best way to prevent headaches in the long run. The result? The beautiful yard you always dreamed of where you can enjoy watching the sunset while eating your low-salt potato chips.

Answered by Meagan Hynes, Teralytic

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