Soil salinity secrets are once again dissolved thanks to Dr. Grant Cardon, our USU Extension Soil Specialist. Grant digs deep into plant tolerances of soil salinity in his most recent edition of Dirt Diggers Digest. Here is his post and stay hydrated out there – it’s hot!
Plant Tolerance to Soil Salinity
In my December 2017 post (see extension.usu.edu/dirtdiggersdigest) I discussed soil salinity and why we deal with saline soils and waters throughout the western U.S. In this post I want to go a little more in depth regarding the tolerance of plants (especially crops, turf, ornamentals, fruits, vegetables, etc.) to saline soil, and provide some good sources of guidance information on the subject.
As I previously discussed, salts introduced into the root zone of any plant, establish an osmotic restriction to water movement into the plant root. Some plants have the ability to tolerate this condition via two primary mechanisms, namely, “osmoregulation” or “salt exclusion.”
Without going into too much detail, osmoregulation refers to the ability of the plant to regulate the concentration of solutes inside the root to overcome fully or partially, the osmotic restriction. Salt exclusion is the ability of the plant to take up the salt with the water and then compartmentalize internally, or even purge excess salt, thereby reducing negative effects on metabolism. Either mechanism “costs” the plant energy otherwise available for growth, hence the plant stunting effect at low to moderate soil salt levels.
The ability, or lack thereof, of any given plant to overcome soil salinity means that there is wide variation in plant salt tolerance between species and even among cultivars within a species. Beginning in the 1950’s and 60’s, much work was (and continues to be) devoted to categorizing the salinity tolerance of plants in order to provide selection criteria for the use of a given plant in a specific soil salinity condition. I have provided below, links to some good summaries for various plant types (crops, veggies, fruits, turf and ornamentals) that one can use to make informed selections for their conditions.
In the sources I have noted, plant tolerance to soil salinity is described by two measures, namely, the salinity threshold value at which a given plant begins to suffer salt stress, and a slope factor that indicates how severely the performance of the plant is reduced by additional increases in soil salinity above the threshold. A tolerant plant generally has a higher threshold and/or smaller slope value. The measure of salinity is deci-Seimens per meter (or dS/m), which is a measure of the electrical conductivity of the saturation extract of a soil, and is a surrogate measure of the amount of salt dissolved in the extract. The higher the conductivity, the higher the salt content.
To illustrate, sweet corn, which is a moderately sensitive plant, has a threshold value of 1.7 dS/m, and a slope value of -11.5, meaning sweet corn production would decrease 11.5% per unit of salinity above the threshold value. On the other hand, beets, a relatively tolerant vegetable, has a threshold value of 5.3 dS/m and a slope value of -7.3. Not only does beet have a higher threshold, but also a much smaller slope, indicating the beet’s ability to tolerate more salinity (NOTE: these values are all contained in the document below entitled: Salinity and Plant Tolerance). As part of a “routine” soil test provided by the USU Analytical Laboratories (see link below), salinity is determined for a soil sample and provides the information one needs to make informed choices of the types of plants that will do well in that specific condition.
If soil salinity values are too high for the types of plants one wishes to grow, the only way to reclaim the soil is through leaching, or washing soluble salt out of the root zone. This requires flushing large quantities of water with a low salinity content, through the soil. The rules of thumb for salt removal are that 50% of the soluble salt can be flushed from the top foot of soil (generally good enough for vegetables, turf, and flowerbeds) if six inches of clean water is applied over the saturation point of the soil. A reduction of 80% is possible with twice that amount. Note that for trees and ornamentals, which have deeper root zones, one may need to leach the top two feet of soil, requiring proportionally more flushing.
A reduction of 50% as noted above is often sufficient to reclaim most moderate saline soil conditions. Please note that this is a reclamation process and is required only periodically if/when soil salinity is too high. Adding a small amount of excess water (15-20%) with each irrigation will help maintain a lower salinity value.
Flushing the soil requires there be plenty of free drainage, meaning infiltration may need improvement through the opening of the soil and amending with organic matter to improve aggregation and soil structure (see the March 2018 and October 2017 Dirt Diggers Digest posts that discuss soil structure and organic matter amendments). Free drainage also requires that the water table be sufficiently deep so that leach water will not raise the local water table within 6 to 8 feet of the soil surface. If the local water table is higher than that, water will continue to wick up to the soil surface and carry dissolved salts back into the root zone. Such situations may require installation of an artificial drainage system to remove excess water.
If one suspects they have a salinity issue–often expressed as chronically stunted plants or leaf burning/drying along the margins or edges of the leaves (or in extreme conditions, completely dried leaves or stems)–then have the soil tested, choose plants that are tolerant of the given condition, and/or take remediation steps to reduce soil salinity levels.
Salts are a natural part of our arid zone soils, but they can be managed—good results always start with good information!
Salinity and Plant Tolerance – USU Extension fact sheet that contains tables of salinity threshold and performance reduction values for a variety of vegetables, turf, fruits, nuts, forages, field crops, flowers and ornamentals commonly grown in Utah: https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1042&context=extension_histall
Soil Salinity and Ornamental Plant Selection – USU Extension fact sheet that contains an extensive salt tolerance classification list for ornamental shrubs and trees: https://digitalcommons.usu.edu/extension_curall/1079
USU Analytical Labs: https://usual.usu.edu