An Index of Salinity and Boron Tolerance of Common Plants

By: Mark McFarland, Tony Provin, Larry Redmon, Diane Boellstorff, Jennifer Peterson, Alyson McDonald, Larry Stein, Benjamin Wherley and Casey Reynolds

Salinity and Boron Tolerance of Common Native and Introduced Plants in Texas

Salinity is a measure of the total dissolved salts in the root zone of a crop (soil salinity) and in irrigation water. It affects plant growth in two ways: osmotic stress and specific ion toxicity. Increased salt concentration in the soil water reduces a plant’s ability to absorb water, resulting in an osmotic or “drought” stress, even when there is sufficient soil water. High soluble salt concentrations can also cause toxic amounts of some ions such as boron, chloride, sodium, and sulfate to accumulate in the plant.

Plant species vary widely in their tolerance to salts in soil and irrigation water. In most plants, salt tolerance is lowest at germination and during early seedling establishment. Tolerance levels can also change with soil and environmental conditions. Minimizing the salt stress on plants, particularly during early development, promotes successful establishment and growth.

Salt tolerance can be a good indicator of drought tolerance. More salt-tolerant plant species are typically better suited to situations where irrigation is limited and are often more resistant to drought conditions.

Some species do not require any supplemental irrigation. Others can tolerate higher salt levels in irrigation water applied to the soil, but not when applied to the leaves (sprinkler irrigation). In such cases, plants can be watered with drip, soaker hose, or furrow irrigation. Selecting more salt- and drought-tolerant plant species can substantially enhance water conservation for landscapes.

About the Index

While the salt tolerance level of some plant species is known, there is limited or no information for many species. Data for this publication came from a wide range of sources, including early research reports. Salt tolerance limits listed in the index are intended as a general reference for selecting plant species for agricultural and horticultural applications. Actual plant response at a given location will vary, depending on the specific composition of the water applied, the soil or growth media, and environmental conditions.

This index contains thresholds for both soil salinity and irrigation water salinity. In addition, thresholds for boron in irrigation water, one of the more common toxic ions, are provided when available. Plant species are grouped by crop type (such as trees, ornamentals, field crops) and tolerance level (such as very sensitive, moderately sensitive, and tolerant).

The list includes species with reported values for soil and water salinity thresholds. Due to limited data, most of the irrigation water salinity thresholds are estimates based on the assumption that evaporation and transpiration that occurs after the application of irrigation water can increase soil salinity by twofold or more. However, changes in soil salinity as a result of irrigation can differ substantially, depending on soil properties and environmental conditions.

Because salt accumulation occurs more rapidly in fine-textured, poorly drained clay and loam soils compared to sandy soils, use estimated irrigation thresholds only as a general guide. If proposed irrigation water approaches the salinity threshold value, manage soils and add sufficient water to leach salts and limit their accumulation in the root zone. This may include adding soil amendments such as gypsum. Test the soil every year or even more often to monitor salt accumulation and guide decisions about irrigation and soil amendment needs.

To search this index for a particular species, press control+f on a PC (or command+f on a Mac) and type a common or scientific name. There is also a table with a full listing of the plant species in alphabetical order by common name.

Throughout the tables, common names in bold text are hyperlinked to the United States Department of Agriculture Natural Resources Conservation Service PLANTS Database, which provides additional information about taxonomy and distribution. Place the cursor on the plant name and press control+click on a PC or, on a Mac, click on the name. This index will be updated as new data and information become available. To report errors or omissions, please call (979) 845-3041.

Because the units that different laboratories use to report salinity vary, you may need to convert these units in order to compare values. This publication reports soil and irrigation water salinity in units of decisiemens per meter (dS/m). Table 1 shows simple conversion factors to use to compare laboratory water salinity and soil salinity results to the suggested thresholds.

Water and Soil Testing

In this index, electrical conductivity of irrigation water or soil is a primary indicator of the impact salinity has on plants. However, electrical conductivity evaluates only the potential osmotic effects of salts. Individual ions in water can be toxic to plants and the balance of positively charged ions or cations (usually dominated by calcium, magnesium, potassium, and sodium) and negatively charged ions or anions (usually dominated by bicarbonate, chloride, and sulfate) is important. You will need a complete irrigation water analysis to fully assess the suitability of a water source. When the total salts or an individual ion are elevated, consult a water professional to interpret the test results and determine whether a water source is appropriate to use for irrigation.

Test water wells at least annually, especially during extended dry periods, since the water quality in a well can change because of pumping demand and limited recharge. Aquifers, lakes, ponds, and rivers subject to rapid changes in depth or flow can experience significant changes in water quality over short periods. More frequent testing may be needed to monitor water quality and make appropriate management decisions.

The Texas A&M AgriLife Extension Soil, Water and Forage Testing Laboratory (SWFTL) can analyze irrigation water for agricultural and urban use. Forms and information for water sampling and testing are available on the laboratory website at https://soiltesting.tamu.edu.

Salts from irrigation water, fertilizer and manure, and decomposed organic matter can all increase soil salinity if leaching is limited and the net removal of nutrients is less than the inputs. Homeowners and land managers should have annual soil tests done for landscapes and fields that receive irrigation water with elevated salinity. A complete, standard soil test includes a measurement of soluble salts. A basic salinity analysis is part of the routine test offered by the SWFTL; a detailed analysis is also available if requested.

For more information regarding managing salts in irrigation water, see:

Fipps, G. 2003. Irrigation Water Quality Standards and Salinity Management Strategies. B-1667. Texas A&M AgriLife Extension Service, College Station, Texas. http://soiltesting.tamu.edu/publications/B-1667.pdf.

Provin, T. L. and J. L. Pitt. 2012. Managing Soil Salinity. E-60. Texas A&M AgriLife Extension Service, College Station, Texas. http://soiltesting.tamu.edu/publications/E-60.pdf.