Soluble salts can accumulate when the application of inorganic or organic fertilizers to soils is accompanied by dry conditions, irrigation water high in soluble salts is used and/or the amount of water received at a site is inadequate to flush salts from the root zone. The result is saline soils. When salt sensitive plants are grown on saline soils plant injury can occur. In Pennsylvania saline soils are rarely encountered because soluble salts typically do no accumulate as rain, snow and other precipitation events effectively flush soluble salts out of plant root zones. However, we have heard from growers and experienced saline soils with the use of stationary, 4-season high tunnels. In this case, the protective plastic covering high tunnels excludes precipitation and leaching of soluble salts which can then accumulate in plant root zones. High tunnels are also often equipped with drip irrigation systems. These systems use water efficiently; however, the small amount of water used does not promote leaching of salts.
Plant response to salinity levels
The table below illustrates the effects of salinity level on various vegetable crops.
|Salinity Level (mmhos/cm)||Effects|
|Less than 0.40||Negligible salinity; salt sensitive cultivars of beans and carrots my exhibit effects|
|0.40-0.80||Very slightly saline; 25-50% decrease in yields of carrots, onions, peppers, lettuce|
|0.81-1.20||Moderately saline; seedling injury possible; 25-50% decrease in yields of broccoli, potatoes|
|1.21-160||Saline; beets tolerant|
|Greater than 3.2||Very strongly saline|
*Adapted from the Agriculture Analytical Services Laboratory, The Pennsylvania State University; based on a 1:2 soil:water test.
In addition to affecting yields, salt injury symptoms include necrosis (burning) of leaf margins, stunted plants, wilting and in severe cases, plant death.
Salt sensitive plants are less able to uptake water from saline soils and can become water stressed. Another way plants are injured is when sodium, chloride or sulfate levels become toxic and cause nutrient imbalances with potassium and calcium.
Salt tolerant plants respond to saline soils differently than salt sensitive ones. Some salt tolerant plants simply do not uptake excess salts. Others uptake excess salts, but then excrete them through leaves. Still others, store excess salts in cells (in vacuoles).
Testing for Soluble Salts in Soils
Penn State’s Agricultural Analytical Services Laboratory will test the soluble salt level of soil samples. To request this, check “soluble salts” under the “optional tests” box and submit payment. Currently the cost is $5.
Electrical conductivity (EC) testing
EC testing is used to determine salinity. Pure water (salt-free water) does not conduct electricity well. As the salinity of water increases, so does its electrical conductivity.
Soluble salt levels are usually expresses as decisiemens per meter (dS/m) or millimhos per centimeter (mmhos/cm). These two units are equivalent and used interchangeably. For example, 0.40 dS/m equals 0.40 mmhos/cm.
Before interpreting results it is important to match the interpretation with the EC method used. The Agriculture Analytical Services Laboratory uses the 1:2 soil:water method. Other methods are the 1:1 soil:water method and the paste method.
Soluble Salts (Conductance) Interpretation for Soils (based on using the 1:2 water:soil EC method)
Source: Agricultural Analytical Services Laboratory, The Pennsylvania State University
|<0.40||Salinity effects mostly negligible, excepting
possible beans and carrots
|0.40-0.80||Very slightly saline; but yields of very salt
Sensitive crops such as flax, clovers (alsike, red), carrots, onions, bell peppers, lettuce, sweet potatoes may be reduced by 25 to 50%.
|0.81-1.20||Moderately saline. Yield of salt-sensitive crops restricted. Seedlings may be injured. Satisfactory for well drained greenhouse soils. Crop yields reduced by 25 to 50% may include broccoli and potato plus the other plants above.|
|1.21-1.60||Saline soils. Crops tolerant include cotton, alfalfa, cereals, grain sorghum, sugar beets, Bermuda grass, tall wheat grass and Harding grass. Salinity higher than desirable for greenhouse soils.|
|1.61-3.20||Strongly saline. Only salt-tolerant crops yield satisfactory. For greenhouse crops leach soil with enough water so that 2-4 quarts (2-4L) pass through each square foot (0.1 m2) of bench area, or one pint of water (0.5 L) per inch (15 cm) pot; repeat after 1 hour. Repeat again if readings are still in high range.|
|>3.2||Very strongly saline. Only salt-tolerant grasses, herbaceous plants, certain shrubs and trees will grow|
Instructions on how to test soil pH using pH strips and a soil-water slurry
Supplies needed: distilled or deionized water, soil sample, paper cup, pH strips
- Place 5 g of air-dried soil into the paper cup.
- Add 5 mL of distilled or deionized water to the cup.
- Stir for about 15 seconds and let stand for about 30 minutes.
- Place pH strip into the slurry and read within 30 seconds.
*Adapted from: Eckert, D. and J. Thomas Sims. 1995. Recommended soil pH and lime requirement tests. P11-16. In: J. Thomas Sims and A. Wolf (eds.) Recommended soil testing procedures for the northeastern United States. Northeast Regional Bulletin #493. Agricultural Experimental Station, University of Delaware, Newark, DE.