Soil Test Note – No. ST003

Dr. Stanley L. Chapman, Extension Soils Specialist

MANAGEMENT OF SOILS WITH HIGH SOLUBLE SALTS

There are numerous extensive summaries on salt-affected soils. This article is intended as a brief overview on soil soluble salt problems and possible solutions for continued agricultural production.

The University of Arkansas Soil Testing Laboratory began testing soluble salt levels of routine soil samples in 1994. The laboratory utilizes an electrical conductivity (E.C.) test on 1 part soil:2 part deionized water mix to measure soluble salt levels. The higher the electrical conductivity, expressed in micromhos per centimeter (umhos/cm), the higher the soluble salt content of the soil. Salt-affected soils (saline, sodic, saline/sodic – see Table 1 for classification) are more common in the semi-arid southwestern U.S. but may also occur in special situations in the mid-South as well.

Table 1. Classification of salt-affected soils.

	Soil pH	1:2 Soil:Water Soluble Salt Level^1 (micromhos/cm)	Sodium Saturation Percentage
Normal Saline Saline-sodic Sodic	below 8.5 below 8.5 below 8.5 above 8.5	below 900 above 900 above 900 below 900	below 15 below 15 above 15 above 15

¹ 1:2 soil:water E.C. calculated from E.C. (saturated paste) numbers using data from Gilmour and others

(1985) and Sriyotai and Gilmour (1976).

Salt accumulate when the rate of addition exceeds the rate of removal. Elevated soluble salt levels in Arkansas soils may be caused by 1) poor quality irrigation water, 2) excessive nutrient addition from fertilizers, manures, or waste materials, or 3) both. Excessive salts may be worse on soils that have slow or very slow permeability, poor internal drainage, and possibly high or perched water tables that restrict downward water and salt movement. Often, plant injury results from excessive salt accumulation near the seed or in the seedling root zone. Frequently, this problem is only temporary.

When soluble salt is mentioned, many think of table salt or sodium chloride. Soils with naturally-high sodium levels occur in northeast and central eastern Arkansas. It is possible to have 1) elevated sodium (above 10 percent sodium saturation of the soil cation exchange sites) with low soluble salts, 2) low sodium with high soluble salts, or 3) high soluble salts and high sodium. In most Arkansas soils and irrigation waters soluble salts are composed largely of the cations calcium, magnesium and sodium (Ca⁺⁺, Mg⁺⁺, Na⁺) and the anions bicarbonate chloride, sulfate, and nitrate (HCO₃^-, Cl^-, SO₄⁼, NO₃^-).

Soils with elevated sodium levels may be referred to as sodic. In Arkansas they include the Lafe, Foley, and Hillemann mapping units described by the Soil Conservation Service in published county soil survey reports. There is also elevated sodium in the subsoil of the Stuttgart soil mapping unit. The depth to the elevated sodium layers varies in each soil. Land leveling may expose sodium (sodic) layers that have poor soil structure, low fertility, and very low productivity. Before land leveling is initiated, extensive soil sampling and testing should be conducted while referring to a published soil survey. Failure to take such precautionary measures may result in exposure of unproductive soil layers that require costly and difficult corrective action to permit acceptable crop production. Recent research by the University of Arkansas has shown that repeated yearly applications of 1,000 to 2,000 pounds of poultry manure, fresh or composted, may help to restore some rice-producing sodic soils to an acceptable productive state. Data are also being collected for soybeans and cotton but are not conclusive at this writing.

In recent years several areas in Cross, White, Monroe, Poinsett, and Chicot Counties have been identified where irrigation wells may be delivering salty (saline) water (above 1,200 micromhos/cm) with particularly high chloride levels (above 100 ppm). Research by Dr. Darrell Widick, soybean breeder with the University of Arkansas Agricultural Experiment Station and Arkansas State University, has identified major differences in chloride tolerance among soybean varieties. His work has led to the public release of the chloride-tolerant variety Crowley and potential future soybean releases that may be better adapted to high-chloride and saline soil conditions.

Rice has been shown to be very sensitive to chloride and nitrate salts in the seedling stage. Rice is more sensitive to total soluble salts than soybeans. If the soil test detects elevated soluble salts, irrigation water should be tested, and pre-plant nitrogen and potash fertilizer applications may need to be delayed until seedlings have developed three to four true-leaves and a reasonably healthy root system. Seedling plants are more sensitive to elevated soluble salts than older plants. Routine broadcast fertilizer applications of recommended fertilizer rates have not been shown to contribute significantly to increases in salinity risks, especially if actual nutrient rates above 100 pounds per acre are applied in two- or three-way splits.

We currently categorize 0 to 6-inch soil soluble salt levels (1:2 soil:water) according to Table 2 for field and forage crops.

Table 2. Relative productivity of agronomic crops to soluble salts (1:2 soil:water) in the root zone.¹

Crop	E.C.1:2 (micromhos/cm)
	300	500	700	900	1100	1300	1500	1700	1900	2100	2500	2900	2400	3800
----------% Relative Productivity ----------
Rice, paddy^*	100	100	100	88	76	63	51	39	27	15	0	0	0	0
Soybean	100	100	100	100	100	80	69	40	20	0	0	0	0	0
Corn, forage	100	99	91	84	76	69	61	54	47	39	24	10	0	0
Sorghum	100	100	100	100	98	90	84	78	70	63	50	36	22	8
Fescue	100	100	100	99	94	89	84	78	73	68	57	47	36	25
Wheat	100	100	100	100	100	100	93	86	79	71	57	43	29	14
Bermudagrass	100	100	100	100	100	100	99	93	87	80	67	54	42	29
Cotton	100	100	100	100	100	100	100	98	93	78	71	64	57	47

¹Data from Carter (1981) cited by Bresler and others (1982) with salinity conversions from Gilmour and others (1985) and

Sriyotai and Gilmour (1976).

^*Seedling rice is very sensitive to soluble salts. Seedling damage may occur at E.C._1:2levels of 400 to 500 micromhos/cm.

At most sites where chloride salts have been recognized as causing significant problems for soybeans, salty irrigation water has been the major culprit. If the irrigation water chloride concentration is known, and if soybeans are in the crop rotation, refer to the following table (Table 3) for interpretive guidelines.

Table 3. Preliminary categorization of irrigation water chloride concentration and potential impact on chloride - including soybean varieties

Water Chloride Concentration
meq/L	mg/L (ppm)	Approximate Lbs of Chloride Delivered Per Acre-Inch	Category of concern or Potential Impact
0.2 0.4 0.6	7 14 21	2 3 5	Minor concern; probably negligible impact with long-term use.
0.8 1.0 2.0	28 36 71	6 8 16	Possible concern; test soil and soybean leaf tissue for build-up. Long-term use of this water may harm certain soybean varieties.
3.0 4.0 5.0	107 142 178	24 32 40	Potentially damaging with long-term use; test soil and soybean leaf tissue. Select chloride-excluding varieties.

Management Steps to Limit Problems from Excessive Salts

1. Soil sample in 6-inch, or even 2-inch increments if salts are suspected to help determine how extensively the soil is affected. Sampling down to three feet may be warranted if “native” salinity is thought to be the problem.

2. Determine your irrigation water quality. If it is saline or has high chloride, minimize its use by substituting with surface irrigation water where possible. CAUTION: tail-water or re-lift water may contain higher soluble salt levels than some well waters. Soluble salts may be “flushed” out of a field and actually increase during storage in a reservoir or canal as water evaporates.

3. Seedling rice samples should be collected at the 2- to 5-leaf stage and analyzed for zinc, chloride, and nitrate-nitrogen levels. Failure to collect samples at this growth stage could result in an incorrect diagnosis. Refer to page 50 in the 1994 Rice Production Handbook for interpretation of the rice seedling concentration of these elements.

4. Shallow chisel-plowing or subsoiling to shatter shallow soil compaction may help allow salts to leach out of the major root zone. However, this may not be feasible for flood-irrigated rice fields or soils with fragipans (natural dense silty layers usually in the subsoil unless soil is badly eroded). Research has shown that attempts to break-up fragipans are not beneficial because such soil layers readily recompact. Also, do not chisel-plow or subsoil fields that have high sodium levels (above 10% sodium saturation of the cation exchange sites) near the surface or within reach of the tillage equipment. This could result in bringing sodium to the surface which could inhibit crop emergence and seedling development. Crop tolerance to high sodium levels is shown in Table 4.

5. Use care in pre-plant applications of nitrogen and/or potassium fertilizers to limit seedling exposure to damaging salt levels. If these fertilizer salts are applied, and if good quality irrigation water is available, keep the root zone moist to limit the salt concentration around plant roots.

6. If planting on elevated beds, check soil soluble salt levels before planting. It may be better to plant below or off the bed shoulder or in middles (furrows) if salts have ‘wicked” to the top of the beds. If beds have been formed for some time, and salts have accumulated near the surface knock the beds down to the soil which has low salt levels before planting. Elevated beds may actually contribute to greater risk of seedling salt injury because of the salt-wicking action in beds, just as frequently observed on levees used in flood irrigation.

7. If feasible, attempt to flush salts out of the upper soil profile with rain water, good quality irrigation water and by providing adequate surface drainage in the field. Displacement of the sodium salts with gypsum is seldom considered economically feasible.

8. Maintenance of a soil cover or mulch may help to decrease moisture evaporation and the associated accumulation of salts near the soil surface. However, later in the year, it may be desirable for excessive soluble salts to accumulate near the soil surface and away from the developing root system. The location of the soluble salts in the soil may be just as important as the concentration and types of salts.

The best way to manage excessive salts is to prevent their development in the first place. If the soil is already salt-affected, consider planting crops that are salt tolerant. Refer to Tables 2 and 3 for guidance and contact your county Extension agent. For more in-depth readings, consult the publications in the list of references.

Table 4. Tolerance of various crops to exchangeable sodium in soils^a.

Tolerance to ESP^b and range At which affected	Crop	Growth response under Field conditions
Extremely sensitive (ESP = 2-10)	Deciduous fruits Nuts Citrus Avocado	Sodium toxicity symptoms even at low ESP values.
Sensitive (ESP = 10-20)	Beans	Stunted growth at low ESP values even though soil physical condition may be good.
Moderately tolerant (ESP = 20-40)	Clover Oats Tall fescue Rice Dallisgrass	Stunted growth due to nutr