Soil Test Note – No. H007

Dr. Stanley L. Chapman, Extension Soils Specialist, Dr. Mike B. Daniels, Environmental Management Specialist – Agriculture,

Dr. Gerald L. Klingaman, Extension Horticulturist – Ornamentals

EXCESS SALTS

The electrical conductivity test (EC) is a measure of the amount of total soluble salts in a soil sample. A certain amount of salts is desirable and necessary in a soil. However, too high an EC value means an excess salts problem exists or could develop by continued use of high rates of fertilizer, animal manure, or irrigation water high in salts. The soil texture or amount of clay in the soil greatly affects the amount of salts that plants can tolerate. The more clay present, the greater the salt tolerance. The soluble calcium is used as a measure of the percent of clay in the soil sample.

A high electrical conductivity test (EC) reading indicates that this particular soil sample contains more salts than might be expected in a normal soil. However, if the soil samples were taken largely from the upper inch of soil from an area not recently plowed, the problem may be only in sampling. Nitrate salts may accumulate at the soil surface during dry weather and can give excessively high readings at the surface while the soil as a whole may not be excessively high in salts. Representative soil samples taken as a slice of soil from the surface to six inches give a better picture of the overall salt condition in a soil than a more shallow sample.

Where high rates of soluble salts are found in a routine soil test sample for a commercial crop, it is usually advisable to resample the field at a later date or in more detail. Try to obtain representative samples from potentially different areas. If the cause of a high reading on a garden sample can be determined from the method of sampling or fertilizer history, no additional sample may be necessary. Irrigation or watering with good quality (low salt) water can often correct potential salt problems.

Salts accumulate in the soil when they are added faster than they can be removed. Only under certain circumstances are excessive salts a problem in areas of high rainfall. The salts that concern us when found at high rates in Arkansas soils are often some of the same ones needed by plants for growth. The problem where it exists is usually only in the amount present. The salts themselves are not toxic in the usual concentrations. The action of the salts is to cause water uptake by the plants to be more difficult as the soils dry out. Certainly highly salty soils may appear “wet” even though plants can remove no more water from the soil. This “wet” appearance is due to certain minerals or organic mater that has dissolved and “wicked” to the soil surface. Too much fertilizer added at one time, or poor soil drainage that does not allow even moderate amounts of salts to pass through the soil and out into the groundwater, are common causes of salt accumulation.

Table salt or sodium chloride often comes to mind when salts are mentioned. A number of other salts can be present and cause the same effects as too much table salts. Commercial nitrogen fertilizers are soluble salts when applied or after they react with the soil. The potash in commercial fertilizer is also highly soluble and must be considered when salts may be a problem. Phosphates are not usually as soluble; or they are not applied in such amounts as to be of concern in the soil.

Fresh manure contains considerable soluble salts and may be a problem if applied in large amounts just prior to planting. With adequate drainage excess salts can be leached out of the soil by normal rainfall over a period of time.

Wood ashes from a brush pile, fireplace, or stove add high amounts of salts to the soil and are not necessarily the ones needed. Unless the soil is very deficient in potash, it is usually better to dispose of ashes elsewhere or spread very sparingly over a large area.

Other indicators besides EC may be used to determine the possible presence of a salt problem. High soil test values for phosphorus and potassium indicate that high amounts of commercial fertilizer or manures may have been applied. If the high rate were the result of an accumulation over a period of years, the excess soluble salts may have been carried out of the soil by rainfall. Heavy fertilizer amounts recently applied may still leave a residue or soluble salts in the soil. Adding further high rates of nitrogen or potash may exceed the amount tolerated by most crops. Certain clay soils and highly fertile garden soils high in organic matter may have high soil test values for phosphorus and potassium but contain only normal amounts of free salts. This is a highly desirable condition. Crops grown under high salt conditions tend to have a dark bluegreen color in addition to stunted growth. Crops stunted by low fertility are often yellowish.

Percent Fertilizer Reduction from Excess Salts or Soil Nitrates

Modification of Fertilizer Recommendation*	Soil Textural Classification (lbs/A of CA)				Nitrate Nitrogen (lbs/A)
	Sands (<1000)	Sandy Loams Silt Loams (1000-3000)	Clay Loams (3000-4500)	Clays (> 4500)
------ EC x 10^6 (micromhos/cm) ------
No reduction	< 150	< 200	< 300	< 500	< 20
Reduce by 25%	150-200	200-300	300-533	500-1000	29-44
Reduce by 50%	201-250	301-400	534-767	1001-1500	45-69
Reduce by 75%	251-300	401-500	768-1000	1501-2000	70-100
No Fertilizer recommended	>300	>500	>1000	>2000	>100

*The % fertilizer reduction used is the larger of the reduction from excess salts or nitrates.

The salts status of the soil sample is determined from the exchangeable calcium and EC determination for each sample. The salt level classification is indicated in the soil test results. The fertilizer recommendation of any garden samples with salt (EC) values above the normal range are lowered by 25, 50 or 75 percent for low, medium or high salt buildup respectively. Soil samples with salts (EC) above the normal range will be referred to the county agent for the recommendation.

October, 1999

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