August 2006

Soil Salinity and Gypsum

Salinity Basics

Soil salinity in Manitoba is estimated to affect 700,00 acres of agricultural land. Usually this is in the form of localized areas around sloughs and potholes, or along roadside ditches. While many of these saline areas occur naturally, some non-saline cropland has become salt –affected over the years or has become more intensively affected as the result of traditional agricultural practices.

The main cause of salinity is excess water. Water tables within 6 ft. of the soil surface can carry dissolved salts, found naturally in prairie soils, into the plant root zone or even to the soil surface by wick action. Salts reduce plant growth by preventing the uptake of water and nutrients on these soils. There are three types of salt-affected soils:

1. true saline soils are those with sufficient amounts of soluble salts to impair crop growth;
2. sodic soils contain sufficient exchangeable sodium to interfere with the growth of plants through undesirable soil structure (very hard and crusted when dry and very slippery when wet);
3. saline-sodic soils contain a combination of soluble salts and exchangeable sodium sufficient to interfere with plant growth.

Sodic or saline-sodic soils represent about 75,000 acres in Manitoba. Soil test information can determine which type of salt-affected soil the producer is dealing with. Electrical conductivity (E.C.) measures total salt content; sodium adsorption ratio (SAR) measures the ratio of sodium relative amounts of calcium, magnesium and sodium; soic soils have considerably more sodium present than calcium and magnesium).

The data can be summarized in the following table:

The greater the values for E.C. and SAR, the greater severity of soil salinity and therefore the greater degree of difficulty in remediating the salt-affected soils.

To reclaim a waterlogged, salt-affected soil, three basic steps are required:

1. The water table must be lowered. This is best accomplished using perennial forages or through improvements in subsurface drainage.
2. The excess salts must be leached out. Rainfall or irrigation should accomplish this over time.
3. Where the sodium content of the soil is high, the sodium must be replaced with at more desirable cation, such as calcium. Prairie soils usually have enough calcium present to replace the sodium.

Is Gypsum the Answer to These Problems

Gypsum has recently been suggested as soil amendment to remediate such problems soils.

But application to saline soils will not be beneficial. In fact, gypsum (calcium sulfate) is a salt itself, and may actually increase the salt content.

Application of gypsum to sodic soils will provide calcium to displace sodium. However, one still needs rainfall or irrigation to leach the displaced sodium down and secondly, internal drainage to allow downward movement out of the root zone. If either of these conditions is lacking, then gypsum application alone is fruitless.

As mentioned previously, our soils tend to be naturally high in calcium. In these instances application of sulfur alone (elemental or ammonium sulfate) will solubilize and generate free calcium to displace the sodium.

Where does gypsum fit? Gypsum is an effective supplier of calcium and sulfur to crops. Since our soils are predominately calcium carbonate based, this nutrient is generally not lacking and calcium applications are not required. Sulfur deficiencies are more widespread, and gypsum is effective in meeting crop needs. Sulfate-S from gypsum is less available than from ammondium sulfate or ammonium thiosulfate, but more readily available than from elemental S.

Gypsum contains 14-20% S and should be compared to other S fertilizers based on the cost per pound of S supplied. At present, sulfur fertilizers cost about 20-22 cents per pound of S. Gypsum may be available in a ground or pelted form. Specialized equipment, such as lime spreaders, may be required to adequately apply the ground form.

References:

Dryland Salinity Investigation Procedures Manual. Alberta Agriculture. 1992.
Management of saline soils. Agriculture Canada. Publication 1624/E. 1983
Stalinization of soils in: The health of our soils – Towards sustainable agriculture in Canada. AAFC. Publication 1906/E. 1995.
Soils ’84, Lesson 3: Salinity and Drainage. Manitoba Agriculture. 1984.
Soil Salinity – Manitoba Map Sheet. Agriculture Canada. Publication 5261/B. 1990.