Science in Action: What Does Sulfur Do In Desert Soils?



Granular sulfur remaining on the surface of a desert soil three
years after its application.

Arid soils and irrigation water in the desert southwest present numerous problems to nondesert plants that are brought in from other parts of the country and the world. These problems include high pH and high levels of sodium and bicarbonates. Sulfur, in one of its many forms, is commonly used to reclaim these soils and water and improve their quality so that a broader range of plants may be used in landscapes. Sulfur, in the form of sulfuric acid, is used for injection into irrigation water to combat pH and high bicarbonates.

 

In some parts of the country granulated or micronized sulfur is used as part of the fertilizer program when it has been found to be deficient through soil or tissue samples or when a lowering of soil pH is needed. The purpose of using sulfur in a maintenance plan, and which form to use, is often confusing to a landscape manager. This can lead to disappointment when the results from the use of sulfur containing materials don’t live up to the expectations of the landscape manager.

In horticulture and agriculture, sulfur is used for two primary purposes: as a nutrient for plants growing in sulfur poor soils and to reclaim poor quality soils and irrigation waters. It should be noted here that all sulfurs and sulfur products are not the same. The form of sulfur that would be picked as a fertilizer is not necessarily the same form that would be picked for reclaiming poor quality soil and water.

Sulfur is an essential plant nutrient. Fortunately it is plentiful in most soils and shortages are not common but they do occur. Plants take up sulfur from the soil in the form of sulfates. Fertilizers that contain sulfur usually contain it in the form of elemental sulfur (soil sulfur) or in the sulfate form such as ammonium sulfate. Sulfur is converted to sulfates through a process called oxidation. When applied to aerated, moist soils, elemental sulfur is oxidized by soil microorganisms to form sulfuric acid. This sulfuric acid in turn supplies the sulfate ion that is taken up by the plants. During this conversion from elemental sulfur to sulfates is when sulfur releases its acidifying power. When elemental sulfur is incorporated into soils the oxidation of sulfur to sulfates is what helps to temporarily lower soil pH.

Sulfur containing fertilizers should be applied to soils when sulfur deficiency has been clearly established through soil tests or tissue analysis. The preferred sulfate fertilizer is ammonium sulfate due to its price, solubility in water and availability. Since the sulfur contained in sulfates has already been oxidized, the acidifying power possessed by sulfur has already been lost. The sulfates contained in fertilizers have little affect on the pH of a soil or water when added to it and so may not be a good choice if lowering the soil pH is desired.

Elemental sulfur, sometimes called soil sulfur, is not a good choice to quickly correct a sulfur deficiency or quickly lower soil pH. To become available to plants and release its acidifying power it must be first converted to the sulfate form by microorganisms. Like all microorganisms they are most active and efficient under environmental conditions that promote their activity. The size of the sulfur particles, soil temperatures, moisture and oxygen levels must be in a range favoring their growth and activity. Peak oxidation levels, provided soils are well aerated, occur at soil temperatures of about 85 degrees F. This conversion from sulfur to sulfates in soil is a relatively slow process even under the best of conditions. Even though acidification around the sulfur particle may be relatively quick, the diffusion of sulfuric acid to the surrounding soil volume is generally slow.

Generally speaking, given warm soil temperatures and good soil aeration, the smaller the sulfur particles the faster the release of acid and the formation of sulfates. This is why elemental sulfur is sometimes combined with bentonite clay to help in the physical breakdown of the sulfur particle. In water, a bentonite-sulfur particle swells, breaking it up into very fine particles. Once broken into small particles, the increased surface area allows soil bacteria to transform the sulfur to sulfate more quickly. However, even in the presence of small particles, transformation of sulfur to sulfate is a slow process often taking months.

Elemental sulfur may be a good choice on a long-term management program provided the economics justify its use. Reasons for continued application to soils include the temporary, but long term lowering of soil pH and the reclamation of soils containing excess sodium. Elemental sulfur is often chosen to lower soil pH, but it must be used carefully since it can have a high potential to burn plant tissue under certain conditions.

The amount of sulfur needed to decrease soil pH is determined from a soil test by the amount of lime contained in the soil. Each 10 pounds of elemental sulfur generates enough acidity to neutralize 30 pounds of lime. Applications are best made when temperatures are warm enough for the bacteria to oxidize the sulfur but not hot enough to burn leaf tissue. Since sulfur does not generally move in soil, surface applications may be tied up in the thatch layer and not move into plant root zones. Generally temperatures above 90 degrees F would be the upper limit for applications.

Incorporation of sulfur into the soil just after coring is a good method for reducing burn, reducing contact with plant foliage, improving aeration and moisture conditions for the oxidation of sulfur and treating a greater volume of soil than just applying it to the surface. Published rates of application for turfgrass are less than 5 pounds of actual sulfur per thousand square feet for bermuda and less than 0.5 pounds for bentgrass per application. Reapplication would depend on soil pH tests.

Sulfur is also used for reclaiming sodic soils (soils high enough in exchangeable sodium to cause plant damage and decrease soil permeability). For sulfur to work there must be enough calcium present in the soil for sodium to be precipitated out and good drainage. When calcium levels are low (rarely in arid soils of the western United States) gypsum may be used if a quick response is needed. In arid soils already plagued with salinity problems it is recommended that a degree of caution should be used when applying gypsum to correct sodic soils. Since using gypsum will add about five times the amount of salt to a soil as using sulfur, the addition of gypsum should be avoided when managing landscapes with salt sensitive plants.

Sulfuric acid injection into irrigation systems is sometimes chosen for lowering soil and water pH and correcting high bicarbonate levels in irrigation water. High bicarbonate levels in irrigation water exacerbate potential plant damage due to high sodium (measured as SAR). Bicarbonates will combine with calcium and magnesium and precipitate them out. This is a problem because calcium and magnesium help to "buffer" problems due to high sodium. As these calcium and magnesium ions drop out of the irrigation water solution, the relative sodium levels may rise to dangerous levels (measured as adjusted SAR). The addition of sulfuric acid to irrigation water helps to lower bicarbonate levels and keep calcium and magnesium in solution.

Directly injecting concentrated sulfuric acid into the irrigation system through an injector or using a device such as a sulfur burner will accomplish the same thing. Sulfuric acid can be made from sulfur by burning sulfur, also an oxidation process, in a combustion chamber. In the case of burning sulfur, the oxidized end products are sulfur gases; sulfur dioxide and sulfur trioxide. When these gases are mixed with water they produce sulfuric acid. Sulfuric acid is produced and can be injected directly into the irrigation system. This is the basic idea behind sulfur burners used for acid injection into irrigation systems. Sulfur burners have become more popular recently due to safety considerations when handling concentrated sulfuric acid.

Sulfur may be a good choice in managing arid soils provided the reasons for its application warrant it and the economics of its use justify it.

This article was originally printed in Southwest Trees and Turf