Sulfur and Soil pH

A soil pH in the range of 6 to 7 provides the best chemical environment for growing plants.

When pH values fall outside of this range, there are potential problems with the availability of nutrients and there may be detrimental shifts on microbial populations. If the pH is excessively low (acidic), the soil can be “limed”, and the pH increased to the proper level. Reliable tests are available to determine how much lime is needed in any given situation, and the process of raising pH is relatively easy and inexpensive in most soils.

Often the problem is not one of an excessively low pH, but actually one of excessively high pH. In this area of Texas, pH values of 7.5 to 8.3 are the rule rather than the exception. The lowering of soil pH (acidification) is generally not as easy as raising pH. Sulfur is the element usually recommended for acidification. Theoretically, this is a sound practice. Sulfur applied to the soil is converted to sulfuric acid by soil microbes that lower pH. The problem is that in many soils, the amount of sulfur required to lower pH is so large that the process is impractical.

The problem soils are the calcareous soils—those that contain lime (calcium carbonate). This is the same lime used to raise the pH of acidic soils. In many soils, calcium carbonate (CaCO3) is a part of the parent material from which the soil was formed. Soil scientists have found that as long as solid lime is present in the soil, it is not possible to decrease the pH. Remember that statement, because it is the key to the decision on whether the use of sulfur will be practical for the lowering of pH. When sulfur is added to a calcareous soil, sulfuric acid is formed and some of the CaCO3 dissolves, but as long as CaCO3 remains in the soil, the pH cannot be permanently lowered. This resistance to change in pH is known as "buffering." It is very difficult to change the pH of a highly buffered soil, whereas the pH of a soil with a lower buffering capacity can be changed more easily. An analogy used by soil scientists to explain this phenomenon is to compare pH testing to pressure testing of tires. Both a bicycle tire and a tractor tire could be measured at 50 pounds of pressure. When air is released for 10 seconds from both tires, the bicycle tire's pressure will drop several pounds, but the tractor tire will remain very close to 50 pounds of pressure. The tractor tire is "buffered" against pressure change, but the bicycle tire is not.

The pH of soils is similar. Two soils may both have a pH of 7.5. One may be readily decreased to pH 7.0 by the addition of sulfur, whereas the other, a highly buffered soil with large amounts of CaCO3, could be treated with large amounts of sulfur and show little decrease in pH. Agronomists have calculated how much sulfuric acid would be required to dissolve CaCO3 in the soil. They have determined that if sulfuric acid costing $1.45 per pound were to be added to a calcareous soil, 68 tons of acid per acre at a cost of $198,000 would be required to dissolve just one percent of CaCO3 in the upper 7 inches of soil.

Temporary decreases of pH in highly buffered soils treated with sulfur are not unusual. However, the detrimental effects of excess sulfur on the plants usually outweigh any benefit from the slight pH decrease. Some sulfur is beneficial for plants, but if used in excess the sulfur will form excessive salts that can easily kill the plants that you are trying to help. Excessive salt accumulation can be tolerated if mixed in a volume of soil such as in a garden area being tilled 8 to 10 inches deep. However, when too much sulfur is applied as a surface application and excess salts are formed and concentrated in a shallow soil zone, grass or any other plants can be killed or severely damaged. Another analogy to explain this phenomenon of temporary pH decrease is that of a coffee pot with a glass stem along its side to show the coffee level in the tank. When coffee is released, the level in the glass stem drops sharply, but as soon as the flow is stopped it quickly returns to the level in the tank. Likewise, a highly buffered soil will show a temporary drop in pH when sulfur is applied, but it will quickly return to the higher, buffered pH when sulfur applications is stopped. Situations exist in which the use of sulfur to lower pH may be practical. A soil with pH normally in the range of 6.9 to 7.0 that has been raised by the application of irrigation water high in calcium to a moderately high pH of 7.3 to 7.4 could potentially be lowered by sulfur.

The main concern when the pH is high is that some plant essential nutrients will not be available for uptake by the root system. Rather than trying to lower pH in these calcareous soils, a better approach may be to apply micro?nutrients to the foliage in the liquid form. This approach has been recommended to avoid zinc deficiencies in pecan trees by spraying the foliage with NZN or zinc sulfate periodically.

The banding (placement under the plant in a narrow band) of minor elements such as iron to prevent iron chlorosis has been recommended for vegetable crops before planting occurs. This procedure involves acidifying only a small portion of soil in the root zone, and will increase the availability of phosphorus, iron, zinc and manganese in the area of the band. The banding procedure begins after planting beds have been formed in the garden area. Simply split open the planting bed so that after application and recovering with soil, the band of sulfur will be 2 inches below the growing plant. For example, if a corn seed is planted 1 inch deep, then the sulfur band should be 3 inches deep. Distribute ½ pound (½ cup) of sulfur or iron sulfate per 10 linear feet in the split planting bed. Iron sulfate is a better source of iron than it is acidifier and is sometimes recommended as a "cure" for iron chlorosis. However, in an alkaline soil, the iron of iron sulfate (copperas) rapidly becomes unavailable for plant use. In a severely iron deficient soil, iron sulfate should be substituted for sulfur in the banding procedure described above.

For plants, such as blueberries, azaleas, camellias and gardenias, which are severely sensitive to high pH soil conditions and the subsequent lack of minor elements, an artificial growing media must be created if successful growth is expected. The best technique is to excavate a 4x4x4 foot hole, dispose of alkaline soil and rock, and refill the hole with a mix of 2/3 spaghum peat moss and 1/3 washed sand. This, too, may not always be practical, but it would be preferable to trying to lower pH with sulfur.