Lower soil pH with sulfur

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Many plants cannot thrive in soil with a high pH. It’s not just blueberries that need an acid soil. Apples, basil, carrots all prefer a slightly acidic soil with pH less than 7.0.

A soil pH over 7.0 allows phosphorus to bind with calcium into an insoluble form, unavailable to plants. As the pH increases, the essential micronutrients iron, manganese, copper, zinc and cobalt oxidize into a form that is not plant available under normal conditions. At a soil pH of 7.0 or higher, hydrogen is not available on the cation exchange sites, reducing the mobility of the major cations: calcium, magnesium, and potassium. The soil is less lively. For garden soils, the optimum pH is in the 6.2 to 6.8 range where nutrient availability is maximum.

If your soil is pH 7.0 or higher, it’s good to understand the reason why. Some possible reasons are:

  • The native soil is alkaline and has an abundance of calcium carbonate and/or magnesium carbonate. If the soil has been surveyed, the native soil composition may be found in a local soil survey database, accessible online here. The presence of carbonates in the soil will make it fizz in vinegar, which is easy to test for at home. Soils with a high level of calcium carbonate are known as calcareous soils.
  • The soil has received mineral amendments which increase the pH. Agricultural lime, dolomitic lime and (non-organic) nitrate fertilizers will all raise the pH. Lime (either type) will make the soil fizzy for about 3 years after its application.
  • The soil mix is based on a high pH manure compost. Composted feedlot or factory chicken manure can be tremendously alkaline with pH over 8.0 and excessive sodium and potassium levels. Check the pH, sodium and potassium content of the compost before you buy it, especially if you are near a commercial animal operation, such as anywhere in central or southern California.
  • The irrigation water is alkaline and contains bicarbonates which remain in the soil, raising the pH. See what can be done with bicarbonates in irrigation water. In arid environments where irrigation is necessary, this is often an issue.

Organic garden farmers have few options when it comes to lowering soil pH. Increasing soil organic matter will tend to bring the soil toward neutral (beware of high pH composted manure!), however building the amount needed to move the pH may involve enormous effort growing cover crops and sourcing benevolent compost materials over a matter of years. Ammonium fertilizers will lower pH but they are not organic and are hard on soil biology [ref 3]. Sulfates such as gypsum (calcium sulfate) do not lower soil pH.

Amending with elemental sulfur is the only practical, organic method of lowering soil pH, and then only for non-calcareous (non-fizzy) soil. In the soil, elemental sulfur is oxidized by bacteria which in turn produce H+ and sulfate ions. The H+ acidifies the soil and the sulfate ion remains as a plant food or else is leached by rain or irrigation. The microbes do their work best in a warm moist environment. The process stops if it is too hot, too cold, too dry or too waterlogged.

The amount of sulfur needed to lower non-calcareous soil pH is shown in these two references [ref 1 and ref 2] as a function of soil TCEC. We have modeled the data in the table and have produced an equation and calculator for this job. Yes, wow!

Calcareous soils require too much sulfur to practically lower the pH. For example, a soil with 1% calcium carbonate content contains 20,000 lbs/acre of sulfur. One third of a pound of sulfur is required to neutralize one pound of calcium carbonate, so 6,700 lbs/acre of sulfur would be required to bring this soil to neutral. We limit the amount of sulfur in one application to 300 lbs/acre in order to maintain soil biological health. These soils require a different approach.

For more about sulfur see our page, here.