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How do I acidify my soil?

Missouri soils vary in whether they are naturally acidic or alkaline. When this soil property is measured, we refer to the value as a level of pH. Most plants grow well with a pH of 6.5 to 6.8 meaning that the soil is slightly acidic. A pH of 7 is considered to be neutral and values greater than 7 are alkaline. In certain areas where limestone materials form part of the subsoil structure, the soil pH may be quite alkaline with values ranging from 7 to 8.5. Alkaline soils can also result when during construction of a home, the limestone-based subsoil is dug out and placed on the surface covering any good top soil. Alkaline building materials like concrete used for foundations and sidewalks will also affect the pH of the soil as they age.

Alkaline soils are not desirable for growth of common garden plants and turfgrass. When soils have a high pH of greater than 7.2, the amount of available manganese, iron, boron, zinc and copper in the soil becomes limited. Nutrient deficiencies caused by alkaline soils will appear as a yellowing of leaves and this is most commonly found with pin oaks, sweetgum trees and river birch.

The only way to determine if the soil pH is right for growing plants is to measure it by doing a soil test. If the test indicates that soil is alkaline, then some steps can be taken to lower the pH and if possible, this should be done prior to planting. You can lower the pH by using elemental sulfur, iron sulfate, acidifying nitrogen fertilizers, peat moss and organic matter. Elemental sulfur is the most effective when you work it into the soil before you plant. Bacteria must oxidize the sulfur before it can lower the pH and the oxidation process will take 3 to 6 months.

The amount of sulfur to add depends upon the initial soil pH, the soil type and the desirable range you wish to obtain. If you are preparing the soil for planting, then add no more than about 20 pounds of sulfur per 1000 square feet in the first year and till this in 6 to 12 inches deep. Wait at least 6 months before you add more if the soil test indicates this. For areas where plants are already established, add no more than 5 pounds per 1000 square feet at each application. This should be done when it is dry to avoid burning the foliage. Wait 4 to 6 weeks before adding more. This should be worked into the surface of the soil, if possible.

When making major pH adjustments of greater than one unit, you should check the pH every 6 months before adding more. Sulfur can burn plants easily if too much is applied. Iron sulfate will react more quickly than elemental sulfur, but you need to add about 6 times more to get the same change in soil pH.

Once the pH has been moved into the approximate range for growing plants, it will be good to use acid forming type fertilizers to help keep the pH in the right range. These fertilizers produce an acid reaction in the soil. Acidic fertilizers include ammonium sulfate and ammonium nitrate. These will add nutrients as well as lower the pH however, avoid burning plants by adding too much. Follow label directions for fertilizer application. Peat moss and other composted organic matter will assist in stabilizing the soil pH once it has been corrected, but should not be depended upon to make major pH adjustments. For garden areas, add a 2 to 4-inch layer to the soil surface and till to a depth of 6 to 12 inches. Organic matter will also aerate the soil and improve drainage.