Part I – What is pH?

Part II – How is nutrient availability affected by pH

Part III – How do fertilizers affect pH?

Part IV – How to adjust pH of soils/media

Part V – How to adjust pH of irrigation waters

The process of adjusting 'soil' pH is the process of changing the amount of H⁺ or OH^- ions that are bound to the colloidal surface of soil particles. This is referred to as the buffer pH or the reserve acidity.  Adjusting soil pH is best done before crop production, since the chemical reactions that need to take place often occur over a season.  Therefore, it is necessary to be proactive.

Steps to adjust soil and media pH:

1. Know the recommended pH for the crop in question.
2. GET A SOIL TEST DONE - CAUTION:  Follow recommended procedures for sampling.  Most labs will provide guidelines. Always take multiple samples from different areas in the field or media heap and combine them, mix thoroughly and take a sample from the mixed multiple sample.  Otherwise, the analyses may not represent the average chemical profile of the soil/media.
3. Follow recommendations of the lab.  However, please remember that the change in soil pH, even after amendments are added can take several days or even months to adjust, depending on the amendment added. 

Methods of adjusting soil pH:

1. Soil amendments - addition of specific soil amendments such as peat moss, pine barks, etc.
2. Chemical amendments - addition of specific fertilizers such as aluminum sulfate, sulfur, sulfuric acid, calcium carbonates, etc.

Soil Amendments:

Lowering Soil pH:

Organic matter amendments

Incorporating organic matter into the soil will usually lower pH.  The degree of pH change will be correlated with:

1. Amount of organic matter added.
2. Duration of time since the organic matter has been in the soil.
3. Components of the organic matter - i.e. peat moss, bark, added amendments such as lime - some companies will add lime and other nutrients to neutral pH, so be aware of product components.
4. Size of organic matter particles - the larger the particles, the longer it will take for organic matter decomposition and lowering of soil pH.
5. Stage of decomposition of the organic matter.

Chemical amendments 

• Sulfur - Sulfur is the primary acidify agent for acidifying media and soils.  However, like nitrogen, the chemical process of acidification is biological (See CORF NEWS Winter, 2003) and will take up to a year to occur, depending on the soil and sulfur product used.  Most sulfur products are elemental sulfur.  However, in some field sites, sulfuric acid has been injected into the soils for instant acidification.  This is done ONLY by trained professionals since use of concentrated sulfuric acid is extremely dangerous and can be deadly if handled improperly.
• Aluminum - Aluminum will acidify the soil.  However, in mineral soils, aluminum can also be toxic to roots.  Therefore, most recommendations of aluminum sulfate are restricted to artificial growing medias for containers where organic matter component makes up from 50-100% of the mix.  Aluminum sulfate is often recommended for hydrangeas, azaleas and other acid-growing crops.
• Ferrous sulfate - Both the iron and sulfur will acidify the soil.  However, the use of this product is limited as iron toxicity can easily occur since the plant requirements for iron are very low.
• Ammonium fertilizers - The reaction of ammonium in the soil as well as the uptake of ammonium into the plant will result in soil acidification.  Fertilizers containing ammonium are often part of a recommended program for acid-loving plants.

Increasing soil pH:

The primary way of increasing soil pH is through chemical additions.  The fertilizer compounds described below will all increase soil pH. The time required for the soil pH to increase after incorporation of the product will depend primarily on particle size of the compound.  The finer the particle size, the more quickly the soil reaction will take place. 

• Oxides - Calcium and magnesium oxides will react with soil to increase soil pH.
• Hydroxides - Calcium and magnesium oxides will react to increase soil pH.
• Carbonates - The liming materials of calcium can magnesium carbonates will increase soil pH.

Unfortunately, we cannot give exact quantities of acidifying and neutralizing agents needed since all production systems are different. However, as mentioned in the previous article (Winter, 2003), the Western Fertilizer handbook (California Plant Health Association, 2002) has a wonderful discussion of soil pH and amendments with some general guidelines.