What is soil chemistry and what does that mean to me?

Ah, the “litmus test.” Do you remember that from your high school science days? You dip a piece of white litmus paper into a solution, and it turns red or blue¹. If it’s red, your solution is acidic. If it’s blue, your solution is basic. Chemists assigned a number to the test called pH. The number can range from 0 (very acidic) to 14 (very basic, or “alkaline”), and a value of 7 is considered “neutral.”

One of my graduate advisors told me “if you can only measure one thing, make sure it’s soil pH.”

What does pH have to do with the soil in your yard or garden?

Soils are a complex mixture of organic and inorganic, living and non-living components. And with this complex mix comes some very complex – yet important – chemistry. If there is one principle that can frame and help simplify soil chemistry, it is pH. The influence of pH over living and non-living processes has earned pH the ‘master variable’ in soil science: change pH, and you likely alter or influence a slew of processes. The reason for this is that pH is intimately tied to chemical, biochemical, and biological processes, and their interactions.

Soils that were once unproductive in Brazil (cerrado) are now good agricultural land, thanks to knowledge of soil pH. Source: http://www.no-till.com.au/about/files/Tomato-harvest-DSCN0079_800.jpg

pH is a measure of how many free hydrogen ions are available in a solution. How and why does pH matter in soils?

Soil pH is critical for soil nutrient availability. Soils provide the nutrients that your plants need to thrive. And whether or not a soil is acidic or basic can influence whether or not the nutrients are available to plants and other things that live in the soil.

Nutrients can be locked by soil minerals if the soil pH is not correct. This affects whether the plant can use the nutrient or not. Credit: ASA/CSSA/SSSA staff

Phosphorus is an excellent example of pH potentially restricting nutrient availability. Beyond a narrow window of pH 5.5 to 8.5 pH, phosphorus can react with other soil minerals such as iron oxides and carbonates. This can ‘lock up’ phosphorus and make it unavailable to plants and soil microbes. In other words, soil pH can be the key that can unlock availability of nutrients that may be already present but not available. For most nutrients, there exists a “Goldilocks zone” of just-right pH in which its availability is optimal. Measurement of pH is not surprisingly routine in laboratory characterizations of soil samples.

For this reason, pH correction is a key strategy to manage fertility in soils with non-optimal pH. Growers – and gardeners – commonly perform this for soils with high acidity. Addition of lime (most commonly calcium carbonate) can raise pH to the near-neutral (pH 6.5) target for plant productivity. Interestingly, using lime on your soil works in two ways. It treats the symptom: a high amount of extra hydrogen ions. And, it gets at the cause of the acidity: aluminum ions in the soil.

Agricultural use of the Brazilian cerrado with very red soils visible. Source: http://curitibainenglish.com.br/wp-content/uploads/2015/05/127110.jpg

For alkaline soils, gardeners can add elemental sulfur (S) to decrease pH to a more neutral value. Sulfur works to bring down pH because soil microbes use oxygen when processing the sulfur and this creates extra hydrogen ions.

Testing your garden soil for pH is a very important step in creating and maintaining the best growing environment. We recommend testing pH when you create the garden and every few years thereafter (check with your local extension office for the best recommendations for your area). Remember that different plants may need different pH values, so be sure to consult optimal pH values for a given plant when you are working in your garden and using soil amendments and/or fertilizers.

Soil pH management has proven key to unlocking productivity of acidic soils, such as liming of weathered soils in the Brazilian cerrado. What was once considered non-productive land for agriculture has been turned into productive land, starting in the 1960’s. Knowing about soil chemistry, specifically pH, has helped Brazil become the second largest exporter of soybeans, as well as providing feeds to supports a thriving beef and poultry industry.^{2, 3}

Answered by Andrew Margenot, University of Illinois, Urbana-Champaign

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1. Those familiar with Universal indicators will know there are ranges of colors for professional testing.
2. https://en.wikipedia.org/wiki/Cerrado
3. “Soils under Cerrado: A Soil Management Success Story” Accessible at http://www.ipni.net/publication/bci.nsf/0/BD82D5423F10863F85257BBA0070C42D/$FILE/Better%20Crops%20International%201996-2%20p09.pdf