Soil pH is a measure of the acidity of the soil which can affect nutrients availability to plant roots, plant growth, and soil microbiome. pH values range from 0 to 14 and most plants grow best when soil pH is between 6.2 and 7.2. Consequently, the more we move away from this optimal pH range, the more difficult it becomes for plants to grow efficiently. In this post, we discuss why pH matters for plant growth, how to measure soil pH and what can be done to adjust soil pH.

Why do most plants grow best when soil pH is between 6.2 and 7.2 ?

Plants need sunlight, water and nutrients to thrive. If the plants cannot efficiently absorb nutrients from the soil, then they will have difficulty completing their life cycle as can be observed with poor germination or root development. More than 70 years ago, Truog E. observed that soil pH influences the uptake of nutrients from plant roots.

The figure on the right is a chart adapted from Truog’s article published in 1947. The horizontal axis shows a range of soil pH from 4 (very acidic) to 10 (very alkaline). Any value of pH below 7 indicates that the soil is acidic while any value of pH above 7 is an alkaline soil. Each colored bar represents a nutrient essential for proper plant growth. The wider the bars become, the more available the nutrients are to the plant roots. For example, iron and manganese are well taken up by roots when the soil is acidic, while calcium and magnesium are best absorbed in alkaline soils.

According to Truog’s study, a soil pH range of 6.2 and 7.2 provides the optimal nutrient absorption explaining why most plants grow best in a mildly acidic environment.

Representation of Truog’s chart

How does pH influence nutrient absorption by roots ?

In order for nutrients to be taken up by plant roots, they must be dissolved in the soil. Nitrogen is one of the most important nutrients for plant metabolism. A major source of nitrogen comes from bacteria in the soil which can fix the nitrogen from the air and convert it into nitrate, a soluble molecule accessible to plant roots. These bacteria (like the rhizobia-symbionts of legumes) grow best when pH is in the neutral range (6.5-7.5). If the pH deviates from their optimal growth rate, then there will be less bacteria in the soil and, consequently, less nitrate available for the plant roots to absorb.

When pH values become too acidic or too alkaline, certain nutrients precipitate, i.e. they form a solid rendering them inaccessible to the roots. This is the case of phosphorus which reacts with aluminum to form a solid at low pH or iron whose solubility is poor at the neutral pH, but increases when pH decreases. Other nutrients like potassium and sulfur are generally soluble independently of pH.

How to measure soil pH ?

Measuring soil pH can easily be done at home using a colorometric test or a pH-probe. For both methods, soil must be mixed with water, usually 1 part of soil with 5 parts of water.

When preparing soil samples for testing, it is recommended to collect soil that is about 6 inches deep because this is where the roots are going to be. Moreover, doing several pH measurements of the area to be checked can help confirm the test results and feel more confident on the next steps to take.

The colorimetric test can be done using a solution which changes the color of the soil/water mixture when added or, a pH-strip which once dipped onto the soil/water mixture, changes color. For both colorimetric methods, the color of the soil/water mixture or the pH-strip is then compared to a color chart under sunlight for measurement. If pH is measured using a pH-probe, then the probe must be added into the soil/water mixture to provide a pH value on the screen.

How to amend a soil ?

In the case of a soil that is too acidic or too alkaline for plants to grow, several additives available at the local retail shop can be added to the soil to improve its pH. For example, dolomite, quick lime or wood ash can be mixed with the soil to increase soil pH while organic matter like decomposed tree leaves or pine needles should be used to decrease soil pH. Commercial test kits often provide advice regarding the amount of additive to use. If the soil has been sent for testing in a laboratory, then recommendations to amend the soil successfully are usually added with the test results. After these additives have been added to the soil, it is important to measure the pH of the soil again to make sure it is in the wanted pH range.

As a last remark, although the optimal pH range is considered to be between pH 6.2 and 7.2, some plants might not grow best in this range indicating that soil amendments must be done in accordance to the needs of the plants we wish to grow.

Conclusion

Soil pH is a central parameter of regenerative agriculture because it helps figure out how to restore our soil. Thus, adjusting soil pH to our plants has positive impacts on plant growth, the soil microbiome, and wildlife which includes beneficial insects and birds.


Sources :

Soil pH testing picture: https://www.gardenersworld.com/how-to/maintain-the-garden/how-to-test-your-soil-ph/

pH absorption nutrients picture: https://planetpermaculture.wordpress.com/2013/07/25/ph-chart-showing-nutrient-availability/

Truog, E. 1947. Soil Reaction Influence on Availability of Plant Nutrients. Soil Sci. Soc. Am. J. 11(C): 305–308.