Soil pH

What is soil pH, and how does it affect plant growth?

Soil pH is a measure of the acidity or alkalinity of soil based on the concentration of hydrogen ions (H+) in the soil solution. Soils with a pH of less than 7 are acidic, and soils with a pH of greater than 7 are alkaline. Since pH is measured using a logarithmic scale, a decrease of 1 unit of pH means that the acidity increases by a factor of 10, so small changes in pH values can have important consequences. 

For most of Illinois, soil acidification is a concern. A surface soil pH that is too high or too low can have several negative impacts, including poor root nodulation of legumes and decreased nitrogen fixation, and decreased biological nutrient cycling. The availability of mineral nutrients to plants may also be affected, as soil pH can affect the capacity of soil surfaces to retain nutrients and plant uptake of nutrients. Soil pH may also affect the interaction of certain herbicides and soil surfaces.

What soil tests are used to determine pH levels?

The water pH test is used to measure active soil acidity, while the buffer pH test measures the soil’s “reserve acidity” or pH buffer capacity, which is its ability to resist pH changes. While it is important to know the soil’s active acidity, it is only a portion, and usually small, of total soil acidity. The soil’s reserve acidity is on clay and organic matter particle surfaces and accounts for a much larger portion of total soil acidity. Soils with high clay and organic matter content have a greater ability to resist pH changes. Knowing both the soil’s active and reserve acidity allows for a more accurate measurement of how much lime is needed to neutralize the soil. For this reason, liming recommendations should be based on measures of reserve or exchangeable acidity using a buffer pH test, while soil pH is used to identify soils that need limed.

How do I raise my soil pH?

A soil test every two to four years is the best way to check pH levels. For cash grain systems, pH levels should be maintained between 6.0 and 6.5. If the soil test shows that the pH is 6.0 or less, apply limestone, which raises the soil pH by neutralizing the soil. Liming rates are determined based on soil buffer pH and the desired pH level. Adjustments may need to be made based on depth of incorporation and the quality of the limestone being used. 

How is limestone quality determined? 

Limestone quality is determined by its effective neutralizing value (ENV), which is a measurement of the neutralizing value and the fineness of grind. 

• The neutralizing value of limestone is determined by its calcium carbonate (CaCO3) equivalent (CCE): the higher this value, the greater the limestone’s ability to neutralize the soil.
• The fineness of grind determines the rate of reaction: finer limestone will neutralize soil acidity faster. 

Relative efficiency factors have been determined for various limestone particle sizes (Table 1). The ENV can be calculated for any liming material by using the efficiency factors in Table 1 and the CCE of the limestone in question. The Illinois Department of Agriculture collects and analyzes limestone samples from quarries that wish to participate in the Illinois Voluntary Limestone Program. These analyses are available in the annual publication, Illinois Voluntary Limestone Program Producer Information.

To calculate the ENV and the correction factor needed to determine rate of application for materials not reported in that publication, obtain the analysis of the material in question from the supplier and use the worksheet for lime-rate calculation on page 98 of Chapter 8 of the Illinois Agronomy Handbook. 

How do I apply limestone?

Since limestone does not react with acidic soil very far from the particle, adjust application rates proportionally to the depth of tillage. For no-till systems, when lime is broadcast on the soil surface, apply one-third of the needed rate to avoid creating extremely high pH at the soil surface. Consequently, liming may be required more often (but at lower rates) in these systems than in cultivated fields. 

Make sure limestone is spread evenly throughout the soil surface by avoiding overlaps. If a mistake was made and very high rates were applied, scraping the material out of the field or increasing the amount of mixing by tillage would be a practical way to reduce negative effects. Limestone can be applied at any time, but fall applications are preferred to avoid soil compaction and concerns about spring planting delays. Fall application also allows more time for limestone to neutralize soil acidity. If high rates of limestone are needed, it can be applied in split applications. 

In no-till fields where lime is not incorporated in the soil, surface applications eventually neutralize acidity below the surface. However, this process is slow, so it is recommended to always maintain surface pH levels at adequate ranges. If pH levels in the surface are allowed to drop, lime applications will take a long time to start to neutralize acidity below the soil surface.

What is the calcium (Ca)-magnesium (Mg) balance in Illinois soils?

Soils in northern Illinois usually contain more Mg than those in central and southern Illinois, both because of the high Mg content in the rock from which the soils developed and because northern soils are geologically younger. This relatively high level of Mg has caused speculation: is the level too high? Although there have been reported suggestions that either gypsum or low-Mg limestone should be applied, no research data have been put forth to justify concern over a too-narrow ratio of Ca to Mg. Furthermore, a recent study done at The Ohio State University showed that balancing Ca and Mg levels in soils did not affect corn or soybean yields.