By Jake Munroe, OMAFRA Soil Fertility Specialist
Correcting soil pH problems is one of the first steps in good soil management.
If you have soil sampled recently and found that pH is low, you should act on it. Soil pH affects the availability of a wide range of nutrients in soil; if it is too low or too high, nutrients are less available to crops. If pH drops too low, e.g. below 5, aluminum toxicity can become an issue. Acid soils can also negatively impact nodulation of forage legumes, persistence of perennial forages and growth of sensitive plants, such as wheat.
There are two values to key in on when reading a soil test:
Soil pH tells you if lime is needed. Typically, if you have wheat or field vegetables in rotation, lime is beneficial when soil pH is below 6.1 on medium or lighter textured soil. For clays and clay loams, it’s needed when pH is below 5.6.
Buffer pH tells you how much lime is needed. The lower the buffer pH value, the more lime will be required to raise the pH to a desired level. For example, soil with a buffer pH of 6 requires two to three times the lime of a soil with a buffer pH of 6.5. Clay and organic matter provide what’s called “reserve acidity,” which is a supply of hydrogen ions held by soil particles. This is why heavier textured soils tend to have a lower buffer pH and require more lime to increase soil pH.
Once you have determined your target pH and the lime requirement, it’s time to compare products.
Limestone quality is determined by how well the lime can neutralize acidity and how finely it’s ground. These values are combined into what’s called the Agricultural Index. OMAFRA lime guidelines are based on an Ag Index of 75. It’s essential to compare prices of lime based on relative Ag Index values. For example, a product that costs $20 per ton with an Ag Index of 95 is a better deal than a product that costs $15 per ton with an Ag Index of 65.
The choice between calcitic and dolomitic lime is simple: if your soil test level for magnesium is below 100 parts per million, use dolomitic lime; if it is higher, use either type.
If you work with a lab or ag retailer to have grid or zone soil sampling done on your fields, the same principles apply for soil pH and liming, just at a finer scale. Prescriptions are based on soil variability and can allow for a more accurate (and potentially cost-effective) lime application across the field.
The bottom line: Soil sample regularly to monitor soil pH if you know it can be an issue in your area. If your soil test calls for lime application, follow the guidelines above to make an accurate and cost-effective decision on rate and product. Addressing soil pH is an essential component of managing soil fertility.