Pasture growth requires access to nutrients from the soil, and managing soil pH is key for influencing whether pasture plants can access nutrients.
In Ohio and much of the temperate eastern U.S., the natural pH of soils is frequently lower than optimal for crop production. Low pH reduces the availability of important macronutrients like phosphorus and reduces the ability of legumes to fix nitrogen. Over time, pastures with consistently low pH have trouble retaining clovers and shift towards grass dominance.
High pH can also cause challenges, especially with micronutrient availability, but it is more common for us to see low pH soils in our region. Both low and high pH conditions can inhibit root growth, reducing overall plant health and increasing bare soil, weed encroachment, and drought susceptibility.
Identify fields with low pH by soil testing. While soil samples can be collected at any time of year in a pasture, it’s easier to observe changes in fields over time if you pick one time of year for soil sampling and stick with that.
Fall is a particularly great option, though, since soil is firm enough to drive over but has enough moisture that the soil probe won’t get bent in half, and it gives us plenty of time to review results over winter and make a plan for next year’s nutrient management.
When sending your soil sample to the lab, always be sure to fill out the submission form accurately. This important step is easy to overlook, but it’s important for interpretation of lab results since some crops have a different target pH.
Determine whether your soil pH results meet the needs of your pasture system. Optimal soil pH for alfalfa fields is 6.8 where subsoil pH is below 6, and 6.5 for alfalfa fields where subsoil pH is above 6.
Other forage legumes and grasses have an optimal pH of 6.5 when subsoil pH is below 6 and an optimal pH of 6 when subsoil pH is above 6. If your soil test results are more than 0.2 lower than the optimal pH for your pasture, it is time to plan for a lime application to raise soil pH.
Soil pH helps answer the question, “Would my pasture benefit from a lime application?” but we need to look at a second part of a soil test to determine how much lime would be useful.
Use the buffer pH or exchangeable acidity results on your soil test report to calculate the lime rate. Buffer pH is usually the test run by labs in Ohio, but Penn State’s lab utilizes exchangeable acidity. Both tests are reliable ways to predict your optimal lime rate, and the report that comes with your soil test report should provide accurate lime rate recommendations as long as your submission form was completed correctly.
Do check that the rates recommended by the lab are intended for permanent pastures or no-till systems, though, since unincorporated lime applications will only adjust pH in the uppermost 4 inches of the soil. Incorporated lime applications can adjust deeper layers of soil and will need higher product rates.
Lime application can occur over fall or winter as long as there’s not a thick layer of snow on the ground when you spread lime. Depending on the particle size/ fineness of the lime source, soil pH increases will occur over the 6 to 24 months following lime application.
Lime applied in November will have at least partially reacted by April, and soil pH will continue to improve throughout the next growing season. Be sure to walk fields that have recently received lime before making nitrogen fertilizer decisions, since any lime remaining on the soil surface will increase volatilization of nitrogen and greatly reduce fertilizer use efficiency.
The benefits of a well-managed pH cannot be overstated for pasture growth. Optimal pH improves root growth and nutrient uptake, and it helps legumes fix more nitrogen to fuel their growth and the growth of companion grasses.
Consider collecting soil tests before the ground freezes to see if your pastures are in the optimal pH range, and contact your soil testing lab or local extension office with any questions related to soil pH.
