The modern sports world debates who is the GOAT — greatest of all time — in every sport, even at every position. If farmers and agronomists started a debate about the GOAT for nutrients, which would win? Nitrogen, phosphorus, potassium, a micronutrient, sulfur?
How about soil pH and lime? The first argument might be whether lime is truly a nutrient. But agronomists like Jamie Bultemeier, supervisor at A&L Great Lakes Lab, argue that getting pH levels right everywhere in a field lays the foundation for the best use of all other nutrients.
“Availability of nearly all macro- and micronutrients is affected by pH levels,” he says. “It is extremely important to get pH levels right.”
Farmer questions about pH, lime
Here are Bultemeier’s answers to questions farmers often ask about pH and liming:
How much more acidic is pH 5.5 compared to pH 6.5? Is it a gradual, linear change? No! It is a logarithmic scale, where each full pH unit, a 1.0 change, represents a tenfold difference in acidity. The scale is basically a measure of free hydrogen ions, which create acidity. The more hydrogen ions roaming freely in the soil, the higher the acidity.
So, pH 5.5 is 10 times more acidic than pH 6.5. Remember that 7.0 is neutral. Above 7.0, things go basic, with more OH- ions than H+ ions at pH levels above 7.0.
What happens if pH moves below 6.5, and especially 6.0, on typical mineral soils? Phosphorus becomes less available, especially at 6.2 and below. Potassium and sulfur become less available below 6.0, and especially below 5.5. However, metals like aluminum and manganese can become toxic because availability increases too much. Molybdenum supplies, critical for legumes, decrease. Some herbicides break down faster, while others stay around longer, threatening carryover to a rotational crop. Others are not affected.
What if pH levels are basic, above 7.0? Raising pH too high can be just as dangerous. This is why testing by soil type and applying lime by variable-rate application is critical. If the pH is alkaline, above 7.0, iron, manganese and zinc may become deficient because availability drops. Instead of being low, molybdenum can reach toxic levels. Phosphorus availability declines again, especially above 7.5. A different set of herbicides are impacted. But again, some don’t last as long, while others stay around too long, and some are not impacted.
What happens as pH changes in muck soils, where organic matter is 5% to 5.5%? Availability of many nutrients, including phosphorus, shifts compared to mineral, timber soils. The ideal zone for availability for most nutrients becomes 5.0 to 5.5, compared to 6.2 to 6.7 for mineral soils. Testing for and managing pH becomes critical on muck soils.
Do changes in pH affect soil microbes? Yes! Some are more active at lower pH and some at higher pH levels. In general, most microbes are “happy” between 6.0 and 7.0, although it varies by organism.
Is acid rain still a threat today? No. In the late 20th century, acid rain, with pH levels in the low 6 range, was common. Sources of acid rain, like sources of free sulfur, were cleaned up. Today, pH level of rain is around 7.2 to 7.4.
Can fertilizers increase soil acidity? Yes, especially fertilizers containing nitrogen. If hydrogen ions are released through chemical reactions, soil acidity goes up and pH drops. For every pound of N applied as anhydrous ammonia, urea or ammonium nitrate, it takes 1.8 pounds of lime to neutralize it. For each pound of ammonium sulfate and MAP applied, it requires 5.4 pounds of lime to offset it, while it takes 3.6 pounds of lime to offset 1 pound of DAP. Triple superphosphate, 0-46-0, without nitrogen, does not produce acidity.
Our lab reports soil pH and buffer pH. Which should we follow? Soil pH indicates the acidity or alkalinity of your soil solution. Utilize that value to determine if lime is needed. When needed, buffer pH determines lime recommendations, or how much lime to apply at once. Soils with higher cation-exchange capacity values have more reserve acidity, requiring higher liming rates to move the needle on pH values.
If my goal for pH is 6.5, and values in a field are 6.2 and below, should we begin applying lime? Yes. Expect pH values in a corn-soybean rotation to drop about 0.1 per year. If you only test every four years, it could be 5.8 next time, acidic enough to impact availability of certain nutrients, herbicide activity and microbe performance.