On Aug. 1, 2018, the Soil Science Society of America published an article titled “Soil Phosphorus Availability and Lime: More Than Just pH?” Those cooperating scientists wrote that plants cannot survive without phosphorus (P). But there is often a “withdrawal limit” on how much P they can extract from soil. That’s because soil P is often in a form that plants can’t take up – this nutrient is biologically unavailable to them. That effective shortage can impact the health and productivity of plants, particularly crops. One influence on P availability is the soil’s pH level. If soils are too acidic, P reacts with iron and aluminum, binding it up, making it unavailable to plants. But if soils are too alkaline, P reacts with calcium and also becomes inaccessible.
Soil scientist Andrew Margenot, Ph.D., described the optimum P nutrient usefulness scenario this way: “Phosphorus is most available to plants when soil is at a ‘Goldilocks’ zone of acidity.” Baseball players might compare that to the “sweet spot” on their bats. The latter is a good comparison, agronomically, as lime applications are often referred to as sweetening sour (acidic) soils.
Margenot pursued that logic further, stating that there are ways to make P more available to plants. For example, adding lime (calcium hydroxide) reduces soil acidity. That can unlock the P that was previously unavailable. This is a common practice. “Liming is a bread-and-butter tool for agriculture,” said Margenot. However, liming can influence other ways by which P might become available to plants. Enzymes, called phosphatases, are also known to influence the amount of P available to plants. Margenot’s study looked at liming and soil management history to see if it influenced the activity of soil enzymes. Working with colleagues, he conducted experiments in western Kenya, a region with acidic, weathered soils. Researchers added varying amounts of lime to long-term experimental plots. These plots had specific fertilization treatments since 2003: One set of plots had been unfertilized, another had received cow manure and a third set of plots had mineral nitrogen and P added.
Twenty-seven days after liming, the researchers measured phosphatase activity. They also measured how much P was available to plants. They found no clear relationships between soil acidity levels changed by liming and phosphatase activity. This was unexpected. “We know that phosphatases are sensitive to soil acidity levels,” said Margenot. “Our findings show that it is more complicated than just soil acidity when it comes to these enzymes.” More surprisingly, changes in phosphatase activities after liming depended on the soil’s history. This suggests that the sources of these enzymes (microbes, plant roots) could have responded to different fertilization histories by changing the amount or type of phosphatases secreted.
Furthermore, in all cases, the increases in P availability were relatively small. “In the soils tested, lime alone was not enough to be meaningful to crops and thus farmers,” Margenot said. “Lime needs to be combined with added phosphorus to meet crop needs in these soils.”
This is where I like to add that with all the geopolitical confusion that drags on in Ukraine, one ray of common sense does shine through: The price of ag lime in the U.S. has increased very little, and its availability has remained fairly constant. Being an almost exclusively domestic product certainly helps with that score, as this minimizes transportation costs. Lime prices have increased about a 30% in the last 10 years. Various components of the “big three” – nitrogen, phosphate and potash – have increased on average about 150% in the last 30 months. The fertilizer scene these days, compared to just a few decades ago, is so increasingly global. In 1961, the U.S. applied 25% of the planet’s fertilizer; in 2021, that figure had dropped to just 10%. While U.S. crop input consumption has increased tremendously in these six decades, crop input demand, elsewhere on Earth, has increased astronomically.
Prudent use of lime, particularly these days, can take a lot of the sting out of the fertilizer ingredient price wound. Like Margenot, most crop growers agree that fertilizer utilization is best when soil pH is optimized for the intended crop. For corn, clovers and most grasses, we get the best return on fertilizer investment by raising pH to 6.2. Alfalfa, more finicky on fighting acid, wants pH of 6.8 – 7.0. When soil test results indicate a certain lime tonnage for alfalfa, subtract one ton from that figure to get a clover recommendation. When we undershoot the pH required by a given crop, the fertilizer utilization efficiency often drops by 30% – 50%. All of the “big three” nutrients are 100% available at pH 7.0. When pH drops to 5.5, nitrogen and potassium availabilities drop to 77%. At that low pH, P is only 48% available. I’ve studied USDA research which shows that at pH 5.0 – 5.2, P is only 10% available. Often, I recommend that with most crops (potatoes and blueberries excluded), growers shouldn’t bother applying the “big three” unless pH has been dragged up above 6.0.
On the global scene, P prices aren’t getting any better, with so much of this nutrient being mined in Russia and no longer being shipped to the U.S. The resulting shortage of this component hits both the fertilizer business and the mineral business. The CEO of one mineral company with whom I work told me that sales of monocalcium/dicalcium phosphate will be limited to historic use patterns for existing customers. Lest we forget, the classic P deficiency symptom is that signature purple leaf-tip dieback – most visible in corn.