This analogy might seem like a stretch, but hang in there: Think of crop nutrients and lime like a football offense. In a high-powered offense, your quarterback, running backs and receivers typically make the highlight reels with exciting plays. In most cases, a successful offense starts with solid blocking from the line. Good line play doesn’t always get the credit it deserves; unless you are really watching, it may go unnoticed. But without it, an otherwise high-powered offense generally struggles.
In corn and soybean production, nutrients like N, P and K are like backs and receivers. They get a lot of credit for success. And if there are deficiencies, sometimes there are visual symptoms. We tend to keep a pretty good eye on them. Lime? Well, it’s like the line — often taken for granted, and in tough financial times, it is often a place where we look to cut back.
What is soil pH?
Soil pH is a measure of relative acidity or alkalinity: a pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline. For corn and soybean production in Iowa, soil pH 6.5 is considered sufficient in areas with low-pH subsoil; pH 6 is sufficient in areas with high-pH (calcareous) subsoil within a 4-foot depth of the surface. The general soil association areas with low or high subsoil pH have been summarized in ISU publication PM 1688 A General Guide for Crop Nutrient and Limestone Recommendations in Iowa. Read the section on testing and managing soil pH.
In Iowa and fields of humid regions, acidity gradually builds from natural nutrient leaching, and mainly from applying ammonium or ammonium-forming fertilizers, manure with high ammonium content, and some sulfur fertilizers (but not gypsum).
If soils become too acidic, we can encounter potential problems. Root function is impaired, risk of aluminum and manganese toxicity increases, atmospheric nitrogen fixation by legumes decreases, while availability of phosphorus, potassium, sulfur and some micronutrients to plants can decrease. Biological activity can take a hit as well. For example, there is decreased activity of soil microbes that contribute to recycling of nutrients and reduced nitrogen.
Soil testing and lime
The game plan against soils becoming too acidic starts with soil testing, and recent research from Iowa State University agronomist Antonio Mallarino has me leaning toward the shorter end of the often mentioned two- to four-year sampling intervals.
His report Evaluation of Agricultural Lime and Pelleted Lime to Increase Soil pH And Crop Yield discusses this in more detail. My takeaway is that the rate of soil pH increase after lime application occurs faster than originally understood, as well as the rate of soil pH decrease. Shorter soil sampling intervals should help us track pH more accurately.
Note that the recommended soil sampling depth for no-till and strip till is 2 to 3 inches (make sure the lab knows the sampling depth). If this isn’t practical for you, the typical 6-inch P and K samples can be used, but lime application rates should be adjusted to about one-half of the amounts recommended for a 6-inch depth.
If soil tests show you need lime, there is a little more info to gather for the game plan. Application rates vary based on soil pH, buffer pH, product quality and type. Luckily, ISU publication PM 1688 walks us through all this. It may not be the most glamorous task, but managing soil pH may help block some blindside hits to yields.
McGrath is the research and Extension coordinator for the Iowa Soybean Research Center at Iowa State University. Email him at email@example.com.