Soil pH: Maybe You Should Sweat the Small Stuff

A career in agriculture never becomes boring, in part because so much is constantly changing, including the weather. We’ve never had two identical growing seasons and wouldn’t recognize a “normal” growing season if we got one. Agricultural technology continues to advance, including more efficient equipment, more effective pest control and higher-yielding crop varieties, all helping to economically squeeze that last bushel of corn from an acre or pound of milk from a cow. And thanks to the Internet, the rate of technology transfer – the speed with which a technological advance makes its way from a university or research facility to the farm – is faster than ever.

An example on the Internet front: Some years ago I was researching the use of chloride as a fertilizer and had a question. So I posted it on an Internet field crops “listserv,” a program that forwards messages and requests from one member of the list to the entire list. I soon got a message from an agronomist at the University of Arkansas stating that he was unable to answer my question but there was someone in Australia who might know, and he included the fellow’s email address. I contacted the Aussie agronomist and had my answer within 24 hours. Before the “Computer Age,” my chances of finding an answer to the question, at least in a reasonable amount of time, would have been just about nil. So what difference does the personal computer (and more recently, the smartphone) make in moving technology from universities and research labs to the farm? All the difference in the world!

The small stuff

An oft-cited quote by Robert Elliot: “Two rules to follow: 1. Don’t sweat the small stuff. 2. It’s all small stuff.” With all the focus on new crop technologies, including reduced-lignin alfalfa varieties discussed in this column last month, it’s easy to focus on what’s new and exciting instead of the basics of crop production, including, for example, soil pH. Let’s face it: Compared to new-and-improved crop varieties (GMOs and those developed by conventional breeding) and shiny new equipment that seems to get bigger every year, maintaining adequate soil pH by the application of agricultural limestone is boring. There hasn’t been any notable advancement in the use of ag lime: It’s the same old material doing the same old job. Soil test labs have an improved ability to determine the correct lime application rate based on soil pH and other soil properties, but that advance was at least a generation ago. Therefore, farmers sometimes consider monitoring soil pH and applying lime as “small stuff.” Example: “Hey, soil pH might be a bit low on this field – probably should have limed it last fall but I ran out of time. But I’m going to seed one of those new reduced-lignin alfalfa varieties this spring. If it does well maybe I’ll topdress the field with lime sometime this summer.” You may smile but I promise you that something very similar will happen this spring. With this in mind, following are a few facts about soil pH, lime and liming, and why you should sweat the small stuff. (When Al Gore titled his book on the environment, “An Inconvenient Truth,” he could have been writing about ag lime!)

  • All lime is not created equal. There are considerable differences in the ability of various sources of lime to affect soil pH, both in the rate and extent of change. That’s why if there’s more than one source of agricultural lime in your region, you should check the applied price and estimated neutralizing value (ENV) of each source. The cheapest lime per ton might not be the best buy. There are also wide differences in ag lime’s magnesium content; some soils are high in this secondary nutrient while soils would greatly benefit from the use of “high-mag” lime, also called dolomitic lime. Even if the dolomitic lime is more expensive, it’s likely to be the cheapest way to supply magnesium to your crops.
  • Lime takes time. Lime application isn’t like applying nitrogen fertilizer to corn or grass, where you can expect a fast response. Ag lime is finely ground limestone and should be applied well before the crop needs it – in the case of alfalfa, preferably the fall before seeding.
  • Low soil pH can tie up some soil nutrients. It’s been a while since I’ve seen a graph in the agricultural press showing how nutrient availability is affected by soil pH, so I’m including one here (see the graph on page 54). As the graph shows, some nutrients such as potassium aren’t affected by soil pH until it’s so low that most crops won’t grow at all, while phosphorus availability starts declining at a soil pH of about 6.5. Iron and manganese become more available in acidic soils – to the extent that they can be toxic to crop plants.
  • Soil pH can impact herbicide performance. Postemergence herbicides are rarely affected by soil acidity, but the effectiveness of soil-applied herbicides may be considerably reduced if soil pH is much below 6.0. This includes at least some of the triazines, including atrazine. Acid soils will tie up the atrazine but not degrade it, so if lime is eventually applied, the herbicide may then become plant-available. This may be OK if corn is still the crop grown, but not if it’s an alfalfa seeding. I saw this happen some years ago: The farmer fought the effects of poor weed control in one corn field for several years despite repeated atrazine applications. He then had his soil analyzed prior to seeding to alfalfa the following spring and was surprised to find that the pH was well below 6.0. So he applied several tons of lime per acre that fall, which increased soil pH and allowed the atrazine to become plant-available – just in time to kill much of his spring alfalfa seeding!

In summary, take full advantage of the exciting new technologies available to farmers, but don’t forget about lime. Sometimes we should sweat “the small stuff.”