There is no bad time to take samples for a soil test.
“You can do it about any time the soil is in good enough shape to pull a sample,” Tim Schnakenberg, regional agronomist for University of Missouri Extension told Ozarks Farm & Neighbor. “I think it’s better to let the soil dry a bit because if it comes in wet, we’re going to have to dry it before we ship it, so that slows the process down.”
The crucial thing is to get the information in time to be able to amend the soil for the upcoming season.
But there are some times, he said, that are better than others.
“If you take your sample in the fall, usually the soil labs are not quite as busy during that time and you can get your results back. If you need to address a serious soil fertility issue such as low pH or low phosphorus or potassium levels, you have time to be prepared for the next growing season, especially with lime,” he said. “Limestone takes six months to a year to get an adequate change made in the pH, so the earlier you start for the next growing season, the better.”
The producer can request test information on a number of soil components. Dr. John Jennings, University of Arkansas forage agronomist, said the most important data are the soil pH, and major and minor minerals like phosphorus and potassium.
“Below that, we start looking at sulfur, magnesium and zinc,” he told OFN. “Also, soil salinity and organic matter can be requested on soil tests, and that’s useful to know, too, for pasture situations. If we can build organic matter, productivity and moisture holding capacity of the soil seems to improve.”
Jennings said good samples should be taken 4 to 6 inches deep, in a zigzag pattern across the pasture, with 20 to 25 cores over 20 acres. If a larger area is to be sampled, it should be split into sections that will be managed in a like manner. He cautioned not to attempt to take samples during hot, dry conditions, especially on rocky soils.
“What happens often is producers take a sample that’s too shallow; it might be only 2 inches deep, and biases the fertility of what a normal, deeper sample would be,” Jennings said.
He recommended forage producers take soil samples every two to three years, with some exceptions.
“If you’re in a really high management situation and trying to make improvements quickly you can sample yearly,” he said. “If they take off a much larger hay crop than normal they may want to sample the following year, just to make sure they haven’t pulled the fertility down too much. But generally, it doesn’t change really quickly; if they stay on that two-year track, they’re going to catch that anyway.”
Regardless of when samples are taken, it should be at the same time of year each time, because fertility varies during the year. Schnakenberg said, “The University of Illinois conducted a trial many years ago where they sampled a field in the same place every month for three years in a row, looking at fertility levels at different times of the year. They found that potassium levels were highest between March and June and lowest between between August and September, and the levels varied from 140 pounds per acre to 310 pounds, which is pretty significant.”
Producers should also look at how the field has been managed over several years. “Whether or not the field is cropped, pastured or hayed has a huge difference in fertility levels and you’ve got to factor that in,” he said. Many Ozarks hay fields have been used for that purpose for many years, and as a result are drastically depleted of nutrients; pastures tend to be in better shape due to the recycled nutrients through the manure.
And landscape position plays a role in nutrient needs.
“Is the field representative of the practices in the past; is it toe-sloped, the crest of a hill?” Schnakenberg asked. “Is it the side-slope, or the bottom? Generally, the subsamples need to be similar in past cropping and landscape position to one another because those factors change it a lot, and they affect the nutrient content…Soil testing is not a perfect science; it’s the best we have and a wonderful tool, but there are too many variables out there. We’re better off testing the same time of year each time we soil test.”
