As farmers look for ways to increase crop yields with strategic fertilizer use and contain costs at the same time, some are turning to sap testing and analysis.
“[Sap testing] helps us fine-tune our fertility program in-season based on what the plants tell us they need nutrient-wise, so we don’t over-apply or under-apply fertilizer,” says Lawrence Hewing, who grows corn and soybeans in Champaign County, Ill.
Over the past three years, he’s been able to reduce nutrient use, improve crop health and boost yields.
“We haven’t seen a large yield increase in corn, but we’re using a half pound or less of nitrogen per bushel – half of what we used to use,” Hewing says. “In soybeans we have gained additional yield – up to 10 bu. more per acre from doing a foliar application in-season. I think that’s helped us hold onto more blooms and then retain pods.”
How Sap Testing Works
A sap test measures the nutrient levels – including minerals, sugars and proteins – in the plant’s vascular tissues, the xylem and phloem, according to Mike Evans, co-owner of Calibrated Agronomy, based in Dow City, Iowa.
“Sap tests have taught us a lot about nutrient mobility in the plant, that plants can reallocate certain minerals and not others,” he says. “This was a big a-ha moment for us and really opened our eyes to the deficiencies we’d been missing with tissue tests.”
Tissue tests are done only on new growth, Evans adds, while a sap test is done on both new and old growth, which provides additional data for analysis.
“You’re pulling a sample from a new leaf, just like you would for tissue sampling, but you’re also pulling a sample from an older, mature leaf on the plant, too, and then comparing the test results between them,” Hewing explains. “The idea is to give the plant enough nutrition it can support new growth without needing to rob nutrients from the old growth. We think this helps us improve overall plant health throughout the growing season.”
Because a sap test sample is pulled from both old and new leaves, the analysis can reveal current nutrient deficiencies as well as those that will soon occur.
“It’s like getting a glimpse into the future,” Evans says.
John Kempf, founder of Advancing Eco Agriculture (AEA) in northeast Ohio, says sap analysis detects upcoming nutrient deficiencies between 21 to 28 days ahead of tissue analysis and 35 to 42 days before visible symptoms appear.
In-Season Nutrient Adjustments
Evans has used sap testing and analysis with his clients for the past several years. He says the most significant benefit is being able to better address corn nitrogen needs.
“The biggest eye opener has been the value we’ve seen from measuring what we call a plant’s nitrogen conversion efficiency,” he says. “We benchmark the nitrogen at the beginning of the season, and then we watch it. We want to keep it at a level above 95%, regardless of how good the plant looks.”
Sap test and analysis also help Evans’ growers fine-tune micronutrient needs in corn, including zinc, boron, iron and copper.
Who Finds Value In Sap Tests?
According to Evans, producers are usually in one of two camps. “A lot of them have been doing tissue sampling for a while and have hit a yield plateau they can’t get past,” he explains. “Then there are the farmers who are really inquisitive about crop fertility and want to achieve better nutrient efficiency.”
Hewing is in the latter camp. “We knew something was missing in our program and that we needed better agronomic insights,” he recalls. The search for those led him to John Kempf and AEA. The 2025 season marks the fourth year Hewing has worked with the Ohio-based organization.
“We started out using just one sap test on a couple of fields,” Hewing says. “This year, we’re going to use the tests across the whole farm.”
Evaluate The Pros And Cons
The upfront cost of doing sap testing and analysis is about $100 per two-part test – more than the price of a tissue test.
To optimize ROI, Evans says he tells growers to allocate 70% of their dollar spend to their baseline fertility program and then retain 30% of their budget for sap testing and in-season nutrient applications.
Another potential issue with sap testing, Hewing says, is the analysis usually requires more turnaround time than tissue testing.
“Depending on the lab you use and who you work with, it can take 10 days or so to get your results,” he says.
Still, Hewing has achieved measurable results he says outweigh the cost and time involved with sap testing.
He offers a common-sense recommendation to other farmers looking to improve crop health and performance: “I’d tell them to pick out their worst field and give sap tests a try. Evaluate the results for a few years and see what kind of results you get and then go from there.”
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