If you were a physician and were trying to assess the health of a patient, you could run through their dietary intake regimen, ask about their activity level, listen to the lungs and check for symptoms, but you’d be missing a key element: the blood test. A blood test will tell a doctor a host of things that observation or a patient’s history can’t provide.

Jaynes lynn
Managing Editor / Ag Proud – Idaho

It’s the same with a plant.

Pat Purdy is a farmer and rancher located near Picabo, Idaho. In the 2022 growing season, he added a sap analysis to his crop management toolbox. This effectually reveals what nutrients are available to (and being used by) the plant compared to a soil or tissue test.

Sap analysis involves taking a closer look at the nutrients that are active in a plant. During a growing season, the crop nutrient demand can shift drastically. An in-season test can reveal any deficiencies and optimize amendment input costs. A sap test will analyze levels of zinc, sulfur, sodium, silicon, selenium, potassium, phosphorus, nitrogen, nickel, molybdenum, manganese, magnesium, iron, copper, chloride, calcium, boron, aluminum, electrical conductivity and pH.

Purdy offers this disqualifier, “I’m not a soil scientist, not a plant scientist, and certainly by no means an expert on sap analysis. This was really our rookie season, and we learned an awful lot in this process.”


He says, “Tissue testing is really a measure of the total accumulation of the nutrients in the plant tissue up to that time that you took that test; whereas sap analysis is a much more dynamic measurement of the nutrients that are moving through the vascular system or the circulatory system of the plant.”

A sap analysis measures all three forms of nitrogen – nitrate, ammonium and total nitrogen. Timely sap analysis will show nutrient deficiencies before they become visibly apparent in the plant.

Purdy used sap sampling on a field of brown mustard seed in three runs – one on June 13, one on June 29 and one on July 19. In his words, he is working with “pretty poor, sandy soil” that was taken out of desert ground many years ago.

56918-jaynes-onions-1.jpgAs growers find more ways to optimize yield while decreasing inputs, sap analysis proved a valuable asset for Purdy Farms on this mustard seed crop. Photo provided by Pat Purdy.


The sap analysis process

When taking a sap sample, the preferred method is to take some old, lower leaves and some of the fully formed new leaves and place them in two separate sample bags. The next step is to submit the two samples together to the lab. These tests can show what is happening right now, along with what happened in the old leaves.

For their first test in June, Purdy says, “My nitrogen use efficiency in my new leaf was only 74.6 percent, and my old leaf nitrogen use conversion efficiency was only 58 percent. Ideally, it'd be 100 percent, and 58 percent is a good solid ‘F,’ and 74.6 percent is a good solid ‘C’. So we're not doing very well in our nitrogen use efficiency.” Purdy says the plants had quite a bit of nitrogen locked up in the ammonium and nitrate forms. He also noticed they were missing some critical micronutrients needed to convert the ammonium and nitrate forms into useful nitrogen forms. He had virtually no molybdenum, was critically low on iron and boron, and lacked copper, phosphorus and potassium. Essentially, the lack of key nutrients was blocking the ammonium and nitrate from being converted into plant-useable forms. Admittedly, the sandy soil undoubtedly contributed to not retaining water or nutrients well in general.

Armed with the results of the sap analysis, Purdy used a ground rig sprayer two separate times to apply an additional 15 pounds of nitrogen, 2.6 pounds of phosphate, 1 pound of potash, and varying amounts of zinc, iron, manganese, copper, boron, molybdenum, cobalt and carbon per acre. Traditional methods would have probably been to apply 100 to 150 pounds of nitrogen on a crop like this in total for the season. Purdy pre-planted with 30 units of nitrogen, sulfur and carbon.

By July 19, they pulled a final sap sample. Those samples showed significant improvement, moving into the “A” range for nitrogen use efficiency. The needle also moved dramatically in boron, copper, iron and manganese, and bumped up the molybdenum and sulfur.

“It takes some training and experience to learn how to read the sap results,” Purdy says. He strongly recommends working with an experienced person to help interpret the results.

The net result was that Purdy produced a mustard crop with above-average yields. He used less than 50 units of nitrogen and put a higher emphasis on micronutrients. “For half or a third of the typical nitrogen application, we were able to meet that plant’s nitrogen needs by focusing on getting the metabolism working in an efficient manner, versus just pouring more nitrogen on the crop, which is what historically would have been done,” he says.

“I think we had very real successful results with this,” Purdy says. “We're very happy with what we learned. We certainly understand a lot more now than we did when we first started. But we're still just one year through our rookie season, so … we've got a lot to learn. It's definitely a process.”