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Sap testing is a new laboratory test that measures the nutrient concentration in plant sap, which may be a better way to forecast nutrient deficiencies in soybeans.

Last week I posted a blog on pulling tissue samples and running tissue analysis in soybeans. This is a tried and true method of seeing how much nutrients the plant has banked away in its tissue. Over the years guidelines have been developed to help interpret the results and decide whether nutrient concentrations are low, normal, high or even excessive. Sap analysis may be a complement to this test or another method to choose from.

Sap analysis is not new. Potato and vegetable growers have reliably used petiole nitrate testing as a means of monitoring the nitrogen status of their crops. Vegetable growers can take leaf petioles, express the sap, then test for nitrate and/or potassium using portable Cardy meters that measure nitrate, potassium, calcium, sodium, pH and EC.

Today, Crop Health Labs out of Ohio offers sap analysis as a new tool to assess nutrient availability and a plant’s metabolic health. In this procedure you collect old leaves and new leaves, bag them, place them on ice and overnight them. And as with conventional tissue testing, the values reported are categorized as either deficient, sufficient or excess.

You will ask: Why old and new leaves? Well, plant nutrients differ in mobility within the plant. Nitrogen, for example, is very mobile and moves from old leaves to new leaves so the plant can always get a supply either from the soil or by cannibalizing it from old leaves. Calcium, on the other hand, is immobile in plants and new tissue always needs a fresh supply.
• Mobile: N, P, K and Mg (oldest leaves most often deficient)
• Partially mobile: S, Cu, Fe, Mn, Mo, Zn (Micronutrients are generally considered relatively immobile, thus the justification for foliar feeding.)
• Immobile: Ca and B (youngest leaves most often deficient)

Conventional tissue analysis measures mainly 11 nutrients: N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu and B. Sap analysis looks at 23 parameters: sugar, pH, EC (conductivity), total N, nitrate N, ammonium N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, B, Mo, Chloride, Silicate, Cobalt, Aluminum and Selenium. And next year nickel will be added. One additional difference with sap analysis is the suggestion to pull samples every 2 or 3 weeks to develop a trend line on what is happening in the plant over the season rather than at 1 or 2 times as is common with tissue testing.

Sap analysis looks at the nutrient concentration in the sap and the nutrients readily available to be used by the plant’s metabolic engine. I refer to this as bio-available nutrients. So how do you interpret the results or even trust that you can rely on them as you would rely on a tissue test? Crop Health Labs and Nova Crop Control in the Netherlands (they pioneered this sap testing methodology) report that sap testing will reveal nutrient deficiencies 2 to 3 weeks earlier than tissue testing will.

So can you trust this test? This is a new test which very few agronomists or university researchers have experience using; many aren’t yet aware of it. Last year I gave it a try on commercial corn and soybeans on two sampling dates and the results indicated that micronutrients were borderline deficient in corn and soybeans, while tissue testing reported they were sufficient. I have done tissue testing in the past and the results are always same—sufficient—so I quit doing tissue test. But the sap test got me to thinking.

This year both Fred Below at the University of Illinois and I are doing another test on corn and soybeans and comparing sap analysis and tissue testing. My tests are on commercial fields of corn and soybeans, and Fred’s are on replicated fertilizer and unfertilized research plots. We are sampling 5 times throughout the season. We will be sharing and discussing results as we begin to understand the numbers and value of this test in better managing corn and soybeans.

The technology intrigues me with its potential to report nutrient availability as well as indicate a plant’s metabolic health. We have a lot to learn, though, both on the technology and how to properly interpret results.

For more information visit http://crophealthlabs.com/.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or by leaving a comment below.