Tissue testing can reveal the ‘hidden hunger’ that sets plants back

It can be a valuable diagnostic tool but not enough growers are taking advantage of it

Jack Legg: “The more material we have, the higher the accuracy.”

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When you want to know how much in the way of nutrients the soil can provide, you take a soil test. But to determine how much of that fertility actually made it into the plant you have to look a little higher.

That’s where a tissue test comes in.

It can be a valuable diagnostic tool but “not enough growers are taking advantage of it,” Jack Legg of SGS Agri-Food Labs in Guelph told the recent Southwest Ag Conference at Ridgetown. Typically the only time a tissue test is submitted is when someone wants to verify a symptom pointing to a nutrient deficiency, he said.

But that’s selling the technology short. It can also detect nutrient shortfalls, especially in micronutrients, that don’t result in clear visual symptoms, the so-called ‘hidden hunger’… Or you can track nutrient imbalances where the important ratio between two nutrients is out of whack.

“It’s a very accurate indicator of the plant’s nutrient utilization,” Legg said. It’s not intended to take the place of the soil test but to act as an add-on to provide more detailed information.

When a lab receives a leaf sample for analysis technicians liquefy the material to measure the macro and micronutrient content, said Chris Roelands of Honeyland Ag Services at Ailsa Craig, who co-hosted the presentation with Legg. Those measurements are then compared to the growth stage of the crop to determine if nutrient uptake is where it should be. “The accuracy becomes very high,” Roelands noted.

Chris Roelands: “Sample in such a way as you’re willing to manage.”

Knowing the stage of the crop is important because a low reading in a rapidly growing crop is not unusual. Also, if the crop is stunted due to other reasons the nutrient content may be high.

The test can also show if nutrients such as nitrogen and sulphur or potassium and nitrogen are present in the proper balance.

While visual symptoms can appear clear cut they sometimes don’t tell the whole story. For example, that yellow corn crop that appeared to be crying out for more nitrogen may actually turn more yellow when that nutrient is applied, Legg said. That’s because the problem was sulphur and adding more N simply put the two nutrients further out of balance, making the problem worse. Testing tissue lets you be sure.

That interveinal chlorosis may suggest potassium deficiency but could also point to magnesium. In one such case the tissue test revealed that both nutrients were low, Roelands added.

Tissue tests can also be used to verify the soil test results. Take the sample from the same spot as the soil test and “you can be doubly confident,” Roelands said.

But just like with a soil test, “sample in such a way as you’re willing to manage”, whether that be on an acre by acre basis or larger zones, he added.

Tissue tests have received more publicity lately as some of the high-yield growers, like Randy Dowdy or David Hula, have endorsed the practice. “These guys want to map out the nutrient uptake curve in their field from year to year,” Roelands said. It enables them to see that nutrients like phosphorus, boron and zinc are all required later in the season.

Just like soil testing, your results will only be as accurate as the sample you submit, Legg noted. That includes sampling the most recent leaves. Taking the lower leaf from a corn plant, for example, may not tell the whole nitrogen story because the N may have already moved out of this part of the plant.

Make sure you provide plenty of leaf material. Legg suggested half of a paper lunch bag is sufficient. For corn, that could mean about a dozen ear leaves. For a crop like strawberries it may be closer to 100. “We’ve had people submit a single leaf. The more material we have, the higher the accuracy.”

Sampling plants from both good and poor areas of the field is a good practice, he said.

While you may want to get an answer on leaves that are clearly showing deficiency, “don’t sample leaves that are too far gone or dead,” he warned.

Also, avoid leaves that may have had soil splashed on them by recent rains or those that may still have residue from a fungicide application, he said.

In a young corn crop it’s best to take the whole plant and cut it up. At pre-tassel, take the most recent fully developed leaf. When the plant is tasseling, submit the ear leaf, he said.

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