- Use tissue testing to identify trends, not diagnosis.
- Geo reference for same point soil and tissue sampling.
- Sample at the same growth stage for multi-year comparisons.
- Consider soil, weather, hybrid and other possible causes of perceived deficiency.
- Compare tissue analysis results under multiple sufficiency ranges.
Does it get any better than being able to take a tissue test, identify deficiencies and treat with a foliar application? It's like having a box of silver bullets...if there were silver bullets. Dan Gehling uses tissue sampling and foliar, as well as in-season soil application of micro and macronutrients.
However, the southeastern Minnesota corn and soybean grower doesn't believe in silver bullets. His tissue sampling is part of a long-term effort to evaluate soil nutrient levels and plant utilization of those nutrients. He is looking for trends, and tissue testing – like soil sampling and yield monitoring – is simply another tool.
Long term trends
"Over the years, I'll see what the plant might be short of," says Gehling. "If it shows up deficient and I can pull the levels up to sufficient for a couple of bucks an acre, I figure it takes some stress off the plant."
Over time he has developed a program that includes zinc and manganese on the seed with his starter fertilizer, as well as P and K plus boron and sulfur in a 3x3 and 28 percent – all at planting. This is followed with more nitrogen, sulfur and boron at sidedressing, then zinc and manganese as a foliar with Roundup. Gehling's confidence in his use of micronutrients is bolstered by his overall fertility program. This is combined with pattern tiling every 40 feet and two dominant, high organic matter, heavy soil types – Oran silt loam and Tripoli silty clay loam.
Tissue sampling protocol
He follows a strict protocol when tissue sampling. Corn is sampled right before sidedressing and right before tassel. Soybeans get sampled before he applies Roundup and five days later. When evaluating a treatment, he uses multiple 16-row replications with control strips in between. Since upgrading his yield monitor a few years ago, he collects and compares results with confidence.
Paul A. Trcka is Gehling's go-to-advisor on tissue sampling and analysis. A CHS Yieldpoint specialist with a local CHS cooperative, Trcka has been working with tissue sampling for more than 30 years and has learned to be skeptical of its use as a diagnostic tool for in-season supplements.
A snapshot in time
"A tissue test is simply a snapshot in time on that given day. Take it a week later, and it could be totally different," says Trcka. "Crops are produced in a biological system with sunlight, heat, moisture and soils all affecting plant growth and uptake."
To better evaluate his "snapshots", Trcka maintains an established database of checkpoints on customer fields to look for trends and response rates in various soils over time. Using GPS, he and fellow agronomists go back to the exact same spots at the same growth stages year after year for soil and tissue sampling, taking the field history and weather patterns into account when evaluating results.
"You need to look over multiple years with tissue sampling and not say that this year we are short of copper or manganese and apply it," says Trcka. "A tissue test can show a possible deficiency, but not the cause. If the soil test shows adequate levels, then you have to look at other possible causes. Is there compaction or is the soil too mellow and too dry for capillary action to allow a nutrient to be diffused through the soil?"
One trend that Trcka and Gehling have identified with tissue testing is a consistent potassium deficiency in corn from V10 through R1. What they don't have is a response that works.
"Common wisdom suggests deficiency should result in yield loss," says Trcka.
Lack of return
Gehling has tried in-season supplements. However, even when a yield increase occurred, it didn't pay for itself. In one case he tried proactively sidedressing an additional 44 lbs. of K2O at the V5 stage. Tissue tests at R1 showed increased (though still deficient) K levels. With a two-bushel increase at $3.10 per bushel prices (October 2014), the gross return per acre didn't cover the $17.68 in additional inputs.
"Before throwing money at a deficiency, you have to ask if you are making a return on it," says Trcka. "You also have to have your other macro and micro nutrients in balance."
"If N, P, K or sulfur are deficient, they are likely to be a cause of apparent micronutrient deficiencies as well," suggests University of Minnesota extension soils specialist Daniel Kaiser.
Trcka and Kaiser warn against taking any one cited deficiency range as valid. Trcka points out that significant differences exist in deficiency ranges based on different university researchers.
"Before purchasing a product based on a tissue analysis, check with another lab or an extension specialist and compare sufficiency ranges."
When considering a suggested product, ask what the yield data is behind the recommendation and where it was gathered, advises Kaiser. He notes that even N, P and K yield responses vary by location, and tissue uptake can even vary by hybrid.