Unfenced Answers: Adequate Micronutrient Levels?


Today's high-yielding crops require more frequent soil testing since their nutritional demands are greater. A heavy rainfall or coarse-textured soils can move these nutrients down beyond the typical soil sampling depth of 6, 8 or 10 inches.

"As much as 60 percent of yield depends on soil fertility," says Curt Woolfolk, Manager of Crop Nutrition Technologies for The Mosaic Company. "Regardless of what we spend on crop inputs, if we haven't taken care of our base soil fertility, we're not going to maximize the yield potential of the crop." 

It is important to continually monitor and replenish the soil. "In order to keep levels where they are today, we need to apply nutrients to replace what we're removing with the crop," adds Woolfolk.

Out of 17 essential crop nutrients, there are eight micronutrients: boron (B), chlorine (Cl), manganese (Mn), iron (Fe), nickel (Ni), copper (Cu), zinc (Zn) and molybdenum (Mo). They are called micronutrients because they are needed in small amounts by the plants. Though they are generally taken up in ounces/acre rather than pounds/acre, they are critical to maximize the yield potential of today's hybrids and varieties. 

"Mobile micronutrients such as boron, chlorine and molybdenum are important to measure during a routine soil analysis, but it is important to note that these nutrients can fluctuate throughout the growing season due to their chemistry," explains Woolfolk. A heavy rainfall event or coarse-textured soils can encourage downward movement of these nutrients and beyond the typical soil sampling depth of 6, 8 or 10 inches. 

Micronutrients that are immobile (Mn, Fe, Ni, Cu, Zn) do not move rapidly through the soil profile, so these levels change more gradually. Universities and soil test labs have compiled a great deal of data to support their recommendations (field calibration to show yield response), and are specific to each region, crop and micronutrient. 

"It is important to remember, no matter how high-tech the lab is, the results are only as good as the quality of the soil samples," Woolfolk says. The goal is to get nutrient recommendations from the soil lab based on the intended yield goal for the crop and the results of the soil test.

While it may seem obvious, it is important to follow best practices:

  • Use a quality soil sample probe rather than a spade.

  • Pull a minimum of 8 to 12 core samples across the entire field and within each management zone. Try to pull 4–6 cores in grid-sampling situations.

  • Core samples should always be pulled from a consistent depth. Standard topsoil depths include 6, 8 and 10 inches.

  • Do not angle sample probe when collecting cores. The probe should be placed at a 90° angle to the ground.

  • Mix sample cores in a clean plastic bucket (galvanized can affect results) and a properly labeled soil test bag – one for each field/area. Write down the crop and a realistic yield goal.

"As a general rule of thumb, the more cores that you take for a sample, the better the analysis will be from the lab," Woolfolk points out. "In the past in the Midwest, we may have only tested every four years, but now we recommend growers test every two years." 

He continues, "It's always important to review the soil test value and associated recommendation, but also look at micronutrient uptake and removal for a particular cropping rotation. Then visit with your retailer or lab about next steps to improve, or at least maintain, a balanced crop nutrition approach." 

Mosaic premium products like MicroEssentials® SZ® and Aspire® with Boron achieve uniform nutrient distribution, which is especially important for micronutrients. This unique feature can help take your crop to the next yield level. Additionally, an on-farm trial is always a great way to quantify the yield response for a given soil type and environment.

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