With increasing crop yields, Minnesota growers are feeling more pressure to apply secondary and micronutrient fertilizers.

Secondary nutrients and micronutrients play an important role in balanced crop nutrition and are an essential part of plant growth. The Agricultural Fertilizer Research and Education Council (AFREC) has funded research projects on the secondary nutrient sulfur and micronutrients boron and zinc.

University of Minnesota researchers from the Department of Soil, Water and Climate recommend taking these factors into consideration when applying S, B or zinc:

INSUFFICIENT NUTRIENT: This corn plant shows a zinc deficiency. In pursuit of larger yields, Minnesota growers are feeling more pressure to apply secondary nutrients like sulfur and micronutrient fertilizers like boron and zinc.

Corn, canola, alfalfa and clovers have the greatest potential response to sulfur. In Minnesota soils, nearly 95% of the total amount of S is found in the organic matter and plant residue left from previous crops. Soil testing for sulfur in Minnesota is recommended only on sandy soils with low organic matter concentrations in the soil surface.

It is not suggested to soil test on medium- and fine-textured soils because research indicates yield responses on these soils are often correlated to soil organic matter (SOM) content in the soil surface (see Table 1). In alfalfa, tissue-testing for S works very well, but it is not as reliable in corn.

Sulfur has the greatest return on investment potential of all secondary nutrients and micronutrients. “When the right rate is applied, sulfur gives growers the greatest return on investment — especially on low-organic-matter soils, poorly drained high-organic-matter soils and in continuous corn. Under these conditions, or when soil tests or tissue analyses indicate a likely response to S, annual applications are suggested,” says Jeff Vetsch, soil scientist, U-M Southern Research and Outreach Center.

For corn production on sandy soils, current U-M research supports the application of a broadcast rate of 25 pounds of S per acre. However, reduce the rate to 10 to 12 pounds per acre if S being applied in a starter band away from the seed row.

On medium- and fine-textured soils with SOM concentrations of less than 4%, the university suggests a broadcast application of 10 to 15 pounds of S per acre for both corn and alfalfa.

For intensively managed alfalfa, it is suggested to increase the application rate up to 25 pounds of S per acre annually. When SOM content is greater than 4%, the probability of a yield response to S fertilization is lower.

When applied at recommended rates, variable-rate application of S fertilizer is oftentimes not cost-effective because of the price of the nutrient.

Choose the right source of sulfur and application time. It’s important to know which sources of S are being applied to a field. Early-season S deficiencies may occur when only applying elemental S, due its slow availability. Additional sulfate-S application may be needed. When using additional sulfate-S sources, the best application time is spring. However, fall is also acceptable.

Soil tests are key when considering the use of boron or zinc. “When considering the need for boron or zinc in a fertilizer program, it’s best to be proactive and confirm deficiencies using a soil test,” says Dan Kaiser, assistant professor, U-M Department of Soil, Water and Climate.

Recent soil tests results best determine if growers should apply B fertilizer. For sandy soils or soils in high-rainfall areas, a hot water B test is appropriate where a response to B might be expected. Under the right circumstances, alfalfa is the most likely crop to show a response to a B application. Soil test interpretations are made with certain crops in mind. Table 2 lists current suggestions for B use in a fertilizer program for alfalfa and selected horticultural crops.

In recent studies, corn and soybeans do not show a response to B fertilizer, even when a soil test suggested a low relative level of B in the soil. Be cautious when interpreting soil test results for B on crops that have not shown a strong need for the nutrient, regardless of the yield produced.

When determining the need for zinc in a fertilizer program, a soil test is the best tool. However, a plant tissue test may be also helpful in confirming zinc deficiency in corn. Since it’s rare for an entire field to have a low or very low zinc soil test, grid sampling is recommended to identify deficiencies. To correct these smaller areas, a variable rate application of 5 pounds of zinc sulfate per acre is adequate for several years, or at least until the next soil sampling. Recent AFREC-funded trials show applying chelated zinc with starter fertilizer was less likely to give an ROI, especially when zinc was applied to the entire field.

In Minnesota, data have shown a correlation of zinc soil test results to where corn yield responses will occur. However, practice caution when interpreting soil tests for other micronutrients. When collecting plant tissue samples, be mindful that the sufficiency of a particular nutrient is related to a specific plant part collected at a certain growth stage.

Whether using secondary nutrients or micronutrients, soil tests and plant analyses can help determine where profitable yield response may occur. Having knowledge of the limitations of soil and plant tissue tests is crucial to ensure a decision will be made with the data that will increase profitability. 

Source: Minnesota Agricultural Fertilizer Research and Education Council

The Minnesota Agricultural Fertilizer Research and Education Council funds various research projects that explore tools to sustain crop production and reduce potential environmental effects from crop nutrients. To see results of the council’s research, visit mnsoilfertility.com.