Look beyond product rates when considering pop-up fertilizers

Cool and wet spring conditions have many growers looking at additional measures to get plants up and out of the ground once planting begins.

Every growing season reports surface of pop-up fertilizers gone wrong. Many times producers are left trying to determine the source of error. To clear up some recent confusion concerning pop-up fertilizer applications, Michigan State University Extension has a few additional guidelines for consideration.

When soil temperatures are cold, surface residue high and soil nutrient levels low, starter fertilizer applications become important for early season plant growth. However, applying fertilizer in direct contact with the seed (pop-up) is generally not recommended as many times the risks of reduced germination and yield loss outweigh potential benefits. Fertilizer placement 2 inches below and 2 inches to the side of the seed (2x2) can improve fertilizer efficiency and poses minimal risk of plant injury.

Product rate and cation exchange capacity

Pop-up fertilizer applications can be successful at low application rates, but additional guidelines should be considered. For corn production, do not place more than 5 lbs N + K2O in contact with the seed when the CEC is less than 7 and no more than 8 lbs N + K2O in contact with the seed when the CEC is 8 or greater. Much of Michigan’s corn acreage is grown on low organic matter (less than 3 percent) soils, which greatly influence these guidelines.

Low CEC soils have less reaction between the fertilizer and soil, causing the soil solution salt concentration to remain high. Some Midwestern states have greater percentages of organic matter which can increase the interactions between soil and fertilizer resulting in higher application rates without additional plant injury. Soybean seed is more sensitive to salt injury than corn and starter fertilizer should be broadcast or 2x2 applied rather than pop-up application.

Environmental conditions

Pay attention to the weather forecast both pre- and post-planting date. Soil moisture conditions can have a profound effect upon pop-up fertilizer performance. Fertilizers contain salts in which the charged molecules can attract water, also a polar or charged molecule. The greater the salt concentration near the seed, the greater the potential to draw water out of the roots, resulting in plant desiccation. Under moist soil conditions, salts can diffuse away from the band of fertilizer through soil solution.

When dry conditions exist, minimal diffusion occurs causing fertilizer salts to remain near the seed for longer periods of time. The longer the fertilizer band remains in contact with the seed, the greater the potential for reduced germination. Warm, dry planting conditions in April 2012 were a great illustration of this point.

Regardless of application rate, many pop-up fertilizer studies resulted in reduced plant stands due to several weeks of warm temperatures and little to no moisture immediately following planting. In extremely dry post-plant conditions, fertilizers placed directly beneath the seed may be drawn upwards into the seed furrow through the evaporative stream of water. If you suspect pop-up fertilizer damage, examine the newly emerging roots from affected plants. Root tissue should be white or have a whitish hue, not brown or black as this would indicate root dieback.

Fertilizer source

Minimize fertilizer compounds that liberate or release volatile ammonia as this is toxic to plant tissue. Urea or compounds containing urea are the greatest risk factors as ammonia is formed during urea conversion to plant available form and will damage plants in close proximity. Urea and UAN are not recommended for pop-up fertilizer applications. Due to high salt index, fertilizers containing ammonium thiosulfate are also not recommended for pop-up fertilizer applications. Ammonium polyphosphate solutions (10-34-0) contain N in the ammonium form, but can still cause significant plant injury at higher application rates. Also, be sure to double-check that the crop you are applying to can tolerate your application rate as tolerance levels will differ between plant species (i.e., corn as compared to sugarbeet).

Despite the recent rains across the majority of Michigan, fertilizer placement further away from the seed as in a 2x2 application is still safer than pop-up applications. Placements further away from the seed will also allow for larger nutrient application rates potentially delaying sidedress fertilizer applications closer to the time of greatest plant need. When applied 2 inches below and 2 inches to the side of the seed, combined N+ K2O applications should not exceed 100 lbs/A for corn or 70 lbs/A for 30-inch row soybeans.

Inclusion of phosphate in the starter mix at up to 100 lbs/A can help maintain soil P levels in low P soils, but inclusion rarely increases grain yield in high P (greater than 15 ppm) testing soils. The limited availability of triple superphosphate means that most P fertilizers will be combined with N. When used for starter applications, do not band in excess of 40 lb N/A for corn or 20 lb N/A for 30-inch row soybeans as nodulation may be impacted. Fertilizer for drilled soybeans should be broadcast before planting.

No fertilizer application is without risk, but knowing the variables involved may help growers minimize the risks for early-season starter and pop-up nutrient applications.

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