Minnesota farmers often have to plant corn in cool, wet soils — an environment that can slow nutrient uptake and early-season growth. Poor early-season growth is often associated with reduced phosphorus uptake. Minnesota also has a lot of high-pH, calcareous soils that tie up phosphorus, making the nutrient unavailable to young plants.

Because of this, many growers apply in-furrow — or pop-up — starter fertilizer to help the crop get off to a good start and ensure the potential for maximum yields. In-furrow liquid starters are appealing because they are easy and cheap to apply.

However, pop-up fertilizers are an expensive form of nutrient, pound-for-pound. Five gallons per acre of ammonium polyphosphate (10-34-0), which supplies 20 pounds of P2O5, costs $12 to $15 per acre.

This spring, as margins remain tight, many farmers are taking a hard look at the profitability of pop-up fertilizers. Recent University of Minnesota research can help you decide when it makes economic sense to apply in-furrow starter.

If you broadcast P, do you need starter, too?
A U-M study from 2012 to 2015 examined corn yield response to different pop-up fertilizer rates, with and without broadcast phosphorus.

The research was done on highly variable soils at 10 locations from southeast to west-central Minnesota. Soil P-test levels ranged from low to very high, which allowed researchers to look at the response to pop-up starter and broadcast P across variable fields and also within soil P test levels. Previous research has shown that a yield response to P fertilizer is most likely on low- and medium-P-test soils.

Pop-up treatments of 10-34-0 were applied to corn after soybeans at rates ranging from 0 to 7.5 gallons per acre. Starter rates were tested both with and without 120 pounds per acre of broadcast P2O5, a non-limiting rate for corn grown on low-P-testing soils.

The study showed that starter fertilizer produced a corn yield response only when broadcast P was not applied, regardless of the starter rate or soil P-test levels. For this reason, starter fertilizer is not recommended if you are already broadcasting phosphorus at recommended rates for corn, says study leader Dan Kaiser, U-M Extension nutrient management specialist.

The study also found that corn yields on these variable soils were not affected by the starter rate, Kaiser says.

“The surprising result was that the 2.5-gallon-per-acre rate was the best in most circumstances for low-, medium- or high-P-testing soils,” he says.

The finding suggests that, currently, variable-rate starter application is not worth the trouble. If you do apply a pop-up, Kaiser recommends a uniform rate of 2.5 gallons per acre of 10-34-0 for neutral to acid pH soils, and 5 gallons per acre for high-pH or low-P soils. He also suggests 5 gallons per acre for 20- to 22-inch rows.

University of Minnesota-replicated trials from 2012 to 2015 at 10 locations in southern and west-central Minnesota evaluated the effects of pop-up starter rates on corn yields, with and without broadcast P. As the chart shows, in-furrow starter fertilizer (10-34-0) increased corn yields significantly only when broadcast P was not applied. The bars represent the percentage of average corn yield for each treatment. The chart also shows that the rate of starter did not significantly affect yield response in either set of treatments, regardless of soil test P levels. Source: Dan Kaiser, University of Minnesota

Is pop-up starter needed for early-planted corn?
An earlier U-M study from 2010 to 2012 found that planting date did not affect pop-up fertilizer response in corn.

The field trials were performed on corn after soybeans at Lamberton and Waseca, where soil test levels for phosphorus ranged from medium to very high.

Ninety-four-, 99- and 104-day corn hybrids were planted at two-week intervals, beginning in late April and extending to late May. This was to simulate early, on-time and late planting dates. Two starter fertilizer treatments were tested for each hybrid and planting date combination: No starter versus 5 gallons per acre of 10-34-0 placed in the seed furrow.

As expected, corn planted with starter did show greater growth in the early spring, Kaiser says. In addition, corn planted with the pop-up silked a couple of days earlier and had slightly reduced grain moisture at harvest.

However, Kaiser found the pop-up did not increase corn yields based on planting date or relative maturity. In terms of economic benefits, lower grain moisture generally covered the cost of the pop-up, he says, but did not produce a profitable return on soils that had medium to high soil-P-test levels.

Applying this research on your farm
What management conclusions can we draw from these research findings?

• There’s little or no economic benefit to using starter on top of recommended rates of broadcast P.
• The decision to use pop-up fertilizer should not be based on planting date or hybrid relative maturity, but rather on nutrient needs revealed by soil tests.
• If you need to cut corn input costs this spring on medium soil P-test fields, consider applying 2.5 to 5 gallons per acre of 10-34-0 instead of broadcasting higher rates of dry fertilizer. While this is not an adequate long-term nutrient management strategy, it is a low-risk option for one or two seasons, especially on cropland in short-term rental contracts.
• If you need to cut corn input costs this spring on high soil P-test fields, consider skipping phosphorus application for corn after soybeans.
• Keep in mind that there’s not one right strategy for P management, which is influenced by many variables, including soils, prices and agronomic factors such as crop rotation and genetics. Your crop consultant can help you sort out what makes sense on your farm.

Burns, owner of C.B. Agronomics, is a crop consultant from Stewart. Find information and links to Minnesota certified crop advisers at mcpr-cca.org.