The Role of Phosphorus in Starter

The Role of Phosphorus in Starter
Building on decades of starter fertilizer experience and knowledge, Farm Journal Field Agronomist Ken Ferrie took his research a step further to learn more about phosphorus’s role in starter fertilizer blends. 

What Ken Ferrie Thinks You Need to Know
  • When shopping for an in-furrow 10-34-0 polyphosphate fertilizer, buy a good clean product. If not available, look into orthophosphates. 
  • Evaluate options based on price per pound of nutrient, not per gallon. Orthophosphate starter fertilizers run $6 to $8 per gallon and polyphosphate fertilizers cost $2 to $4 per gallon.
  • Plants respond in yield to a higher amount of phosphate applied closest to the plant.
Before we explore the data, let’s review the two sources of phosphorus in starter fertilizer. Polyphosphates are orthophosphate molecules joined together in the manufacturing process by heat and water removal. Poly fertilizers have a higher analysis compared with the ortho form. When added to the soil, the poly fertilizer combines with water to break down; the ortho fertilizer is readily absorbed by the plant. 

Test Plot #1
To analyze the data, Ferrie and crew measure starter fertilizer on a gallon per acre basis. In central Illinois, they compared 3 gal. 10-34-0 Zn Avail (poly) and 3 gal. 3-18-18 Zn Avail (ortho). That breaks down to 3-10-0 Zn Avail and 0.9-5.4-5.4 Zn Avail per acre. 

“This is where it’s easy to get confused,” Ferrie says. “If you applied 3 gal. 3-18-18, this does not mean you applied 54 lb. of phosphorus in your starter fertilizer.” 

The 3-10-0 Zn Avail (poly) yielded 16 bu. more than the check. The 0.9-5.4-5.4 Zn Avail (ortho) application yielded 5 bu. better than the check. As shown on the chart at right, the additional 5 lb. per acre of phosphorus in the poly fertilizer resulted in an additional 10 bu. per acre gain compared with the ortho fertilizer. This yield response is directly correlated to the pounds of phosphorus applied. 

In addition, Ferrie also evaluated starter response to phosphorus rates and placement. As a rule of thumb, data shows plants directly respond to applied phosphate. Therefore, the higher amount of phosphate applied closer to the seed, the higher the response. These findings often spur debates regarding ortho versus poly starter fertilizers.


When applying starter in-furrow, don’t forget about the risk of starter burn, Ferrie cautions. To avoid starter burn, limit in-furrow rates to reduce the potential harm from applying too much salt around the seed, which can affect germination and ear count. Typically, Ferrie recommends a 10-34-0 poly starter fertilizer to avoid starter burn (no potash). 

However, if you want potash in your blend, ortho starter fertilizers have the potential to shine. These blends use potassium hydroxide, which is not a salt, so it’s considered safer and allows for higher rates in-furrow. However, using an ortho starter fertilizer does not guarantee you avoid starter burn, Ferrie adds. 

“Orthophosphate starter fertilizers do open the door to new possibilities in-furrow,” Ferrie says. “But you have to make sure the jump in fertilizer costs can be made up in yield.”

Test Plot #2 
Ferrie also studied three in-furrowapplications: 4 gal. 6-18-6 Zn Sulfur (ortho), 4 gal. 3-18-18 Zn Avail (ortho) and 4 gal. 10-34-0 Zn Avail (poly). As shown in the chart below, averaged across all management zones, the first (ortho) application yielded 3.1 bu. compared with the check, the second (ortho) received a 4.9-bu. gain and the third (poly) had a 4.7-bu. response. On average, the poly fertilizer was the only application to pay the fertilizer bill. 

Ortho fertilizers are worth the extra cost if you can’t buy a quality 10-34-0 poly starter fertilizer in your area. Because ortho costs twice as much as poly, it’s also a good idea to know your ROI for using one fertilizer versus the other. 

A good clean 10-34-0 poly starter fertilizer should be green with no sediment or offensive smell, Ferrie says. 

Across all starter fertilizer plots in central Illinois, yield data consistently shows the direct correlation to starter response and pounds of phosphorus applied. The higher amount of phosphate you can apply closer to the plant, the higher the response. Farm Journal Test Plots data suggests higher rates in starter placed 2" to the side and 2" down from the surface (2x2) can yield more than low rates in-furrow. 

On average, past data shows there’s a 3-bu. to 5-bu. response to in-furrow applications and a 7-bu. to 10-bu. response to 2x2 applications. 

“Due to starter attachment costs, many farmers only apply starter fertilizer in-furrow, which works,” Ferrie says. “However, by only applying in-furrow at a lower rate, you’re admitting on the front end your responses will be lower.”

In this case, Ferrie recommends using the highest rate possible (whether that be a poly or ortho starter fertilizer) to achieve enough yield response to pay the bill but not burn the seed. However, in 2x2 applications, Ferrie says the economical choice is a poly starter fertilizer with potash. 

“If you’re applying 2x2, you don’t have to play it as safe as in-furrow,” Ferrie says. “Therefore, using a polyphosphate blend such as 7-22-5 or 6-18-6 will help you achieve a higher yield response at less cost.” 

Test Plot #3 
To take the results a step further, Ferrie also evaluated the relay effect. In past Farm Journal Test Plots, dual placement has shown a 15-bu. to 20-bu. yield increase. In 2016, the crew evaluated 4 gal. 3-18-18 Zn Avail ortho in-furrow as well as 10 gal. 7-22-5 Zn 2x2 and 4 gal. 3-18-18 Zn Avail ortho in-furrow. The in-furrow application had an average 5-bu. increase and the relay effect boosted yield by 10 bu. per acre. 

“Once again, you can see the power of the relay effect,” Ferrie says. “It’s like the fertilizer placements hand off a baton, so the roots find the nutrients as they need them,” he explains. 

When you adjust the phosphate amount in your starter fertilizer blend, don’t forget to reduce the amount in other applications to maintain your total goal, Ferrie says. For example, if you apply 10 gal. 7-22-5, you will need to pull back 50 lb. of DAP. 

Regardless of the placements or blends, it’s essential to consider the economics of starter fertilizer in all situations. Choose the combination that best fits your yield goal, soil types and pocketbook.


Thank You to Our Test Plot Partners

Case IH, Jay Barth, Bill Hoeg and CJ Parker; Great Plains, Mike Cleveland and Doug Jennings; New Holland, Mark Hooper, Daniel Valen, Ken Paul, Mike Kizis and Sheldon Gerspacher; Precision Planting and Cory Muhlbauer; Versatile and Adam Reid; Central Illinois Ag and 
Kip Hoke; Kinze Manufacturing, Susanne Veatch and Phil Jennings; Marco N.P.K. Inc; Schaffert Manufacturing and Paul Schaffert; SFP; Unverferth Manufacturing and Jerry Ecklund; Trimble, Frank Fidanza and John Pointon; AirScout and Brian Sutton; Geovantage; Ag Leader and Luke James; Yetter Manufacturing, Pat Whalen and Scott Cale; Blu-Jet and Nick Jensen; Yamaha; Don Schlesinger and Dan Reynolds; Shorty Olson; Crop-Tech Consulting, Isaac Ferrie, Matt Duesterhaus, Mike Carl, Logan Koester, Aaron Herrmann and Chelsea Ferrie

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