Features Agronomy Fertility and Nutrients
Putting the K in corn

For the starter fertilizers with nitrogen, phosphorus and potassium, either in-furrow or 2×2 placement resulted in profitable yield responses at the project sites.   Photo courtesy of Greg Stewart, OMAFRA.

Greg Stewart calls potassium “the Rodney Dangerfield of nutrients” that gets no respect, at least in comparison to phosphorus and nitrogen, in starter fertilizers for corn. But his current research is showing that it can pay to give attention to potash.

Stewart, corn specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), is leading a three-year project to evaluate the yield and economic responses of corn to various starter fertilizer options. “We know that proper use of starter fertilizers can increase corn yields and net profits. But growers are faced with choosing among many options for products, placement and rates. And these choices affect yields and/or application costs,” he explains.

The project is funded by the Ontario Corn Producers’ Association (Grain Farmers of Ontario), Innovative Farmers Association of Ontario, Ontario Research and Development Program, University of Guelph, John Deere and OMAFRA.

When Stewart started the project in 2008, he was focusing on nitrogen (N) and phosphorus (P), which are the two usual nutrients for starter fertilizers (see box). He included potassium (K) in some of the treatments because the field sites with low P levels also often had low K levels. 

The project’s sites are in Ontario, with four sites in 2008 and 10 in 2009. The sites have loam, silt-loam or silty clay-loam soils with medium to medium-low soil test P and K levels.

The starter fertilizer treatments compare several products: ammonium polyphosphate (10-34-0); urea ammonium nitrate (UAN, 28 percent); mono-ammonium phosphate (MAP, 11-52-0); 6-24-6; and 5-20-20, at various rates (see chart). The treatments are also comparing in-furrow placement versus a band two inches to the side and two inches below the seed row (2×2). 

To meet the crop’s nitrogen needs, 150 lbs of N per acre of bulk N fertilizer was applied at all sites. To try to meet the crop’s K needs, 200 lbs per acre of pre-plant broadcast potash (K2O, 0-0-60) was applied to all treatments at the Elora site in 2008 and 2009 and at the Ancaster site in 2008.

Unexpected yield boost
Not surprisingly, data from the project’s first two years showed that yields and returns varied widely, depending on the starter fertilizer option. However, the results for the starters with K were a bit of a surprise.

“The results show that on fields with medium to medium-low soil P and K tests, profitable yield responses are more likely to occur when N-P-K-based starters are applied, as compared to starters that contain only N and P,” says Stewart.

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“In some cases you could say the starter N and P investment was wasted if you didn’t address the low K levels.” For instance, at the Elora site in 2009, yields with the N-P starter treatments were not significantly different from the yields with no starter fertilizer at all, whereas the yields with the N-P-K starters were significantly better than with no starter (see graph).

The effect of starter K was apparent even at the sites where 200 lbs per acre of broadcast K was applied. “Two years ago, I would have expected that if I had broadcast 200 lbs of potash, then potash in the starter fertilizer would have been irrelevant. That doesn’t appear to be the case. Even with 200 lbs of potash, when soil test K is down into the 60 to 80 ppm range, the starter fertilizers that included potash still tended to out-perform the starters without K,” notes Stewart.

Generally, either in-furrow or 2×2 placement of the N-P-K starters provided profitable yield responses, although the 2×2 placement produced a larger response.

Starter fertilizer based on soil test levels
Stewart says that falling soil potassium levels may not be all that uncommon in Ontario corn fields these days. High-yielding corn crops and frequent soybean crops both use a significant amount of K. As well, producers may have been limiting their K applications because of high prices for potash fertilizer in recent years.

High-yielding corn, frequent soybean crops and higher potash prices in recent years have resulted in lower soil potassium levels. Photo by Ralph Pearce.	Both Greg Stewart and Keith Reid were surprised to see project results showing a yield response to K in the starter fertilizer. Photo by Ralph Pearce.

For K applications for a corn crop, he recommends: “If your field has over 121 ppm of soil test K, you don’t need to include K in your starter. If your field has soil test K in the 80 to 120 ppm range, the OMAFRA recommendation is to apply 50 to 30 kg of K2O per hectare (44.6 to 26.8 lbs per acre), respectively. In terms of starter versus broadcast K, the best approach is to apply K in the most cost-effective manner for your operation. If your soil test K levels are in the 60 to 80 ppm range, the OMAFRA recommendation is to apply 80 kg of K2O per hectare (71.4 lbs per acre). The project results suggest this K application may be critical to corn yield and, in some cases, starter K may increase yields even when broadcast K is applied.”

The only way for a grower to tell which nutrients the soil needs is by soil testing. OMAFRA soil fertility specialist Keith Reid says, “No farmer or agronomist can look at a handful of soil and tell which nutrient concentration is there. A soil test is a fairly inexpensive way of getting a snapshot of what’s in the soil.”

Reid explains that a soil test will tell a grower about both starter fertilizer needs and bulk fertilizer needs. “A plant might respond to a little bit of potash near the seed, but if you’ve got a low soil test, the crop is going to need a lot more than that in total to meet its needs. So you have to balance both; what do you need for that starter effect, and what do you need to meet the needs of the crop to get optimum yields? The soil test gives you both of those pieces.”

Like Stewart, Reid was initially surprised to see the project results showing a yield response to K in the starter fertilizer. Reid says, “Two years ago I would have said you don’t need much K in the seedling because the seedling is not very big. When you track potassium uptake, it pretty much matches the plant’s growth pattern. So when you have a small plant, it doesn’t take up a lot of potassium. As it gets bigger during periods of rapid growth, it’s taking up a lot of potassium. And as it matures, the plant is actually losing potassium back to the soil.”

Reid says the response to starter K at the project sites is likely because soil K was in such short supply. “The seedling’s root system is small enough that even the relatively modest K requirements of the seedling can’t be met quickly enough from the soil in low K fields unless there’s a little bit extra potash.”