Watch out for falling potassium


unlike the weather, corn prices, and taxes – fertilizer strategies are something that the average corn producer feels they have some control over.  Although it is not exactly rocket science, there are generally lots of questions about rates, timing, liquid versus dry and band versus broadcast.  Add to that the volatile prices over the past few years and most growers have done some recent re-evaluation of their approaches to corn fertility.

Several years ago, researchers from the University of Guelph and OMAFRA set out to accomplish a couple of things in regards to fertilizer. First, we set out to build a database of all the phosphorous research data, similar to the effort on nitrogen, so that we could re-evaluate recommendations. Second, we wanted to examine, with plot work, some of the nagging questions around starter fertilizer for corn. 

Although both of these projects are on-going, some of the field results from 2008 and 2009 have drawn some unexpected attention to the potential impact that declining soil test levels for potassium might have on starter fertilizer strategies for corn.

the potassium factor
Potassium (K), the Rodney Dangerfield of nutrients (it gets no respect) may be at play more than you might expect depending on where your K soil test levels have fallen to over the past decade.

There are some obvious reasons why we might expect K soil test values to have slipped.  Soybeans are quite heavy users of K and many Ontario acres see soybeans frequently. In addition, corn yields have increased dramatically in many areas thus increasing the amount of K removed and finally, high potash fertilizer prices may have recently constrained K applications.

As usual, we didn’t set out to study potassium as we were more interested in nitrogen (N) and phosphorous (P) effects.  However, as we looked for responsive sites that had lower P soil test levels we naturally found some fields that were relatively low for K as well.

Some of these results are illustrated in Table 1.  The conclusions are not overly complicated or perhaps surprising. That is, when soil test levels for K were down in the 60 to 80 parts per million (PPM) range, starter fertilizers with some K generally out performed starters without K.

Table 1. The impact of starter fertilizers on corn yields in 2008 and 2009.

Site Elora 2008 Elora 2009 Alma 2009 Bornholm 2009
Soil Test Values P:11 K:61 P:8 K:68 P:35 K:77 P:18 K:80
Starter Fertilizer Treatments Corn Yields bu/ac)
Control (no starter) 188 168 179 125
10-34-0 @ 5 gal/ac (in furrow) 195 169 180 116
11-52-0 @ 75 lb/ac (2×2) 198 165 180 114
11-52-0 @ 75 lb/ac + UAN @ 10 gal/ac (2×2) 199 163 171 114
6-24-0 @ 5.0 gal/ac (in forrow) 204 177 181 128
5-20-20 @ 200 lb/ac (2×2) 220 184 183 150
Note: At both Elora 2008 and Elora 2009, 200 lbs/acre of 0-0-60 was broadcast in addition to starter treatments

At the Elora sites this was evident even though we tried to address the low K levels by broadcasting 200 pounds per acre of 0-0-60 fertilizer.  In some cases you could go as far to say that the starter N and P investment was wasted if you didn’t deal with the low K values.

The most extreme results were from a site at Elora which was very responsive to starter fertilizer.  Figure 2 highlights the dramatic impact of starter fertilizers; especially it seems for those treatments which had some K, even if in very small quantities (i.e. five gallons per acre of 6-24-6 fertilizer).  Potassium deficiency symptoms were extreme at this site in those treatments that did not receive K (see Figure 1).


so what?
Potassium does behave differently than N or P.  Research has generally supported the idea that even when soil test levels are high, corn yields can be boosted with a relatively small amount of N and P placed close to the seed (i.e. in furrow or 2×2).  K does not generally behave this way, so the results in this article should not make you go out and include K in your starter if soil test K levels are already high. If your soil tests reveal K levels over 121 PPM, your field will probably not benefit from an application of K.

If you have K soil tests in the 120 to 80 PPM range the OMAFRA recommendations are to apply 30 to 50 kilograms K20 per hectare, respectively. You probably need to pay attention to these fields so that soil test levels do not slip further. However, these results do not deal with the question of starter versus broadcast K for soil tests in this range.  The best recommendation is to apply the potash in the most cost effective manner for your farming operation.

If you have K soil test levels down in the 60 to 80 range the OMAFRA recommendation is to apply 80 kilograms K20 per hectare (that would be 117 pounds per acre of 0-0-60).  These results suggest that this potassium application may be quite critical to corn yield and that in some of your lowest testing soils, K in the starter may add to yields even when a broadcast application of K is performed.

However, please don’t expect yield responses as dramatic as some of those reported here; we may never duplicate them in the 30 years of plot work we’ve got left in us. Secondly if you are not sure what your soil test levels are for P and K, make that job number one this spring!

This research was made possible through the support of the Grain Farmers of Ontario, Innovative Farmers Association of Ontario, the Ontario Research and Development Program, OMAFRA and the University of Guelph.

About Greg Stewart 24 Articles
Corn Specialist, Ontario Ministry of Agriculture, Food and Rural Affairs