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What caused low bean yield?

Rod Swoboda Soybean field in fall
SOIL FERTILITY: Phosphorus, potassium and pH levels need to be in the appropriate range for your soil types. Best bet is to regularly test soils to make sure.
Soybean Source: Disappointing yields on newly rented ground prompt questions and possible solutions.

I’m not sure if it was a side effect of all the political ads or COVID-19 fatigue, but writer’s block has had me in a headlock lately. Luckily for me, editor Rod Swoboda came to the rescue, by asking what interesting questions have come up this fall. Suddenly, the challenge was trying to narrow down the focus since harvest generates a lot of good questions!

I received this interesting question from a farmer recently. “Our soybean yields on a newly rented farm were way off from what we expected in 2020. What is the first thing we can do to help improve yield? We have the farm rented for two more years. The landowner isn't sure when it was soil-tested the last time.”

Folks who know me or read my articles know I rarely miss an opportunity to answer their questions with more questions of my own, so here we go. We can’t go into every possible scenario, but we’ll hit highlights that are common to this discussion.

Do you feel the low yields were due to weather issues, soil compaction, soil fertility, diseases or some other factor or combination of factors? The reason I ask farmers these questions is to get them to share what they think, their gut reaction. They are the boots on the ground, have years of experience and usually have a pretty solid idea of what some of the issues were, so their input is at the top of my list.

Is soil compaction the culprit?

Since the weather is out of our hands, let’s dig into some areas where we have more control. How was the soil handled prior to you taking over the farm? What sort of tillage program was it in? Was it worked wet, perhaps compacted at some point?

Digging some holes to check soil structure and looking at some old roots this fall may help show if there are signs of soil compaction. You can also work with your local agronomist and use a soil penetrometer. If tillage history, or your fall soil and root inspections lead you to suspect that compaction is a problem, consider some fall tillage or sowing a cover crop to help eliminate soil compaction layers.

Has the field had soybeans in the previous three years? If not, did you inoculate the soybean seed you planted in 2020 to increase nodulation? Were there any disease issues, either discovered through scouting or suspected issues? Sometimes diseases hit yield without a lot of easily seen symptomology.

Test for the big one

And a biggie that you knew I had to ask about … the suspense is too much … but I want you to take a guess at what three-word pest could be an invisible yield robber in our bean fields. Yup, soybean cyst nematode. If there aren’t current soil tests for fertility, I’d bet that SCN tests haven’t been done in a while — if at all. So I’d recommend getting some SCN soil tests done and take a look at the Iowa State University SCN-Resistant Soybean Variety Trials.

Even if you planted SCN-resistant beans this year, a quick study of the variety trials will show that not all SCN-resistant beans are created equally.

Now that we’ve hit some of the easier topics, it’s time to tackle the elephant in the room” Let’s take a hard look at soil fertility. If the landowner isn't sure when the last soil samples were taken, it's likely been neglected for some time. So prepare yourself for the potential that pH and soil fertility levels could be low.

To understand where to start improving the field, soil samples should be taken this fall, so we can get results and have some time to evaluate issues in the field and take steps to improve them, like spreading any needed lime, phosphorus or potassium. In my experience, correcting low pH levels with lime prior to spending money on additional fertilizer has been a solid way to prioritize budgets.

Correcting low pH

Start with pH since it drives the soil activity. Sure, we’d prefer to get any needed lime spread this fall, but if the weather doesn’t cooperate, there is still time to apply lime even if you must wait until spring. Applying lime to achieve the correct soil pH is one of the most neglected but important factors involved in soil productivity. Next, look at your basic soil fertility levels for P and K, and decide how you want to apply fertilizer for your situation. Look for ways to apply fertilizer that are efficient for your farming operation.

So, once we have good soil tests (along with your yield maps), where do we go from here? Are we looking at low soil test levels that could limit yields? This is where ISU Extension publication PM-1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa, comes in real handy. Since we suspect there may be some low P and K levels, I’ll share this bit of info from PM-1688 that will, hopefully, want to make you read more. “When we move to the low range of soil test results, odds of a yield response rise to 65%, and for very low-test levels the percentage moves to 80%.”

It’s easy to overlook the nutrient requirements for soybean plants because we tend to fertilize every other year for corn. That approach is OK as long as fertility and pH levels are in an appropriate range. Best bet is to regularly sample and test soils to make sure.

Communicate with landlord

Once you have the soil test results and a fertility and pH plan in mind, it might be worth a shot to sit down with the landowner and discuss the yield problem — fertility and lime needs — before you make a large investment in lime and fertilizer. Some landowners address cost share of lime, and occasionally P and K, in their lease agreements. Since that seems to be the exception rather than the norm, there may be another angle to approach from.

If you commit to improving the condition of the farm, perhaps the landowner would agree to commit to letting you rent the farm over a longer time period to help recoup the money spent fixing low pH and soil fertility issues. If the landowner is unwilling to work with you on this, you may consider giving up this rented farm and find some land that's more productive. It’s a tough call, and land is hard to come by, so hopefully with some creativity and good communication between landlord and tenant, it can be worked out so both sides win.

McGrath is the Extension coordinator for the Iowa Soybean Research Center at ISU. Email


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