Dr. Dan Talks Agronomy

Growing continuous corn has been popular since the run-up in commodity prices eight years ago. High corn prices, accompanied by genetics and technologies that help corn yields outpace soybean yields, led more growers to increase their acreage of corn.

The shoe is on the other foot now with soybean prices outpacing corn. We may see a switch back to growing more soybeans in 2015 to improve overall farm profitability.

First-year soybeans planted back into a field after a long string of corn crops can yield well. Dan Arkels, LaSalle County, Illinois, produced 103.95-bushel-per-acre soybeans in 2014. He attributes part of his record-setting yield to planting soybeans into a field that has been in beans only twice in the last 15 years with the most recent soybean crop grown three years ago.

If you are planting more acres of soybeans back into fields that were in continuous corn, you are likely to experience certain benefits, but also may face challenges.

Growers following two years of corn followed by one year of soybeans won't need to make adjustments. Soybean yield benefits when a field is out of soybeans for two years and the soil has enough residual rhizobia bacteria that inoculating seed isn't necessary.

The following are some factors you do need to consider.

-- Soybeans are more sensitive to an acid pH than corn. If you haven't paid attention to pH after years of planting corn, now is the time to see if the pH is at least 6.2. The time to soil sample and apply lime to correct an acid pH is generally in the fall. However, if you missed fall application, you can still apply lime over the winter or spring before planting. Just remember that lime isn't very soluble and it takes some time to dissolve, react and raise pH.

-- Corn requires more phosphorus than potassium, and soybeans require more potassium than corn. If you have been fertilizing for a corn crop, your soil test levels for potassium may have eroded over time and may not be able to meet the need of the soybean crop. Take a look at the most recent soil test: If levels are medium or lower, apply some potassium. Even if the soil test is medium to high, an application of potash based on crop removal is a good idea before planting.

-- Growers are planting soybeans earlier (or in the same time frame as corn). Cool and wet conditions, either from earlier planting or no-till, create an environment for soil pathogens to flourish. If cool and wet soil are a risk, delay planting until soil conditions are more favorable or use strip till or tillage to blacken the soil so it warms and dries quicker in the spring.

-- The general rule of thumb is that if a field hasn't been in soybeans for four years, treat the seed with a rhizobia inoculum to re-establish a viable soil population to trigger nitrogen fixation by the plant. There are many inoculant products on the market today with single or multiple species. It is easy to treat the seed along with other seed treatments just before taking seed delivery.

-- It may be time to rethink fungicidal seed treatments. Soybeans are more prone to seedling diseases and blights than corn when the soil is cool and wet or warm and wet. Select an appropriate treatment package that offers the protection against the pathogens that reside in your fields.

-- Soybean variety selection isn't exactly influenced by following a long period of corn, but placing the varieties in the right field is important. Consider the characteristics of each field when selecting varieties and where to place them.

-- Expect going back to soybeans to bring a yield bump of approximately 10 bushels per acre. To protect that potential yield, make sure you have the right agronomy in place to get the crop off to a good start.

Dan Davidson can be reached at AskDrDan@dtn.com

(Ps/AG/CZ)

Copyright 2014 DTN/The Progressive Farmer. All rights reserved.