Northern growers continue to get hammered for the second year with awful planting weather. Over the past two weeks we’ve discussed maturity switch dates for corn grain, corn silage and soybean cultivars. Here we’ll discuss switching corn ground to soybean.

1. Check your herbicide label.

Several burndown or early pre-plant programs have labels for both crops. However, tankmix partners or rates may differ between the two. Make sure you verify rates, timings and plant-back restrictions before you make the switch. 

2. Fall-applied anhydrous or spring-applied urea.

How much nitrogen is too much to not even consider the switch? Biologically that number doesn’t really exist. Economically that number is a moving target given yield penalty, corn drying costs, etc. Over the last four weeks the amount of nitrogen readily available to the late-planted corn crop, or in this case soybean crop, has declined, though some nitrogen is still readily in the soil profile. 

We were somewhat prepared for this question a year ago given the likelihood of residual nitrogen following the 2012 drought-stricken corn. In that article we said:

In excess situations, soybeans will generally utilize the background nitrogen prior to initiating maximum nitrogen fixation. This may lead to luxurious early season growth, which in fields with a history of white mold may cause problems if weather conditions are conducive. High soil nitrogen reserves may also lead to increased lodging. This can be accomplished through variety selection (e.g. white mold tolerance, short-statured soybean cultivars or good lodging tolerance), decreasing seeding rates and proper scouting to time fungicide applications if needed. 

The only change we would make to this paragraph would be to increase seeding rates to compensate for delayed planting. Remember, this article was written in 2013 under the assumption soybean would be planted in the first week of May, not the first week of June.

3. Should I use an inoculant even when excess nitrogen is present?

The simple answer is yes. Excess nitrogen limits nitrogen fixation. (Lit review excerpt quoted with permission from Eric Wilson; M.S. Thesis; Shaun Casteel Adviser; Purdue University):

Nitrate uptake of soybean plants didn’t appear to directly damage the BNF capacity. It was concluded that carbohydrate deprivation and nitrate toxicity didn’t inhibit BNF. It’s hypothesized that additional nitrate increased the oxygen diffusion barrier of the nodule, which limited oxygen supply and restricted nitrogenase activity and nodule respiration. However, additional oxygen supplied to the nodules didn’t markedly increase BNF.