Nitrogen management for a nitrogen fixing legume like soybeans should be easy right? Well, it isn’t always a black or white issue. If a field has a long history of soybean production and you have observed high numbers of nodules with pink centers in the past, you probably don’t need to apply any nitrogen fertilizer to achieve a high yield. If you work in an area that is relatively new to soybean production, you may want to consider applying nitrogen fertilizer to achieve higher yields. University research has shown that fields with no history of soybean production require a moderate amount of nitrogen in the 0-24″ profile for good production. The total amount of nitrogen required is about 50-60 lb/a. If the soil nitrate test is 20 lb/a, you will need to apply 40 lb/ of actual N as fertilizer. The gray area for applying extra N comes up when fields have some history of soybean production (1-2 years). In these situations, it is critical to know if a high level of healthy nodules was observed when soybeans were grown the first or second time. If the amount of nodulation is questionable, I would apply enough fertilizer N to bring the soil level up to 60 lb/a. For soil samples tested in our Northwood, ND laboratory, we assume all fields have little history of soybean production, and make a nitrogen guideline based on a 60 lb/a total between the soil and the actual N applied. For samples tested at our Benson, MN laboratory, we assume fields have many years of soybean history and good nodulation and do not require additional fertilizer nitrogen. You and the grower need to make the final decision based on the actual history of each field.
The option of top dressing N to a growing soybean crop if nitrogen deficiency symptoms are obvious should only be used as a rescue treatment. Nitrogen applied to soybeans in June may not get into the crop in a timely manner. If rainfall is not received, the N will stay on the dry soil surface and the soybean roots will not have access to it. If the weather turns wet, you may not have a chance to apply nitrogen fertilizer and yield may be lost. A good plan is always better than a rescue treatment!