Can you fertilize with nitrogen to boost yield?

Alberto asks: Gustavo, we are in the north of Santa Fé and we have group 8 soybeans that are 30 cm tall, on wheat stubble, and with a lack of plants due to successive mortality during the cycle. They are about to start flowering or flowering according to the December/January planting date. It rained 67 mm a few days ago. Could it be fertilized with nitrogen to boost yield? In case you see it feasible, with which products do you think it is better (foliar or solid)? I would be interested to know if you see the use of urea as feasible, especially since it would solve the issue of availability because we already have it purchased). We also would not want to make an investment without much support at the current low potential.

Answer from Gustavo Ferraris: Alberto, I understand that, as in almost the entire country, the nodulation is quite scarce and of poor quality in this campaign. The repeated pulses of drought and heat are lethal for the nodules, since it is one of the first processes affected (metabolically costly), but it does not affect the survival of the plants, it does affect productivity.

Nitrogen (N) use efficiency in soybean is low. We have seen some cases of response to fertilizing soybeans (frequently 100-150 kg/ha increase in yields is obtained). However, the nitrogen use efficiency (NUE) is 7-8 kg grain : kgN, while in wheat or maize it can reach 30 kg grain : kgN. For this reason, if the field had soybeans in any of the last 3 campaigns, and for this reason we can presume the presence of naturalized fixing flora in the soil, I would wait for a good rain to allow re-infection of the plants. A millimeter like the one you describe should be enough, if the rains don’t stop. In addition, phenologically it is on time, fixation rates increase from R2.

In case of not seeing reinfections and formation of new nodules, I would try to use some source of foliar N. I would look for punctual, strategic fertilization, in quality rather than quantity. They are less expensive and more practical than base, soil-applied sources. It seems to me that they do not make sense in soybeans. Biological fixation (FBN) is a highly regulated process. If the plant has conditions to grow, it will generate the necessary mechanism to nodulate. At most a specific application can support the process, without interfering with the FBN.

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