Applying nitrogen to N-stressed soybeans
Although soybeans can fix good part of their own nitrogen (N) if they are well nodulated, this is no guarantee that soybeans will not suffer from nitrogen deficiency at a crucial time in their development. Soybeans are heavy users of N, removing about 3-4 lbs of N per bushel of seed. They need all the N they can fix plus N from the pool of N available in the soil.
Planting soybean without inoculation into soils where soybean has never been grown can result in very poor nodulation and N deficiency. Similar problems can occur when inoculation fails, or if soybeans are planted on severely acid soils that limit nodulation. In these situations, it is logical to ask if soybeans will respond to N fertilizers.
In both 2009 and 2010, a number of fields planted into “virgin” soybean ground or into returned Conservation Reserve Program ground in north central Kansas were observed to be poorly nodulated and N-deficient, even though the seed was commercially inoculated. A field study was conducted in 2009 and continued at a different location in 2010 to determine whether these poorly nodulated, N-deficient soybean would respond to applied N fertilizers. And, if so, how much N could successfully be used.
In 2009, this study was conducted on a farmer’s field near Solomon that had noticeably N-deficient soybean. This field was planted no-till into sorghum residue from the previous year on May 20 at 140,000 seeds/a. A liquid inoculant was sprayed on the soybean seeds as they were loaded into the planter. This field had no history of soybean production. Examination of the root system showed few or no nodules present. Nitrogen fertilizer was applied on July 20, 2009, to soybean displaying N-deficiency symptoms at the R1 to R2 (flowering) growth stages. The N was applied as urea co-granulated with a urease inhibitor and nitrification inhibitor (Super-U) by surface banding the material between the soybean rows. Rainfall occurred within a few hours of N application.
This study was repeated in 2010 on a farmer’s field near Gypsum that had very poorly nodulated, N-deficient soybean. In this case, the soybean was planted into conventional tilled soil at 130,000 seeds/a on June 19, 2010. Soybean seed was treated with an inoculant prior to planting. This field had no history of soybean production. The N was again broadcast-applied as urea co-granulated with a urease inhibitor and nitrification inhibitor (Super U) on July 22, 2010. Rainfall did not occur until 14 days after treatments were applied.