By John Craft and Alex Lindsey

In Ohio, applying N fertilizer to soybeans is not an advisable practice, as the combination of nodulation and soil mineralization are adequate to meet the plant N demands for yield in most cases. The question partially stems from a paper that was published in 2008 that suggested N may be limiting soybean yield in high-yielding environments (greater than 67 bushel per acre).

How do soybeans take up N?
Soybeans form a complex symbiotic relationship with rhizobia (Bradyrhizobia japonicum), the soil bacteria responsible for nodule formation and biological N2 fixation. Nodules form on young roots within a week of soybean emergence, with active N2 fixation beginning 10 to 14 days later (around the V2 growth stage). Nodulation activity peaks around the R5 (beginning seed) growth stage, with approximately half of the N in the seeds obtained after R5. The rest of the N in the seed is remobilized from vegetative tissue after R5. Soybeans receive 36% to 74% of their N needs (58% on average) from N fixation, with the remainder coming from residual N in the soil.

In fields that produced soybeans within the previous three to five years, adequate rhizobia populations should be present for sufficient nodulation. In fields with history of low nodulation or lack of a recent soybean crop, seed may need to be inoculated prior to planting for up to two consecutive years to sustain adequate residual rhizobia populations. Optimal N fixation can be limited by environmental conditions such as elevated soil nitrate or ammonium levels, soil moisture (both too wet and too dry can inhibit fixation), soil compaction, soil pH (optimal range is 5.7-7.3), and root pathogens. Applying N when planting conditions are favorable for seed germination (i.e., warm, not excessively wet) may delay nodule formation and development, and is not advisable.

What is the N requirement for yield production?
Soybeans require approximately 4.6 pounds of N to produce 1 bushel of seed. For example, to produce 54 bushel per acre (Ohio’s average yield), the soybean crop would require a total of 248 pounds of N per ac. If the soybeans have average nodulation and receive 58% of the N needs from fixation (144 pounds of N per ac), the soybeans would need to get the other 104 pounds of N per ac from the soil. In a given year, plant available N following a corn crop may account for 120-165 pounds of N per ac (dependent on residue levels, soil organic matter, and mineralization rates). In most cases, the soil would be able to supply the remaining N balance, and would be adequate to produce Ohio’s average yield. Assessing soybean roots for nodulation can help with N application decisions. If there are more than 10 nodules within the first two inches of the root system, it is expected the plants will have good potential for N2 fixation.

Recent research conducted at Ohio State across a wide range of tillage practices and weather conditions (six environments) examined the effect of N applications (30 pounds of N per acre-1 soil applied at V1 and R1 and two commercial foliar-applied N treatments at R3 for total rates of 5.9 and 10.8 pounds of N per acre-1) on six modern soybean cultivars. The results suggested that the N inputs evaluated were not sufficient to provide agronomic gains or an economic return when considering the cost of fertilizer and application. These results were consistent with a recent paper by Mourtzinis et al. (2018) that compiled results from 105 studies across the US (including Ohio), and concluded that soybeans vary in their response to supplemental N, and yield gains from supplemental N (< 3 bushels per acre-1) were unlikely to provide an economic return.

So, what does this mean from a crop advising standpoint?
At this time, N is unlikely to be the greatest yield limiting factor to soybeans. Growers are encouraged to look to other practices to increase soybean yield (i.e., planting in late April or early May if possible, monitoring soil pH and P and K levels) rather than applying N. Coupled with the lack of economic return and an increased risk of off target nutrient movement from supplemental synthetic N inputs, N fertilization is likely not an advisable practice under most management and environmental conditions.

Craft (CCA Candidate) and Lindsey (CPAg/CCA, PhD), Department of Horticulture and Crop Science, Ohio State University.