Soybean Fertility/Inoc

Rescue Nitrogen Fertilizer for Poorly-Nodulated Soybeans

Nodulation failure and nitrogen (N) deficiency has appeared in soybeans across Manitoba, particularly on fields without a history of soybean, in high residue production systems and on acidic soils (pH <6.0). A possible explanation for poorer than usual nodulation and/or nitrogen fixation could be the dry seeding and growing conditions encountered this spring and early summer. Drought conditions can limit N-fixation and cause desiccation of more vulnerable inoculant formulations (e.g. liquid compared to granular).

Impact on Yield

Over the course of the growing season, a 40 bu/ac soybean crop will need 180-230 lbs N/ac. Soybeans can biologically fix 40 – 60% of their N requirement, acquiring the remainder from soil reserves. On fields without a history of soybean production, research in Manitoba found uninoculated soybeans had an average yield loss of 15 bu/ac and reduction of 4.8% compared to all inoculated treatments (MPSG 2018).

Management Options

Without adequate nodulation (<10 nodules/plant), farmers should consider a rescue N fertilizer application to ensure an adequate supply to the crop through pod filling (R4). This is when peak N uptake occurs (Figure 2). A general recommendation is to apply rescue N fertilizer at 50 lbs N/ac at the R3 (early pod) growth stage.

Direct fertilizer below the canopy to eliminate leaf burn, which can reduce yields. This recommendation is applicable if soil N reserves have adequately supplied the crop with N up to the flowering stage (Heard 2013).

Recent research in Saskatchewan found that 50 lbs N/ac as UAN applied at R2-R3 increased yield of soybeans (at 4 of 9 site years) that received only liquid inoculant on fields with a limited soybean history. Unresponsive sites were extremely dry and had high residual soil N (IHARF 2017a). Even in dry conditions such as those encountered in 2017, a rescue N application increased soybean yield by 36% compared to an uninoculated control, and by 13% compared to the liquid-inoculated control (Table 1) (IHARF 2017b). However, rescue N fertilizer application never resulted in yields greater than those of well-nodulated soybeans treated with granular inoculant.

In Ontario, where soil organic matter levels and residual N levels are low, the recommendation is to apply a rescue fertilizer earlier—at  R1 (early flower) if soybeans are pale green and stunted (OMAFRA 2017). If soil N reserves are low, higher rates of rescue N fertilizer (100 lbs N/ac) may also be required to maximize yields (NDSU 2015).