In fall 2014, we drilled 13 cover crops (6 mixes) into small plots at four cooperator field sites in Delaware with varying fall soil nitrate (NO3-N) levels (Table 1). Urea fertilizer was applied to plots at the rate of 0 and 30 lb N/ac within 24 hr of planting. The goal of the study was to determine biomass response of cover crops to fall fertilization when planted on soils with low and high soil NO3-N levels.
Table 1. Fall soil nitrate (NO3-N) concentrations and cover crop planting dates at four Delaware locations in Fall 2014 to evaluate cover crop biomass response to fall N fertilization.
|Location||Fall Soil NO3-N (ppm)||Planting Date|
|Middletown 1||2.5||20 Oct|
|Middletown 2||2.7||20 Oct|
Fall photos in Figure 1 show examples of how the drilled rye cover crop established at all locations. Cover crops emergence and establishment was best at the Georgetown site location due to the early planting date (Figure 1, top left). Fall biomass production and soil coverage at the Millsboro site was less than Georgetown but was still relatively good considering the warm fall temperatures (Figure 1, top right). However, emergence of the late planting crops at Middletown was spotty due to cooler temperatures (Figure 1, bottom). Visual analysis of cover crops at the Georgetown site suggested that the application of 30 lb N/ac may have improved fall biomass development, even when soil NO3-N concentrations were high. (See examples for rye in Figure 1 and tillage radish in Figure 2.) In contrast, there was no visual response of cover crops to N fertilization in fall at the low soil nitrate sites, again likely due to poor establishment (Figure 1).
Except for the Georgetown site, spring greenup of cover crops was poor due to late planting and an extremely cold winter. Cover crops planted at the Middletown and Millsboro sites produced minimal and highly uneven biomass, which made collecting a representative biomass sample impossible; biomass was not sampled from these sites. Figure 3 shows examples of the sparse and uneven spring cover crop stands at the Middletown 2 site for several species.
Despite some minor winter dieback at the Georgetown site (Figure 1), biomass production and stand uniformity were good, allowing collection of biomass samples prior to termination in March 2015. Overall, N fertilization did not affect biomass generation when averaged across all cover crop species (average dry weight = 1.64 tons/ac for unfertilized plots vs. 1.76 tons/ac for fertilized plots). However, we see a different story when we look at N fertilization effects on biomass within species (Figure 4). The no-till barley, rye-radish mix, and wheat-crimson clover mix all produced more biomass with N fertilization. In contrast, the tilled barley and rye-crimson clover mixes produced less biomass when fertilized. Yet, for six cover crop types, there was no significant effect of N fertilization on total biomass (Figure 4). At this point, we are unable to explain the reason(s) for the mixed biomass response.
We also measured total N in biomass and adjusted this value (total N uptake – total N applied as fertilizer) to get the fertilizer adjusted N uptake by crops. This adjustment allows us to estimate the amount of residual soil N that was taken up by the cover crop. As was noted with biomass, N fertilization had a mixed effect on cover crop N uptake (Figure 5). The tilled barley, rye-crimson clover mix, drilled wheat, and wheat-forage radish mix removed less residual soil N when these crops received 30 lb/ac of fall N fertilizer than the same crops that were not fertilized. In other words, the application of fall fertilizer did not increase growth enough to allow the crop to scavenge more N than the unfertilized crop. With the remaining species (drilled barley, crimson clover, rape, drilled rye and mixes, soil N uptake was not statistically different when crops were fertilized or unfertilized.
Based on results from the Georgetown trials, we do not currently recommend application of fall N to cover crops planted on soils with high residual soil NO3-N (e.g., soil NO3-N is greater than 8-11 ppm, which are the MD thresholds for fall fertilization of small grains), especially when cover crops are planted with the goal of scavenging excess soil N. However, we recognize the need for more than one site year of results to validate these findings. Also, since we were unable to collect samples from the low fall soil nitrate sites in Millsboro and Middletown due to lack of biomass production, we cannot comment on the value of the fall soil nitrate test to predict the need for fall fertilization to establish fall cover crop species and enhance N uptake. However, we strongly believe that these sites failed to sustain biomass through the winter due to the late planting date and colder than normal temperatures, rather than due to the lack of available N.
The authors would like to thank Maryland Grain Producers and Utilization Board for funding this research. Note: Many cost-share programs prohibit fertilization of cover crops. Please consult your contract terms and conditions prior to applying fertilizer to cover crops.