Fall Fertilization of Cover Crops Did Not Increase Biomass Production when Residual Soil Nitrate Was High

Amy Shober, Professor and Extension Specialist, Nutrient Management and Environmental Quality, ashober@udel.edu; Jarrod Miller, Assistant Professor and Extension Specialist, Agronomy, jarrod@udel.edu

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.

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).

Figure 1. Comparison of drilled cereal rye cover crops planted in 2014 with and without N fertilizer (urea) at four Delaware locations with varying fall soil nitrate levels. Plots were planted 11 Sept (Georgetown), 24 Sept (Millsboro), and 20 Oct (Middletown, both). Fall coverage photos were taken in Nov/Dec 2014; spring photos were taken in Apr 2015. Photos show some evidence for increases in fall biomass and greenness at Georgetown, but not at other sites. Note the significant amount of winter dieback at the Georgetown site. Spring greenup resulted in uneven stands with vegetation shorter than 1 inch tall at Millsboro and both Middletown sites making it impossible to collect a representative biomass sample. Photos courtesy of Shawn Tingle, University of Delaware.

Figure 2. Visual comparison of tillage radish cover crop stands planted at Georgetown, DE on 11 Sept 2014 and receiving 0 (left) and 30 (right) lb/ac N. Note that the stand color and density appears similar between the two treatments; however, the radish fertilized with N was larger (in-set).

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.

Figure 3. Photos comparing spring biomass for selected cover crops in Middletown, DE (planted 20 Oct 2014) with and without fall application of N fertilizer (0 or 30 lb N/ac). Biomass samples were not collected from this site or the other sites in Millsboro and Middletown due to scant biomass production and non-uniformity within replications of the same species and fertilizer combination.

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.

Figure 4. Fall N fertilization had a mixed effect on cover crop biomass production at Georgetown, DE as measured in March 2015; cover crops were no-till drilled on 9 Sept 2014. Cover crop species included: B, barley no-till; BT, barley tilled; CC, crimson clover; R, rape; RCC, cereal rye – crimson clover mix; RY, cereal rye; RYR, cereal rye-radish mix; W, winter wheat no-till; WCC, wheat-crimson clover mix; WFR, wheat-forage radish mix; WT, wheat tilled. Letters indicate statistical differences between fertilized and unfertilized crops within the same species.

Figure 5. Application of fall N fertilizer to cover crops established in fall 2014 in Georgetown, DE with high concentrations of residual nitrate reduced N uptake (adjusted for initial N fertilization) for some cover crop species and mixes. Cover crop species included: B, barley no-till; BT, barley tilled; CC, crimson clover; R, rape; RCC, cereal rye – crimson clover mix; RY, cereal rye; RYR, cereal rye-radish mix; W, winter wheat no-till; WCC, wheat-crimson clover mix; WFR, wheat-forage radish mix; WT, wheat tilled. Letters indicate statistical differences between fertilized and unfertilized crops within the same species.

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.