- Legume cover crops can serve as important sources of nitrogen (N) in sustainable agriculture and can be economically beneficial when fertilizer inputs are reduced without a yield reduction. Synchronizing N mineralization from organic materials with the needs of the subsequent crop is a challenge for organic growers.
Predicting plant available nitrogen from cover crop residue enables N fertilizer inputs to be adjusted for optimum economic yield and reduced environmental risk. An experiment was conducted near Corvallis, OR in 2006 through 2008 to evaluate cover crop and N effects in organic broccoli production in western Oregon.
The specific objectives of this experiment were to: 1) evaluate biomass production and N accumulation from selected cover crop treatments; 2) compare the effects of selected cover crops grown as sole crops and as mixtures on broccoli yield, yield components,
and net economic benefit; 3) estimate the quantity of feather meal N replaced by cover crops 4) estimate plant available nitrogen from cover crop residue in an organic broccoli production system; 5) evaluate soil NO₃₋N and petiole NO₃₋N as predictors of broccoli
yield; and 6) evaluate models derived from laboratory incubation of cover crop residue to predict apparent nitrogen recovery (ANR) from cover crop residue in the field. The cover crop treatments included common vetch (Vicia sativa), phacelia (Phacelia
tanacetifolia), 'Monida' oats (Avena sativa L.), phacelia plus common vetch, and 'Monida' oats plus common vetch. A fallow treatment was used as the control. Prior to incorporation, cover crop samples were collected from each block and frozen for later
use in laboratory aerobic incubations. After the cover crops were flail-mowed and incorporated, four N rates (0, 100, 200, and 300 kg N ha⁻¹) were randomized within each cover crop treatment in a split plot design. All cover crop treatments produced much less biomass and accumulated less N in 2008 than in 2007. Planting vetch with oats or phacelia increased biomass production and N accumulation compared to sole crops in 2007 but not in 2008.
Common vetch as a sole crop or in a mixture increased broccoli yield with 0 and 100 kg N ha⁻¹ applied compared to fallow. Legume cover crop mixtures with 100 kg N ha⁻¹ produced similar net economic returns for organic compared to fallow treatments with
300 kg N ha⁻¹. Nitrogen fertilizer input can be reduced by at least 100 kg N ha⁻¹ if common vetch is in the mixture and produces more than 5000 kg ha⁻¹ of biomass (130-180 kg N ha⁻¹). Vetch as a sole crop produced higher levels of soil NO₃₋N than the fallow treatments up to 80 days after soil incorporation in 2007. Oats and phacelia as sole crops, however, reduced soil NO₃₋N compared to fallow for up to 68 days after incorporation. Vetch mixtures with oats or phacelia produced intermediate levels of soil NO₃₋N between vetch as a sole crop and the fallow. Treatment effects were similar in 2008, but differences were less due to reduced cover crop biomass compared to 2007. Broccoli petiole nitrate levels were not affected by cover crop treatment in 2007, and
there was no correlation with yield. In 2008, the oat cover crop treatment reduced broccoli petiole nitrate levels compared to the fallow. Petiole nitrate levels were strongly correlated with broccoli yield, with highest yields associated with petiole NO₃₋
N greater than 10,000 ppm. In the aerobic incubations with cover crop mixtures, a quadratic model described the relationship of percent N in the mixture to the apparent nitrogen recovery (ANR) at both 4 and 10 weeks. The highest ANR (about 40 percent) was similar for both extraction days. Net mineralization occurred when the percent N of the cover crop mixture was 1.5-1.8 percent. There was a strong correlation in 2007 between the ANR predicted by the incubation-derived model and the ANR in the soil and in the aboveground broccoli biomass. The model over predicted the ANR in the field soil, however the model more accurately predicted ANR in the broccoli biomass. The incubation model correctly predicted negative ANR values for the oat and phacelia cover-crop treatments. In 2008, the laboratory-predicted ANR and the field soil ANR were correlated (r²=.45), and the laboratory model over predicted the field ANR. The incubation model gave a poor prediction of broccoli biomass ANR.