Authors: Nick Andrews, Dan M. Sullivan, and Kristin Pool, Oregon State University
Organic vegetable growers rely on legume cover crops as an economical source of plant-available N. This research evaluated N contributions to summer vegetable crops by cover crops (CC) residues by monitoring soil nitrate (NO3-N) concentrations during the summer crop growing season. Replicated field plots were established with three CC mixes: solo common vetch (V), phacelia + V (PV), and cereal rye + V (RV), plus a winter fallow (F) control in grower fields in the north Willamette Valley OR. Soil textures were silt loam (7 sites) and sandy loam (2 sites). Cover crops were seeded in a randomized complete block design in mid-October and killed in April (vegetative) or in early May (just prior to flowering). Cover crop biomass was incorporated by tillage 1-3 wk. after CC kill. Soil samples (0-12 in depth, 4 in beside the row) were collected on 2-4 wk. intervals, beginning at summer crop planting (Year 1 sites) and at cover crop kill (Year 2 sites). Net soil NO3-N contribution from CC in the field was estimated by difference in soil test nitrate concentrations (CC treatment minus winter fallow control). Summer crops monitored included winter squash (3 sites), lettuce (2 sites), and table beet, snap bean, kale and popcorn (1 site each). All sites received overhead sprinkler irrigation. Phacelia did not establish reliably as a winter cover crop. It emerged in fall, but did not survive the winter at most sites. At time of cover crop kill in spring, phacelia biomass exceeded 25% of total CC biomass (phacelia + V) at only 2 of 7 sites where it was seeded. Maximum net soil NO3-N contribution was observed near time of crop planting for lettuce, kale, snap bean and table beet, and just prior to the first heavy irrigation for corn and winter squash. Across 9 sites, median net soil NO3-N contribution (treatment minus control) was 28 lb/acre (range = 3 to 95) for RV and 46 lb/acre for V (range = 9 to 130). Soil nitrate monitoring had the most practical value for adjustment of organic fertilizer input rates for crops that receive limited early season irrigation (corn and winter squash), or for short season crops (lettuce and kale) planted in July. Median net soil nitrate contribution in the field was 50 to 80% of that measured in 10-wk laboratory incubation at 22oC. This suggests that N credits based on measuring soil nitrate (0-12 inches) following a cover crop will usually underestimate actual plant available N supplied by a cover crop.