Broccoli Rotations Role in Reducing Nitrate Leaching in Vegetable Production Systems.

A survey of the nitrogen (N) uptake in broccoli biomass was conducted in commercial production fields in 2012-2013 in the Salinas Valley, California, USA.  Summer-grown broccoli took up an average of 377.8 kg ha^-1, but average applied fertilizer was 204.0 182 kg ha^-1. It appears that broccoli scavenged 173.8 kg ha^-1 from the soil. Rooting depth evaluations indicate that broccoli roots grow deeper into the soil as the crop develops. Halfway through the growth cycle, roots extend to 41.1 cm and at the end of the growth cycle they extend to 99.3 cm deep. There can often be substantial quantities of nitrogen in the top three feet of soil after the production of lettuce due to unused nitrogen from applied fertilizer and mineralization of crop residues. The average nitrate in the top foot of soil was 208.5 kg ha^-1 and in the second foot, 130.9 kg ha^-1. Evaluations of the third foot were conducted in a separate project and are not included here. These observations may help explain why summer-grown broccoli takes up more N than is routinely applied. The overall quantity of water applied to the surveyed broccoli fields was 54.9 cm, and average crop ET was 22.9 cm indicating the potential for nitrate leaching due to water percolation during the crop cycle. At harvest, 111.3 kg ha^-1 is removed in the heads and 266.9 kg ha^-1 remains in the field as crop residue. Summer-grown broccoli has the potential to provide one of the services of a winter-grown cover crop by scavenging nitrate from the soil profile that might otherwise be lost to leaching. However, for broccoli to effectively reduce nitrate leaching, the large quantity of nitrate that remains in the field as crop residue would have to be efficiently captured by subsequent crops.