Highlights

• Fertilizers with a narrow C:N ratio released nitrogen quickly, leading to a buildup of nitrate in the soil early in the season
• Between 13 and 40% of the N in pelleted material became available during the broccoli growing season
• Total nitrogen in the aboveground biomass of heirloom processing tomatoes was 140 kg/ha
• Nitrogen in the aboveground biomass of broccoli reached 190 kg/ha at the most fertile site

The Team

Makena Savidge, Chris Hight, Margaret Lloyd, Charlotte Decock, Daniel Geisseler.

Background

Organic vegetable production relies on organic sources to supply nitrogen to the crops. Depending on the properties of these sources and environmental conditions, the amount of N that becomes crop-available during the growing season can vary considerably. This project will validate previous research that was conducted in laboratory trials. The validations will be done in replicated field trials conducted in commercial organic fields located in the Sacramento Valley and on the Central Coast. The results shall be incorporated into online tools for organic growers and consultants. The overall goal of the project is to make organic vegetable production in California more competitive and reduce the risk on nitrogen losses to the environment.

Objectives

The objectives of this ongoing project are to determine the amount of N released during the growing season from organic sources such as organic fertilizers, soil organic matter, crop residues and cover crops. The results shall be incorporated into N management worksheets and online tools that can be used by growers and consultants.

Our Approach

Replicated field trials are conducted in commercial fields in the Sacramento Valley and on the Central Coast of California. Treatments include plots with and without the application of an organic fertilizer. In some trials several treatments with different organic fertilizers are being compared. The crops grown include processing tomatoes and broccoli. Plant available nitrogen in the soil is being determined at the beginning of the growing season, throughout the season and again at harvest. In addition, crop yield and plant biomass and their nitrogen content are being determined at the end of the season. Laboratory studies under controlled conditions complement the field trials.

Results

Compared to an unfertilized control, the increase in nitrogen in broccoli plants fertilized with a pelleted organic fertilizer corresponded to 13 and 40% of the total N contained in the fertilizer. The pelleted materials had nitrogen contents of 6-8%. In trials at three different sites, the calculated uptake of fertilizer nitrogen increased with increasing soil clay content. In a laboratory incubation at a constant temperature and ideal moisture content, the nitrogen mineralized from the same materials was between 40 and 50% with little difference across soil types. Nitrogen in the fertilized aboveground biomass of broccoli was 105-190 kg/ha (95-165 lb/ac) at harvest. Without fertilization, biomass nitrogen was only 60-150 kg/ha (50-135 lb/ac).

In a replicated field trial in the Sacramento Valley, four different organic fertilizers were compared with an unamended control. The application rates of the fertilizers were chosen so that the estimated nitrogen availability would be the same across treatments. Heirloom processing tomatoes with indeterminate growth were grown during the summer. In this field with a long history of organic management, the amendments did not affect total tomato yield, which ranged from 100-110 Mg/ha (45-50 t/ac). On average, total nitrogen in the aboveground biomass was 140 kg/ha (125 lb/ac), of which 60% was removed from the field with harvested tomatoes. However, at the end of the season, the amount of nitrate in the soil profile was higher in the plots where fertilizers were applied compared to the control.

Amendments and fertilizers with a narrow carbon to nitrogen ratio release nitrogen faster and release a larger proportion of the total nitrogen compared to fertilizers with a wider carbon to nitrogen ratio. These results from the field trials confirm previous research conducted under controlled conditions in the laboratory.