Highlights

• Most N uptake (~70%) occurred between flowering and first harvest
• Yard-waste composts have long-term benefits but may not provide any N for this season's crop
• Generally, an amendment's ratio of carbon: nitrogen was a good predictor of its N release

The Team

Patricia Lazicki, Margaret Lloyd, Daniel Geisseler.

Background

To maintain sufficient nitrogen (N) for plant growth, California organic growers use a wide range of amendments. Most of the N contained in these amendments is not immediately available to plants, but must first be transformed by soil microbes into a mineral form. Amendments differ as to how quickly this transformation takes place. Once in mineral form, nitrogen is very mobile in the soil and can easily be lost if it is not quickly used by plants. Thus, ideally the timing of N release from amendments should match that of plant N demand.

Objectives

1. Understand nitrogen release dynamics of different types of organic amendments used by California growers
2. Measure the timing and amount of N uptake by organic heirloom tomatoes (cv Brandywine)

Our Approach

1. Establish plots on fields of local organic growers who use different amendments
• Amended, unamended beds
• Regularly monitor soil and plant N status, yields
• Observe effect of amendments on soil available N, plant growth
2. Set up a replicated field trial at UC Davis
• Match treatments to grower amendments
• Regular soil, leaf, fruit sampling
• Regularly harvest of whole plants to determine seasonal N uptake
3. Incubations
• Same amendments as field trial; additional determined by surveys
• Incubate at optimum moisture and temperature
• Measure N availability at regular intervals over 12 weeks

Results

Some of our interesting results include:

• In the field trial, average Brandywine tomato uptake was about 250 lbs N/acre (roughly 9 lbs N/ton of harvested fruit). The maximum uptake rate was about 3-5 lbs N/acre/day and occurred between green fruit and harvest stages. However, our crop had a higher N status compared with the grower fields, and these rates are probably higher than normal. A second year of data will help determine how typical they are.
• Brandywine tomatoes took up about 70% of their N between flowering and the first harvest. Little N uptake occurred after harvest began.
• The organic amendments we tested had a wide range of potential N release rates, from nearly all the added N as guano, to almost nothing from the yard waste composts. Some composts actually immobilized N, so the amended soil had less available N than the unamended soil. We also saw this result in our field trial and grower sites.
• For the different amendments we incubated, the ratio of carbon to nitrogen in the amendment was a reasonably good predictor of N availability
• We incubated the amendments in soils from one of our organic grower fields and also in soils from our field trial, which did not have a history of organic management. The amendments behaved similarly in both soils.