Note: This is part of a Research Update series that highlights projects funded by the California Department of Food and Agriculture’s (CDFA) Fertilizer Research and Education Program (FREP) annual grant program.
Project Title: Immobilization of Nitrate in Winter-Fallow Vegetable Production Beds to Reduce Nitrate Leaching
Project Leaders:
- Richard Smith, UCCE Vegetable Crops Farm Advisor, Monterey, San Benito and Santa Cruz counties
- Michael Cahn, UCCE Irrigation Farm Advisor, Monterey, San Benito and Santa Cruz counties
- Joji Muramoto, UCCE Organic Specialist, Dept. of Environmental Studies, UC Santa Cruz
- Daniel Geisseler, Associate CE Specialist, Dept. of Land, Air and Water Resources, UC Davis
Project Status: Completed in 2023
Project Location: Commercial farms located on the Central Coast
Overview:
This project evaluated the use of high carbon (C) amendments to reduce nitrate leaching in winter fallow vegetable crop beds. The researchers identified two primary high C materials, a local, inexpensive and abundant green waste material called Forest Mulch and almond shells from the Central Valley. Laboratory incubations and large-scale field trials in commercial vegetable fields were conducted to look at how effectively different particle sizes and application rates of these materials immobilized nitrate and prevented leaching. In-field starter fertilizer trials were also completed to examine the impact of high C amendments on the yield of the subsequent crop. Finally, an economic analysis of the costs associated with using Forest Mulch and almond shells was carried out to examine the feasibility of their use.
Locally procured Forest Mulch made from tree branches and trunks (above left) and almond shells ground to less than 0.5 mm particle size (right) were both tested as high C amendments in this project.
Key Takeaways:
- Ground almond shells are a viable high C amendment, while the Forest Mulch green waste is not effective at immobilizing soil nitrate.
- The optimal particle size for using ground almond shells as a high C amendment are ¼ to ½ inch. Applications need to be 5 to 10 tons/acre (T/A) to be effective.
- Applying starter fertilizer can adequately overcome immobilization occurring after high C material application rates with no adverse effect on yield.
- Trucking and grinding costs are likely to make the practice of applying almond shells too expensive at the rates required to effectively immobilize soil nitrate-N for most growers at this time.
Background:
On the Central Coast, growers incorporate vegetable crop residues, which have a high nitrogen (N) content, into the soil after harvest. Due to mild temperatures that support microbial activity in the soil, the incorporated residues contribute to high residual soil nitrate-N in the winter fallow period that is vulnerable to leaching by winter rains. This project evaluated whether high C amendments, which have a C:N ratio > 40, can be used to immobilize the excess nitrate in the soil from incorporated residues. The ideal amendment would immobilize nitrate during the six- to eight-week winter fallow period, and then begin to mineralize during the cropping season, when the risk of leaching has diminished. If immobilization of nitrate continues into the growing season, it may reduce early plant growth unless another source of N is provided. Central Coast growers commonly apply compost in the fall to improve soil tilth and health. Therefore, a high C amendment could be easily substituted if economically viable materials are available that meet the desired immobilization timing.
Methods:
Several incubation studies and field trials were conducted during the project. Table 1 briefly describes the treatments, research duration and types of measurements that were collected.
| 2020-2021 | 2021-2022 | 2022-2023 | ||
| Incubation studies: Examined rates of N immobilization of high C materials and compared rates of almond shells at different rates and particle sizes | Treatments | Almond shells, green waste and other possible high C materials | Almond shells (C:N = 78): unground, 2 mm, 1/4” and 1/2” applied at 2.5, 5 and 10 T/A | Almond shells (C:N = 107): unground, 2 mm, 1/4”, and 1/2″ applied at 5 T/A. Unground, 2 mm and 1/2″ applied at 10 T/A |
| Duration | 12 weeks | 24 weeks | 24 weeks | |
| Additional data collected | C/N ratios of materials, application rates See Figure 1 | Treatment with walnut shells (C:N = 248) ground to 2 mm applied at 5 T/A; percent immobilized N | Percent immobilized N See Figure 2 | |
| Commercial field trials: Evaluated different rates of Forest Mulch and almond shells and incorporation methods to immobilize N following a crop of broccoli | Treatments | Forest Mulch (C:N = 37) at 0, 5, 10, 15 and 20 T/A | Forest Mulch (C:N = 97) and unground almonds (C:N = 88) at 0, 5 and 10 T/A | Almond shells (C:N = 107) ground to ¼ inch at 0, 5, and 10 T/A incorporated with one or two passes of a chisel |
| Duration | October – May, includes spring lettuce crop | October – May, includes spring lettuce crop | November – March, ended due to Salinas River flooding plot | |
| Additional data collected | Soil nitrate down to 3 feet, crop biomass, N uptake and yield | Soil nitrate down to 3 feet, crop biomass and yield | Soil nitrate down to 3 feet, crop biomass and yield See Figure 3 | |
| In-field starter fertilizer trials: Evaluated the effect of high C amendments on yield and N fertilizer requirements of the subsequent crop | Treatments | Starter fertilizer (6-16-0) at 0, 7.8, 15.5, and 31.2 lbs/A applied onto beds treated with Forest Mulch (C:N = 186) at 0, 5, 10 and 15 T/A. Forest Mulch was applied by mulching into top 3 inches of soil with bed shaper. Starter fertilizer was sprayed prior to seeding | Starter fertilizer (6-16-0) at 0, 31 and 62 lbs/A sprayed onto beds treated with unground almond shells (C:N ratio = 88) applied at 20 T/A. Shells were incorporated into beds with a harrow and power mulcher to 3” depth prior to seeding | |
| Duration | Jan. – May, during spring lettuce | Jan. – May, during spring lettuce | ||
| Additional data collected | Soil nitrate, tissue N, phytotoxicity, plant stand and mean head weight at harvest | Soil nitrate, tissue N, crop biomass and mean head weight at harvest |
Project Results and Discussion:
Quality and particle size affects high C amendment efficacy
Results from the first winter field trial showed little reduction in soil nitrate levels in the first, second and third feet of soil at all treatments. An incubation study carried out in the lab comparing different high C amendments also showed that Forest Mulch green waste had a low immobilization rate compared to other materials like ground and unground almond shells and was therefore not a suitable high C material (Figure 1). Researchers attributed this finding to the quality and size of the mulch, which was composed of a triple-screened material of various-sized woody pieces that likely contained a significant amount of lignin. They hypothesized that for this reason, it didn’t have sufficient labile C to effectively immobilize soil N.
Figure 1. Overall immobilization rate of N in a range of high C amendments after 12-week incubation.
In the second- and third-year trials, the researchers evaluated ground and unground almond shells at various sizes to determine their effectiveness as a high C amendment. Incubations of 5 T/A of almond shells of various sizes showed that peak immobilization occurred two weeks into the incubation period. Greater immobilization occurred with smaller particle sizes and residual immobilization persisted longest with the unground almond shells (Figure 2).
Figure 2. Incubation study showing N immobilization by almond shells that are unground (NP) or ground to ½”, ¼” and 2 mm particle size that are applied at the equivalent of 5 T/A.
In the third winter field trial, sampling showed reduced residual nitrate in the 10 T/A almond shell treatment that was incorporated with two passes of a soil chisel. A similar trend indicating less soil nitrate in the second foot of soil was observed, but no differences among the treatments was observed at the third foot of soil (Figure 3).
Figure 3. Soil nitrate levels in the first foot of soil in the ¼” almond shell treatments applied at 0, 5 and 10 T/A until flooding terminated the field trial. Dotted lines show one pass of the chisel to incorporate the shells, and solid lines indicate two passes.
Overall, the results from the project showed that unground almond shells immobilize nitrate at a lower rate and remineralize nitrate more slowly than smaller particle sizes. Almond shells ground to ¼ and ½ inch immobilized soil nitrate effectively and remobilized it at a rate that is useful for reducing nitrate leaching during the winter fallow period.
Starter fertilizer can adequately overcome immobilization
During the first year of the starter fertilizer trial there was no immobilization from the application of the Forest Mulch at the tested rates. Thus, there was no need for starter fertilizer to counteract the immobilization and no reduction in the yield of the spring lettuce crop was observed. During the second starter fertilizer trial, when an elevated rate of 20 T/A of unground almond shells was applied, treatments with the use of starter fertilizer showed increased yields. This result indicates that increasing rates of starter fertilizer can offset residual immobilization caused by applying high C amendments (data not shown).
Cost of transporting and grinding almond shells reduces feasibility of adoption
The expense of purchasing almond shells and transporting them from the Central Valley to the Salinas Valley is about $60.50/ton (Table 2). Grinding the almond shells adds an additional $20.00/ton to the cost. In the incubation and field trials, 5-10 T/A were shown to be an effective rate for applying ground almond shells. At this rate, the cost to growers would be $402.50-805.00 per acre. This cost is likely to reduce the feasibility of adoption for growers.
| Expense | Ground almond shells | Unground almond shells |
| Material | $15.00 | $15.00 |
| Trucking | $37.501 | $37.501 |
| Grinding | $20.002 | $0.00 |
| Spreading | $8.00 | $8.00 |
| Total Costs | $80.50 | $60.50 |
Project Impact:
These findings support the goal of educating Central Coast Water Board staff, growers, and crop consultants about the benefits and feasibility of applying high C amendments as a best management practice for reducing nitrate leaching in winter fallow vegetable beds. As a result of these efforts, this practice was included in Ag Order 4.0 as an N removal credit. An Agricultural Expert Panel that is being convened in late 2024 by the State Water Board will consider the efficacy and applicability of this and other N credits and discount factors and is currently collecting data from related research for the panel to consider. In addition, these findings provide important information about the efficacy and cost of applying high C amendments that growers can use in the future as they navigate the changing regulatory and economic conditions of their agricultural operations.
To learn more about this project and other current and completed FREP projects please visit: https://www.cdfa.ca.gov/is/ffldrs/frep/Research.html.