Fine-Tuning Fertilizer Applications in Organic Cool-Season Leafy Green Crops Can Increase Soil Quality and Yields

Project Director: Richard Smith, University of California Cooperative Extension

Project Overview

Managing soil fertility is at the heart of every organic farm, and organic growers invest a lot of resources into building soil health. For growers who rely heavily on organic fertilizers, balancing nitrogen and phosphorous rates can pose an ongoing challenge. The high level of phosphorous (P) relative to nitrogen (N) in many organic fertilizers frequently results in an overapplication of phosphorus, as fertilizer application rates are often determined based on a crop’s nitrogen requirements.

This overapplication of phosphorus can have negative consequences on the environment, particularly when excess P enters waterways as runoff, contributing to algal blooms and hypoxia (a lack of oxygen) in aquatic ecosystems. In response to these effects, the United States has implemented several regulations on fertilizer use.

Nevertheless, organic growers can’t simply base their fertilizer application rates on crop P requirements, as this often results in an underapplication of N (and, subsequently, significant reductions to crop quality and yields). Organic growers need balance. Improved N management practices could facilitate reduced fertilizer application rates that simultaneously meet a crop’s N requirements and minimize P overapplication. Region-specific research is needed to understand how different fertilization methods perform under varying soil conditions/types.

Researchers in the Central Coast of California performed a series of separate field trials to understand how organic fertilizers affect N and P mineralization rates in cool-season organic vegetable production systems. Their goal was to improve growers’ ability to use soil nitrate (NO3-N) tests to make more informed fertilizer decisions.

Farmer Takeaways

Organic vegetable growers in the Central Coast rely heavily on supplemental N fertilization, as mineralization rates of soil organic matter (SOM) from unfertilized soils is low.
Organic fertilizers can significantly reduce the N deficit (the difference between the amount of N that becomes available to a crop throughout a growing season, vs. the amount of N that the crop uptakes), but additional sources of N may still be needed to prevent a gradual depletion of soil N levels over time.
Organic vegetable growers may be able to improve soil fertility and crop nutrient management by regularly testing soils for nitrate (NO3-N) and by stimulating microbial activity.
In this study, P mineralization rates from organic fertilizers were very low and did not result in an excess of soluble P.

Project Objectives and Approach

Measure how quickly nitrogen (N) and phosphorus (P) become available to/are taken up by organic cool-season vegetable crops in unfertilized Central Coast soils and soils receiving organic fertilization

Researchers at the University of California Cooperative Extension established multiple field trials on organic vegetable farms in Monterey County, CA. Field trials lasted four years and encompassed a variety of rotating vegetable crops, including spinach, broccoli, kale, chard, and multiple kinds of lettuce.
One trial measured the nitrate (NO3-N) mineralization rates of the natural soil organic matter (SOM) present in unfertilized organic vegetable plots, and a second trial measured the N and P mineralization rates of two different organic fertilizers (poultry litter and feather meal) when applied to organic vegetable plots.
- - No-Fertilizer Trial: Researchers collected weekly soil samples from unfertilized organic vegetable plots. These samples were analyzed for nitrate (NO3-N), and mineralization rates* were calculated over time. Soil samples were also collected from nearby conventionally-managed plots, and mineralization rates were compared.
    - - *Mineralization rate: the rate at which microorganisms break down soil organic matter, converting complex organic forms of nitrogen into inorganic, plant-available forms such as nitrate (NO3-N) and ammonium (NH4-N). Mineralization rates are important for estimating when nutrients will become available to/be taken up by plants and for preventing excess nutrient runoff.
  - Fertilizer Trial: Researchers measured exact quantities of two organic fertilizers (4-4-2 poultry litter and 12-0-0 feather meal) into mesh bags, which were placed either on the soil surface (to simulate a top-dressed application) or buried three inches deep (to simulate an incorporated application) in organic vegetable plots. Each week, fertilizer bags were removed and analyzed for remaining N and P; from these data, respective N and P mineralization rates for each fertilizer type was determined.

Key Findings

In both the unfertilized and fertilized trials, N mineralization rates were lower than crop N uptake rates, P mineralization rates were low (soluble P was not in excess), and vegetable yields remained economically viable

No-Fertilizer Trial:

On average, the natural soil organic matter (SOM) present in the unfertilized organic vegetable plots mineralized at a rate of 0.63 lb N/ac/day, whereas crop N uptake averaged 2.45 lb N/ac/day. This suggests that, in the absence of supplemental N fertilization, organic vegetable production systems in the Central Coast will quickly deplete existing soil N levels.

Fertilizer Trial:

Applied organic fertilizers (poultry litter and feather meal) supplied an estimated 83 lb N/ac per growing season, whereas crop N uptake averaged 97 lb N/ac per growing season. Despite this deficit, vegetable yields across all sites were economically viable.
- - These findings indicate that – while organic fertilizers can significantly reduce the N deficit in Central Coast organic vegetable systems and support economically viable yields – other sources of N may still be needed to prevent a gradual depletion of soil N levels over time.
Depending on the type of organic fertilizer applied (and whether it was applied at the soil surface or buried), only 1/3 to 1/2 of the total available N was mineralized during a given growing season, with the rest entering the soil organic N pool. The soil organic N pool can be drawn upon during future growing seasons, but makes predicting N availability in subsequent growing seasons difficult.
On average, only 9-17% of the total P content of the 4-4-2 poultry litter fertilizer was released during a given growing season; in other words, there was not an excess of soluble phosphorus in the system. The researchers were not entirely clear why the release of soluble P from this material was so low, so additional research is needed to understand which factors (i.e., soil lime content, manure composition) impact P mineralization in Central Coast soils.

Resources

Smith, R., Cahn, M., Hartz, T., Geisseler, D., & Love, P. (2022). Fine-tuning fertilizer applications in organic cool-season leafy green crops can increase soil quality and yields. California Agriculture: The Journal of UC Agriculture and Natural Resources, 76(2–3).

Location

California

Collaborators

Michael Cahn, UC Agriculture and Natural Resources (UCANR)
Tim Hartz, University of California, Davis
Daniel Geisseler, University of California, Davis
Patricia Love, UC Agriculture and Natural Resources (UCANR)

Region

West/Southwest

Topic

Soil Health, Crop Nutrient Management

Year Published

2022