So you’ve decided to use organic growing methods and are trying to determine how to best feed your plants. Organic fertilizers replenish the elements (nutrients, minerals, etc) required for healthy soil and plant growth that are lost through natural processes. This discussion will describe organic fertilizer choices.

Commercial Organic Fertilizer Blends

Until recently, quality organic fertilizers were not widely available to the consumer although this is now beginning to change. While you have more choices than in the past, you should do your research before selecting your products.

Just because a package says ‘natural’, ‘organic’, or ‘derived from organic sources’ does not mean that it is a product that you want to use for your soil and plant management.

  • Urea – a synthetically derived organic compound, prohibited for use in organic agriculture
  • Look for ingredients you do not recognize – often an indication of synthetic components
  • Extremely high NPK claims – beware of anything that sounds “too good to be true”, this may be an indication of synthetic additives
  • Country of origin – due to lax regulations, products from other countries have been found to contain unwanted (and undocumented) substances

Single Components

A wide range of organic and safe sources exist that can provide the elements necessary for healthy soil and plant growth. Unfortunately, it requires a proper formulation of these sources to deliver a complete balanced plant diet since any single element cannot do it alone.

  • Plant-based components:  typically one of your safest nutrient sources. There is a risk of pesticide and GMO contamination.
  • Livestock by-products:  often a rich source of nutrients. There is a risk of contamination due to livestock management practices (hormones, antibiotics, pathogens, gmo’s).
  • Fish-based products:  the American Indians had the answer; an excellent, active source of nutrients and enzymes.
  • Mined components – as natural as you can get but not always good for your garden. Beware of heavy metals, source contamination and salts.

Compost – compost can be an important component in organic growing practices. Compost contains organic matter, trace elements, plant growth hormones and beneficial microbes. However, nutrient and mineral analysis is normally low (less than 1-1-1 NPK) and compost does not typically provide these in sufficient levels to support robust plant growth and yield. Also, compost can be a source of residual pesticides from the composted materials.

Worm Castings – similar to compost, worm castings are an excellent addition to your organic environment. They can offer a wide range of important elements (particularly microbes) but are not a major source of nutrients.

Manures – A wide variety of feedstock manures are used as soil amendments in both chemical and organic agriculture including cattle, horse, poultry, swine, goat, etc. Usage is typically dependent on what is available locally or on-farm.

NPK values are typically below 1 but vary across manures. As with most organic fertilizers and amendments, manures also provide a broad range of trace minerals and organic matter. Manures break down rapidly, meaning their nutrients are available to plants relatively quickly.

Caution should be used when applying manures:

  • Manures may contain antibiotics, synthetic growth hormones, chemical contaminants, heavy metals, salts and weed seeds depending on source animal and animal management practices
  • Nutrient levels are a direct relationship to what we feed our livestock – as we continue to move towards corn-based and processed feed, manure nutrient levels drop
  • Manures have risk of biological contamination such as e coli passed from source animal
  • The USDA restricts the use of raw manure for crops intended for human consumption
  • Properly composted manures minimize some of these health risks

Guano (Bats and Seabirds) – guano is the excrement of bats and seabirds and can contain high levels of nitrogen or phosphorous, dependent on the bats’ or birds’ diet. It has been used as a natural fertilizer for centuries. However, mining and usage of guano is not without risk.

Mining of fresh guano (high nitrogen) disrupts natural habitats and has led to the loss of bat species, associated invertebrates and fungi. Additionally, a serious human illness (histoplasmosis) has been associated to guano.

Guano is considered a ‘manure’ by the USDA National Organic Program with similar restrictions of use.

Sewage Sludge – Several commercially-available products (including Milorganite) are manufactured from human waste stream sewage sludge. These products are prohibited for use in organic agriculture.

Risks of using sewage-based products include:

  • high levels of heavy metals
  • pathogen contamination
  • toxins from items commonly disposed in our sewage systems

Research

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