Supplementing soil fertility

By Desiree Jans, Ph.D.

One of the principal goals of organic production is sustainability. To that end, soil fertility is largely managed by recycling of farm nutrients and organic matter by incorporating green manures, crop residues, compost and manure and by using varied crop rotations. A healthy population of soil microbes ensures that nutrients become available to following crops. However, a completely self-sustaining system is an ideal. At some point, off farm inputs will likely be needed to supplement specific crop requirements. But which soil amendments are acceptable in organic systems?

Organic standards include lists of permitted substances. It’s important to check with your certifying body before using any amendment because these lists may vary with certifying body and over time. The 2005 draft of the National Standard of Canada for Organic Production Systems states some criteria for identifying permitted substances. In general, permitted substances should not: harm beneficial organisms within the agroecosystem, deplete non-renewable resources, compromise water and air quality, or endanger human health. Certain substances are outrightly forbidden. These include any materials produced from genetic engineering, fertilizer or compost that contains prohibited synthetic substances, and sewage sludge.

Before using a soil amendment, it must be deemed “necessary”. That is, even though all the best organic management practices are being used (crop rotation, green manures, etc.), there is still a specific deficiency. For example, a soil test shows that additional manganese is needed to satisfy a potato crop’s micronutrient requirements. Judicious use of an organically approved source of manganese would then be justified.

The next consideration is the source and production of the substance. Soil amendments may be of plant, animal, microbial or mineral origin and may be processed by physical (mechanical or thermal), enzymatic, or microbial (composting or digestion) means.

Substances derived from living (renewable) sources are favoured. These could range from composted manure to various by-products of processing operations. Examples of permitted plant-derived substances and approximate nutrient content include: alfalfa pellets (3.5% nitrogen, 1-4% potassium, and trace elements), soybean meal (7% nitrogen, small amount of phosphorus; may need to be from organic source), and kelp meal (1% nitrogen, 2% potassium, and micronutrients). Permitted animal-derived substances include blood meal (11% nitrogen; must be sterilized), fish meal (10% nitrogen and 6% phosphorus), and feather meal (7-13% nitrogen).

A variety of mined minerals can be used on organic farms. Approved substances are not processed or fortified with synthetic chemicals and have not undergone any change in molecular structure through heating or combination with other substances. Examples of permitted mined minerals include calcitic lime (70% calcium carbonate), colloidal phosphate (20% phosphate), and langbeinite (27% sulphur, 22% potash, and 11% magnesium).

When selecting a soil amendment, be sure to understand how much of the desired nutrient you are getting. With slowly decomposing substances, there is a big difference between the total amount of nutrient and the amount that will be immediately available to the crop. For example, mineral forms of phosphate contain 20-30% total phosphate but only 2-3% may be available in the first year.

Also be aware of the form in which nutrients are listed on product labels. For example, the fertilizer industry standard is to list phosphate (P2O5) and potash (K2O) while the feed industry lists elemental phosphorus and elemental potassium. Labels for the same product would show 2.3 times more phosphate than elemental phosphorus. Similarly, potash would be 1.2 times higher than elemental potassium. Just because a number is bigger doesn’t necessarily mean you are getting more nutrient!

Here’s how to figure out how much amendment you will need for this year’s crop demand:

Of course, a final consideration is cost. Many amendments are cost prohibitive on a larger scale. But remember, depleting your nutrient reserves now will result in serious fertility issues in the future. Consider including more soil building crops and fewer high demand crops in rotation.

Much of the product information in this article was taken from the ACORN Directory of Allowable Organic Inputs available at http://www.acornorganic.org/acorn. Check out this excellent resource for product details and Canadian suppliers.


Desiree Jans, Ph.D, is a Web Course Instructor for the Organic Agriculture Centre of Canada (OACC). For information about OACC courses or to send comments, contact us at 902-893-7256 or oacc@nsac.ca.


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Posted October 2005