LSU AgCenter's Weekly Message

Organic Gardening, Part 2: Soil Fertility and Plant Nutrition

My last article focused on some basics of what organic means in gardening and farming. This one addresses ways to provide nutrients to plants in a way that’s compatible with organic standards.

As mentioned in the last article, the regulations that govern certified organic production in the US require that soil fertility be managed in a way that maintains or increases organic matter and doesn’t cause contamination of soil or water by plant nutrients, pathogens, or substances that aren’t allowed in organic production. They also specify that crop rotation, cover cropping, and the use of plant and animal materials must be used as part of managing soil fertility.

Cover crops are plants grown not to eat but to build up soil organic matter, take nitrogen from the air and put it in the soil (in the case of plants in the bean family, or legumes), prevent loss of nutrients from the soil during the off-season (after adding compost or manure to the garden, for example), suppress weed growth, prevent erosion, or a combination of these purposes.

Some leguminous cover crops that, because of the bacteria associated with their roots, can help add nitrogen to the soil include clovers, hairy vetch, Austrian winter peas, cowpeas, soybeans, velvet beans and sunnhemp. The first three would be planted in the fall, and the latter four would be planted in the spring. For nitrogen-fixation to occur, seed sometimes needs to be inoculated with the appropriate bacteria. Cover crops should generally be killed at the time that they flower, by mowing or in another manner. Leave enough time for them to decompose before you plant your crop.

While they don’t have the bacteria associated with their roots to fix nitrogen like legumes do, grasses can be part of managing soil fertility, too. For example, if you apply manure to the soil in the fall but don’t plan to plant until the spring, you can seed cereal rye or wheat, for example, to hold onto the nitrogen in the manure so that it doesn’t leach out of the soil over the winter.

Nitrogen and other nutrients are released from organic materials over time, as they break down, rather than all at once. This makes it somewhat difficult to make apples to apples comparisons between organic and synthetic fertilizer recommendations. However, their value isn’t just in the nutrients that they provide but also in the organic matter that they add to the soil.

Especially in some locations, poultry manure is often one of the more economical sources of organic nutrients. These products have nutrient levels around 2 to 5 percent nitrogen, phosphate, and potash, as well as varying levels of secondary nutrients and micronutrients.

When animal manure is used on land for organic fruits or vegetables, it must either be composted according to specific standards laid out in the law (ones that would not be met in a typical home compost pile), or it must be mixed into the soil a specified number of days before harvest. The number of days depends on whether the edible part of the crop does (120 days) or does not (90 days) have direct contact with soil particles.

Some stores offer bagged organic fertilizer products made up of various materials and sold with a nutrient analysis of 5-3-3, 3-4-4, or similar. Typically, these are quite a bit more expensive, on a per-nutrient basis than comparable non-organic products such as 8-8-8 or 13-13-13. To get the same amount of nitrogen from a 3-4-4 product, one would need to use a little over twice as much as they would of 8-8-8 and about four times as much as they would of 13-13-13. Also, generally only some

percentage of the nitrogen is water-soluble, meaning that some will not be immediately available to plants. However, such products offer a convenient option for those who have a small garden and want to follow organic practices.

Another option is to use separate nitrogen, phosphate, and potash materials. For example, one could use feather meal (14-0-0), bone meal (2-14-0), or mined potassium sulfate (0-0-50), as needed. (Nutrient analyses are approximate and may vary by source.) Whether or not such products are organic-compatible depends on their source. An OMRI label indicates that they have been determined to be compatible by the Organic Materials Review Institute.

Those looking for an organic alternative to fertilizers dissolved in water, such as Miracle Gro, might consider OMRI-listed fish emulsion products. As with fertilizers applied dry, you’ll generally need to use more to get the same amount of nutrients as you would from the comparable synthetic product. Most fish-based fertilizers have nitrogen concentrations between 1 and 5% while the synthetic water-soluble products have nitrogen concentrations around 15 to 30%.

Note that this isn’t intended to provide all the information a commercial grower would need to know to sell organic produce. If you’re interested in doing so, you can contact me for more information.

I’ll discuss organic weed, insect, and disease management in the next article.

Let me know if you have questions.

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Dr. Mary Helen Ferguson is an Extension Agent with the LSU AgCenter, with horticulture responsibilities in Washington and Tangipahoa Parishes. Contact Mary Helen at mhferguson@agcenter.lsu.edu or 985-277-1850 (Hammond) or 985-839-7855 (Franklinton).