Gardening While Isolated: Fertilizing Your Seedlings

Series of gardening tips continues with a look at how to use fertilizer

May 1, 2020, 10:44 am | Kathleen Delate, Chris Currey

AMES, Iowa – The "Gardening While Isolated" series from Iowa State University Extension and Outreach continues with a discussion on fertilizing seedlings.

Once seeds have been planted in flats, it will soon be time to fertilize them, to ensure the highest quality crop for transplanting into your garden. The Integrated Pest Management team at Iowa State University has created a YouTube video to explain planting.

Seedlings should be fertilized after they are three inches tall and can be fertilized weekly after that until transplanting. All plants need nitrogen (N), phosphorus (P) and potassium (K), which are commonly stated on fertilizer bags as whole numbers, such as 2-2-3. These numbers are actually percentages, representing 2% nitrogen, 2% phosphate (P2O5) and 3% potash (K2O). Fertilizing with organic materials is somewhat more complicated than fertilizing with conventional, synthetic formulations that are derived from petroleum products, as their nutrient concentrations are exactly the stated percentages on the label.

fertilizing seedlings.Organic fertilizers are formulated from natural, animal- or plant-based substances, whose nutrient concentration can vary, based on the particular raw materials. The release of nutrients from organic sources also is highly variable, so crops should be monitored to determine if any deficiencies develop.

Jaden Gimondo, graduate student in horticulture at Iowa State, demonstrates the types of organic fertilizers that can be used for transplants and how best to apply them.

The most generic fertilizer source is compost, which is best used by mixing in potting media on a 1-to-1 basis. Composting stabilizes nutrient content of manures and other organic materials and releases nutrients slowly, thus, minimizing nutrient loss and potential environmental contamination.

In the presence of oxygen, microorganisms decompose the organic matter present in manure and other raw materials. Some commonly used raw materials in compost include animal manure, crop residues, straw bedding, and processing or kitchen wastes. The elemental composition of the final compost largely depends on the chemical composition of the feedstock.

General guidelines suggest that 10-25% of compost N will be plant-available during the first year of application. Estimates for P and K availability in the first year are higher, at 40% and 60%, respectively. Organic fertility amendments like compost and manures have been shown to improve physical, chemical and biological properties of soil, so adding them to your soil before transplanting is also recommended. Use only well composted manure, as raw manure is strictly forbidden in organic production, unless turned into the soil at least 120 days before harvesting any crops.

All products used in organic production should be organic-compliant, as stated in the USDA-National Organic Program rules. Compliance can be verified if products contain an OMRI label—Organic Materials Review Institute.

Other fertilizers used in organic greenhouse transplant production include commercial dry formulations, which consists of aerobically composted turkey litter, feather meal and sulfate of potash.

Commercial formulations require calculations based on the level of fertilization you desire, ranging from a low to high dosage. Follow label instructions based on your plant needs. A similar commercial organic fertilizer that contains oilseed extract and comes in a liquid formulation is Nature's Source Organic Plant Food.

Liquid formulations rates are often stated as parts per million (ppm) with the recommended nitrogen rate of 150 ppm for vegetable transplants.

One of the easiest, most economical liquid organic fertilizers for small-scale production is fish emulsion.

Composed of recycled fish processing wastes, this fertilizer’s nitrogen content usually ranges from 2 to 3% N. Although the product is sanitized, it’s best applied in an outdoor setting to dissipate any fish smell.

Once your seedlings have reached six inches tall, and temperatures have warmed to 65 degrees at night for tropical crops, like tomatoes and peppers, you are ready to get them in the ground and growing.

The products and suppliers mentioned in this release are for example only. No endorsement of these products or suppliers is intended.

For more information, contact Kathleen Delate, professor and organic extension specialist at Iowa State University, at 515-294-7069, or


Original photo: Fertilizing seedlings.

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