Organic Fertilizer Options for Potassium

Asked April 29, 2017, 3:33 PM EDT

Hello, I am fairly new to organic gardening, and this year I tried developing my own fertilizer blend for general-purpose fruit and vegetable container gardening (strawberries, cherry tomatoes, peas, and green bush beans). However, after much careful research and effort, I was informed from a salesperson at a garden store that my special fertilizer blend may be deficient in potassium.

My fertilizer blend is composed of bone meal (Whitney Farms, Organic, 3 lb. bag, 6-8-0), blood meal (Whitney Farms, Organic, 3 lb. bag, 12-0-0), and kelp meal (Dr. Earth, 2 lb. bag, 1-0.5-2). I also added 4 dozen powdered eggshells (dried at 200-degrees for several hours), and about 2 cups of Epsom salt (I understand that magnesium sulfate helps with blossom production). I have amended the soil with store-bought compost ("Black Gold" brand; N-P-K unknown), and I am going to add worm castings from my own worm bin.

I was preparing to add dried, powdered banana peels for more potassium, but I was informed that this may not be a valid source of potassium (apparently it is an "old wives' tale").

I accessed an article on Oregon Extension website which described the N-P-K values of organic fertilizers( I am very interested in "cucumber peels (burned)", as this article proclaims that the estimated N-P-K value of this fertilizer is 0-11-27. However, I am not clear how to properly create burned cucumber peels, in what amount to apply per square foot, and most importantly, whether this particular fertilizer presents a risk of harm to my garden when added to my already amended containers.

So, after all this explanation of my particular predicament, I would like to know if there are alternative, perhaps "safer" organic options for adding potassium to my garden, or would burned cucumber peels be a good solution? Also, am I misunderstanding the importance of a "balanced" fertilizer? What I mean is, do I have to aim for a fairly equal ratio of N-P-K values, or is it sometimes ok if one of the values is a higher proportion than the other components? (For example, in my situation, I am estimating that if I were to add burned cucumber peels in the correct proportion to my current fertilizer blend, the N-P-K values would roughly be increased from 19-8.5-2 to 19-19.5-27. Would this be a safe or desirable balance for general fruit and vegetable gardening?).

Final question: I have heard varied advice about how best to apply worm castings to potted plants. I have previously allowed the castings to dry out, as this seems to encourage most of the insects from the ecosystem to vacate the castings (which I imagine is in the best interest of the plant). Is this best practice when applying worm castings, or would it be better to apply the castings straight from bin to plant?

Thank you for any advice you can offer me! Have a beautiful day!

Lane County Oregon fertilizer horticulture

1 Response

Hello, and thank you for your question! Based upon the bone meal, blood meal, and kelp meal amounts you mixed, the analysis (before adding the other ingredients) would be approximately 7-3-0.5, so it is relatively high in nitrogen (N) and low in potassium (K).

Banana peels are approximately 7-8% potassium, so compared to other organic ingredients they are relatively high in potassium. I am familiar with the publication you mentioned, and burned cucumber peels are an obscure ingredient and simply are not readily available.

The need for a balanced fertilizer depends on plant needs and the soil’s ability to supply those needs. Depending on management history, many native soils may have significant amounts of phosphorus (P) and/or K, so little or even no P and/or K may be needed on those soils. Note that P doesn’t tend to leach appreciably, but K does leach slowly over time, so potassium is needed periodically but not necessary annually. However, you mentioned containers, so the P and K content is entirely the result of what was in the potting soil you started with plus any fertilizers you have since added (minus crop removal and leaching over time).

Since soils and management histories vary, a soil test generally is the best way to determine fertilizer needs. Soil testing often is related to economy of scale: it makes sense for a farmer with many acres to test soil so as to avoid both over- or under-fertilizing; many homeowners choose not to spend the time and/or money to perform a soil test and instead add balanced fertilizers with the idea that whatever the plants may need probably is contained in the fertilizer. As you can imagine, this approach inevitably results in over-application of some nutrients and under-application of others.

Because N leaches readily, N generally needs to be applied annually. For vegetable gardens, tilling in N at the rate of about 1 pound of available N per 1,000 square feet (about ½ gram of available N per square foot). Using your starting fertilizer mixture with 7% N (which was further diluted as you added more ingredients), 0.5 grams per square foot divided by 0.07 (decimal form of 7%) equals about 7 grams of fertilizer per square foot, which is roughly half of a tablespoon (using 2 cups per pound as a rough conversion factor).

You can safely apply worm castings directly to the soil, whether to the surface or worked in. There is no need to dry the material before applying it to the soil.