OrganicFertilizers_edited-1Years ago (in 1996 to be exact), I took a soil science class at Colorado State University. It was fascinating. The physical make up of soils (sand, silt and clay), the chemical make up of those same particles, and plant nutrition fit right into my nerdy chemistry loving brain. Of course, we did all kinds of fertilizer calculations, all of which I have completely forgotten how to do 20+ years later. And unfortunately, we spent all of about one day out of the semester talking about the micro-organisms that live in soil. If you are an organic gardener/farmer, giving so little attention to this critical ecosystem, which is literally the driver of the entire nutrient cycle, is just sad (not to mention short sighted).

Over the intervening years, I’ve collected a couple of recipes for DIY organic fertilizers. THIS year, I decided to sell one of those fertilizer mixes (actually, two of them) at our local farmers market this spring, formulated specifically for tomatoes. So I went back down the rabbit hole, trying to reconcile what I had learned in my soil science class, those DIY recipes, which called for cups of this and cups of that, but didn’t disclose the actual NPK (Nitrogen, Phosphorus and Potassium) in said recipe, and what I could find on “optimum” ratios for growing tomatoes, peppers, and other garden vegetables. Here’s what I’ve learned (or relearned) and how you can apply it to your own garden this year.

First, a review. Plants need three basic nutrients, nitrogen, phosphorus and potassium (NPK), in substantial quantities, to grow. They also need some micronutrients (calcium and magnesium are two), and trace minerals – things like sulfur (S), iron (Fe), copper (Cu), manganese (Mn), boron (B), Zinc (Zn), cobalt (Co), selenium (Se), silicon (Si) and molybdenum (Mo) – in much smaller quantities. Think of the NPK as akin to the fats, carbohydrates and proteins YOU need in your diet, and the rest as the important vitamins and minerals. The numbers you see on the fertilizer bags (its a requirement that they be labeled), like 4-5-3? That represents the percent NPK (in that order) in that fertilizer. So a 50 lb bag of 4-5-3 contains 2 lbs of actual nitrogen (50x.04), 2.5 lbs of phosphorus (50x.05) and 1.5 lbs of potassium (50x.03).

BloodMealNitrogen (N) is used by the plant to grow big and have lots of leaves. Too much nitrogen, especially combined with a lack of other nutrients, will inhibit flowering and fruiting. Plants with extra high nitrogen needs include lawns and the grains/grasses like sweet corn. If you’ve ever had a tomato plant that was all green growth and little fruit, too much nitrogen might have been the problem. Good sources of organic nitrogen include blood meal, fish meal, feather meal, alfalfa meal, grass clippings and composted manures of all types, including bat guano.

Phosphorus (P) is used by the plant to produce flowers and fruits, and to form strong roots early in the season. Bone meal is a common organic source, as are the rock phosphates, which are much slower to release.

Try not to think too hard about where a bag of blood or bone meal might come from, and how you reconcile a factory farming waste product with your “living lightly on the planet” gardening ethic. Or plan on increasing the size of your own compost pile, a LOT.

Potassium (K) is used by the plant to process all nutrients more efficiently, improves the quality of fruits, and helps plants resist stress. Organic sources include wood ashes (be VERY careful applying these if your soil is already high in pH, as the wood ash will increase pH even more), green sand, kelp meal (which only has K in small amounts, but contains other good stuff), langbeinite (a potassium magnesium sulfate mineral that is mined from the earth) and sulfate of potash (crazy strong, with a K of 60).

Good sources of calcium include various limes, including agricultural lime (primarily calcium carbonate – will raise pH), microna lime (like agricultural lime, but much finer, so accessible to the plants faster in the short term), dolomite lime (contains both calcium carbonate and magnesium carbonate – generally only recommended if the soil is magnesium deficient – will raise pH), gypsum (calcium sulfate – will lower pH), and oyster shell flour (contains both calcium carbonate and small amounts of phosphorus – will raise pH).

Good sources of magnesium include epsom salts (applied overall at about one cup per 100 square feet at the beginning of the season, or used as a foliar spray by mixing 1 tbsp epsom salts to 1 gallon of water and spraying on foliage). Dolomite lime can also be used (see calcium sources above). Note: be careful not to overdue this one. More is NOT better. Too much salt in your soil = bad.

AzomiteGood sources of trace minerals include kelp meal (which also includes a small amount of nitrogen and potassium) and azomite (mined from a unique Utah deposit that contains a combination of volcanic ash and marine minerals. Supposedly over 70 trace minerals, though their website does not list them).

Note that when I say “organic sources” what I mean is sources that are acceptable if you were growing a certified organic garden. Some of these I know from reading about organic gardening practices for 20+ years. And some are officially “approved” sources/brands, given the stamp of approval by the Organic Materials Review Institute (ORMI). You will sometimes see the ORMI stamp on boxes of organic fertilizer (or pesticides) at the store. (I have a whole other blog post in my head about how the organic approval system actually works. It’s actually pretty cool and grass roots – but that’s for another time).

You’ll commonly see, in big box stores and nurseries, fertilizer where the NPK number is the same. So a box or bag of 10-10-10. This appeals to our human desire for balance, and they sell well. In fact, we’ve become so indoctrinated to this idea that some manufacturers make them just because their customers demand them, when in reality, plants don’t generally use these nutrients in equal amounts, and how the nutrients are broken down by the microbes in the soil is not always equal either.

What IS the ideal? Well, there seems to be no universal consensus, since different plants will need these nutrients in different amounts. Advice on tomato fertilizers tends to be more of a 1-2-2 ratio, ie twice as much P and K as N. This is to avoid that issue of lots of green, with low fruit. One analysis of tomato plant tissue I ran across indicated a 3-1-3 ratio, indicating that the P is the least important nutrient. Mike McGrath, former editor of Organic Gardening Magazine back when the articles were more than 500 word sound bites, and then of the WHYY Radio “You Bet Your Garden” fame, (who I love as much for his humor as his gardening expertise) says a good general purpose ratio for a vegetable gardens is 3-1-2.

A note about ratios, which often seem to completely stymie those who are math averse. A ratio is just a part. So, if I had 3 ounces of nitrogen, 1 of phosphorus, and 2 of potassium, I’d have a 3-1-2 ratio. If I had 6 ounces of nitrogen, 2 of phosphorus, and 4 of potassium, I’d STILL have a 3-1-2 ratio. If I had 9 lbs of nitrogen, 3 of phosphorus, and 6 of potassium, I’d STILL have a 3-1-2 ratio. If I had 12 tons of nitrogen, 4 of phosphorus, and 8 of potassium, I’d STILL have a 3-1-2 ratio. See how that works? It doesn’t matter what the unit is. It matters how it relates to its other parts.

The problem with those magazine DIY recipes? They give you the measurements in cups of this and cups of that. So unless you have a scale, and can measure what a cup of blood meal weighs, you don’t really know what percentage of each nutrient you are applying, because those NPK numbers on the bag are percent by weight, not volume. And you really need to measure this for yourself, as one brand of blood meal may not weigh the same as another, depending on the size of the particle.

Here are a few weights vs volume measurements, should you decide to take this on, but don’t have a scale.

  • Blood Meal: 1 cup = 5.1 oz.  1 lb = 3 cups plus 1 tbsp
  • Feather Meal: 1 cup = 4.8 oz. 1 lb = 3 1/4 cups.
  • Bone Meal: 1 cup = 8.5 oz. 1 lb = 1 7/8 cups
  • Green Sand: 1 cup = 10 oz. 1 lb = 1 1/2 cups.
  • Kelp Meal: 1 cup = 6.9 oz. 1 lb = 2 3/8 cups
  • Sul-Po-Mag: 1 cup = 13.3 oz. 1 lb = 1 1/3 cups
  • Gypsum: 1 cup = 9.8 oz. 1 lb = 1 5/8 cups
  • Oyster Shell Flour: 1 cup = 7.3 oz. 1 lb = 2 1/4 cups
  • Epsom Salts: 1 cup = 8.1 oz. 1 lb = just under 2 cups

Here’s what the math might look like for calculating your own blends. Plugging this into a spreadsheet will make playing with the numbers easier.

fertilizer calculations

As a general rule, for organic growing, it is suggested that you don’t apply nitrogen fertilizers at more than 5% strength. So no 10-10-10 fertilizer from the big box stores if you want to stay organic. Super concentrated chemical fertilizers are fast acting, but can fry the microbes in your soil, and so have long-term consequences. As McGrath says, it’s like taking a vitamin versus eating a good meal. These highly concentrated fertilizers also tend to move THROUGH the soil, because there is too much for the plants/microbes to use, and they run off into our groundwater, streams, rivers and oceans causing all kinds of issues. Our drinking water well here on the farm has slightly elevated nitrate levels (below the 10 milligrams per liter cut off), no doubt because we are surrounded by conventional farms, including wheat fields.

It’s important to remember that all fertilizers—chemical and organic—rely on soil life to make their nutrients available to plants, and that high levels of organic matter are the best thing for healthy microbes. As the age old gardening advice goes, compost mitigates all soil issues. So all fertilization plans should include high-quality compost or green manure applied to the soil. Then, as the season progresses, you can give your plants a boost if you think they need it, by adding more compost or using a well-balanced organic fertilizer.


The ultimate goal. A 2 lb delicious organic tomato!

Lastly, the very best thing you can do is get a soil test. Two labs that are often recommended, particularly if you are interested in organic production, are Logan Labs and Midwest Labs. In some areas, your local extension office or Natural Resource Conservation Service office may offer this service as well. Expect to pay between $25 to $50 per test, and be sure to follow their instructions carefully. I’ve learned, from my own soil tests, that my soils are low in calcium, high in magnesium, have plenty of available phosphorus and potassium, and have a pH of about 7.5. From this I know that I do NOT want to apply dolomite lime or epsom salts (I don’t want more magnesium) and that gypsum would be a better additive than ag lime to add calcium, since I don’t want to raise my pH any more than it already is. This year, in addition to composted chicken manure, I’m using feather meal to supply my needed nitrogen.

Miles Away Farm Blog © 2017, where we do our best to minimize our off farm inputs, and use only organic fertilizers when they are necessary. Because that’s just the way we’ve rolled for 20+ years.