If you worked hard to create your ecosystem both above and below ground, to create an environment where nature does much of the work for you, take a moment to consider what synthetic fertilizers do to the soil before you apply them.
The Benefits of Organic Fertilizers
Now, let’s talk about organic fertilizers. These amendments range from plant-based materials such as alfalfa, to minerals such as rock phosphate, to animal by-products such as bone meal. Each has its own properties, and typically several nutrients are combined to create “balanced” fertilizers for different plant groups. On nursery shelves, you will find a box of fertilizer for vegetables, one for roses, another for acid-loving plants, and yet another for fruit trees. Each brand has a proprietary blend that it feels is best for its customers. The blend of nutrients appears on the box as the ratio of our Big Three, and those numbers will range between 3 and 7—much lower than synthetics. For example, a box of organic citrus fertilizer may list a ratio of 7–4–2, meaning that the total content of each nutrient is 7 percent nitrogen, 4 percent phosphorus, and 2 percent potassium.
With numbers that low, the total nutrient content is more likely to be taken up by plants, leaving little behind to infiltrate the water table. Still, any fertilizer should be used with caution; more is not necessarily better.
Available Nutrient Options in Fertilizer
Nitrogen: Green Leafy Growth
Animal-based sources of nitrogen include blood meal, feather meal, hoof and horn meal, and fish meal, which generally have a rate of 9 to 15 percent in pure form. Both hoof and horn meal and fish meal also contain some percentage of phosphorus, fish meal being higher. Blood meal and feather meal may have little to no phosphorus, depending on its source.
Nonanimal-based sources of nitrogen are alfalfa meal, cottonseed meal, soybean meal, and kelp meal. These fertilizers offer a lower rate than their animal-based counterparts, between 1 and 7 percent. Another great source of nitrogen can be found in your coffee maker. Spent coffee grounds supply nitrogen at a rate of about 2 percent, with trace levels of phosphorus and potassium. Compost is a source as well, but it may surprise you that compost is relatively low in nitrogen. However, as we discussed earlier, plants take up nutrients in small amounts, so the available nitrogen is sufficient in many cases. Plus, with the bonus of the vibrant, nutrient-building microbial life forms that compost provides, it makes a fantastic fertilizer.
Animal manures, which straddle the aforementioned categories—being from animals but not by-products of slaughter—are higher in nitrogen when dried, according to North Carolina State University research. Dried cow, chicken, and hog manures and fresh rabbit manure all provide nitrogen at a rate between 1 and 2.2 percent.
Potatoes growing in a container
Phosphorus: Roots, Fruits, and Flowers
Bone meal is the predominant animal-based source of phosphorus but, as mentioned, many of the animal-based nitrogen fertilizers also supply some phosphorus. Bone meal supplies phosphorus at a rate ranging from 11 to 22 percent.
Rock phosphate and soft rock phosphate are nonanimal sources of phosphorus. They are both mined from sedimentary rock: rock phosphate as tricalcium phosphate, and soft rock phosphate as a by-product of the mining industry. They supply 2 to 3 percent phosphorus.
In the manure category, seabird or bat guano (a nice way of saying poop) is excellent. These resources range from 10 to 15 percent phosphorus and also have between 1 and 3 percent nitrogen.
Potassium: Overall Vigor and Fruit and Flower Development
Animal-based fertilizers supply virtually zero potassium, so we’ll skip right to nonanimal-based options. Sulfate of potash, also known by the commercial name Sul-Po-Mag (meaning sulfate of potassium-magnesia) is the most common source of potassium. Potassium remains in the soil for years, so make sure that you test your soil first before applying, because it may not need potassium. Other options include greensand, which supplies between 3 and 7 percent potash. It is marine sediment mined from ocean-adjacent rock formations. If you have very sandy soil, avoid greensand, because it will make soil texture even sandier. Kelp meal and liquid kelp emulsions generally supply low levels of potassium but offer the benefit of trace minerals as well. Wood ashes, depending on what is burned to create them, can provide small amounts of potassium and phosphorus. Don’t use wood ashes if you have highly alkaline soil, because it tends to raise pH.
Animal manures, both in dried and manure tea forms, supply potassium as well as the other aforementioned nutrients. Like liquid kelp emulsions, animal manures and manure teas also supply a broad spectrum of trace minerals.
Is It Organic?
Remember, as mentioned earlier, that even if that box of fertilizer says “organic,” it doesn’t mean that the source of that bone meal or manure was raised organically. It just means that the fertilizer was derived from organic matter and that it is safe for use in organic agriculture.
pH Adjusters and Trace Minerals
Trace minerals play a role in nutrient uptake, and, depending on geographic location and farm practices, these minerals can become depleted. As mentioned, some organic fertilizers supply a small amount of trace minerals, so a fertilizer may be all you ever need to use. But you might be wondering what all of those other boxes on the nursery shelves are for. It’s important to know what they are and what they do before applying them to your garden. Often, products that provide trace minerals also serve as soil pH adjusters. They will raise or lower your pH, and sometimes this is a good thing. Sometimes, not so much.
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