You’ve probably heard the arguments for organic produce and food versus inorganic varieties, but what about organic fertilizers ? Walk down a garden center aisle and you’re faced with a dizzying array of choices when it comes to fertilizers. Sometimes, choosing the right one can be downright impossible.
Here’s the low-down on organic versus inorganic fertilizers . First, both supply nutrition to plants, primarily in the form of nitrogen, phosphorus, and potassium. Synthetic fertilizers are usually made using petroleum products, which makes them less environmentally friendly. In fact, excessive runoff from synthetic fertilizers after storms can actually damage the environment. In addition, they usually don’t add organic matter to the soil or build soil microbes, and they may also contain fewer micronutrients.
Organic fertilizers have potential drawbacks, as well. Manure, for example, is an excellent fertilizer because it’s free and widely available. However, it’s high in nitrogen and low in other nutrients, so you might have to supplement it with other fertilizers. Hauling and spreading it is obviously more labor intensive than opening a bag of synthetic fertilizer and sprinkling the little bits everywhere. Price is a consideration for many organic fertilizers, which can cost considerably more than their synthetic counterparts. However, many farmers feel that the benefits their plants and the environment receive outweigh any extra work or cost they might endure.
Regardless of which one you choose, keep in mind that both organic and synthetic fertilizers can cause damage if improperly used. Both can leach into ground water or burn plants. Many gardeners opt to use both – manure and compost to improve soil texture and small amounts of synthetic fertilizers to boost growth. Whichever fertilizer  you opt for, get a soil test so you know exactly what your soil needs and you don’t overfertilize. Below you’ll find information on the most common organic fertilizers available.
Alfalfa Meal. This agricultural product, usually used as animal feed, can also be used as a fertilizer. Like compost or cover crops, it adds organic matter to the soil, which improves texture and drainage. Alfalfa meal is a good source of trace minerals and has a nutrient profile of 2-1-2. It breaks down over one to four months.
- Advantages: Inexpensive and widely available at feed stores.
- Cons: Might contain weed seeds.
Corn Gluten. Corn gluten is usually touted as a pre-emergent herbicide, but it is also a fertilizer. This natural product, as the name implies, is made from corn. Spread it on your lawn or soil and it dissolves to form a thin layer over the soil when moistened. This layer can inhibit seed germination, which is why it is used as a pre-emergent weed treatment. As the product breaks down, though, it also adds nutrients, and in particular, nitrogen, to the soil. It has a nutrient profile of 9-0-0.
- Advantages: Very safe product and a rich source of nitrogen.
- Cons: Some products contain GMOs. Also, corn gluten is relatively expensive and not yet available nationwide. It inhibits seed germination, so don’t use it until after vegetable seeds have emerged.
Cotton Seed Meal. Cotton seed meal is made from ground cotton seed. This product is high in nitrogen and has a nutrient profile of 6-1-1. Unfortunately, most cotton plants are heavily sprayed with pesticides, which does affect the seeds. Some may be GMOs.
- Advantages: High nitrogen source, widely available in some areas.
- Cons: GMO and pesticide contamination.
Green Manures. Green manures, or cover crops, are planted in late summer. They overwinter and are dug into the soil in the spring a few weeks before spring planting. As the crops break down, they add organic matter and nitrogen to the soil.
- Advantages: Inexpensive, can reduce weed growth. Builds soil while adding nutrients.
- Cons: More work intensive than some methods.
Seaweed. Kelp products, such as kelp meal and liquid kelp, don’t contain macronutrients, but they are a good source of micronutrients and growth enzymes. Use them to rejuvenate stressed plants, such as after a severe storm. Typically, you’ll see new growth within a few days. Unfortunately, these products are often expensive.
- Advantages: Provides nutrients unavailable in other products.
- Cons: Expense.
Soybean Meal. Soybean meal, like alfalfa meal, is most often used as an animal feed. In the garden, though, it is an inexpensive source of nitrogen. It has a nutrient profile of 7-2-1, and it breaks down in one to four months.
- Advantages: Widely available at feed stores. Inexpensive source of nitrogen.
- Cons: Most soybean meal contains GMOs.
Animal Manures. Animal manure is a readily available, renewable source of nitrogen. Cow, horse, sheep, rabbit, and poultry manures can all be used.
- Advantages: Inexpensive. Builds soil texture and improves drainage.
- Cons: Work-intensive. Can burn plants or cause human illness if not used correctly.
Bat Guano. Bat guano is a euphemistic term for bat poop. Harvested from caves, this product is sold as a dry powder. It can be added directly to the soil or mixed with water to make a tea. Bat guano is either high in nitrogen or high in phosphorus, depending on the source and processing method. Nutrient content is either 10-3-1 or 3-10-1.
- Advantages: Easy to spread, breaks down slowly, and stimulates soil microbe growth.
- Cons: Harvesting and processing bat guano is an intensive process; hence, the price for this product is high. It isn’t widely available in all areas.
Blood Meal. A by-product of the slaughterhouse industry, blood meal has more nitrogen than almost any other natural fertilizer product. In fact, if not used carefully, it can burn plants. It is fairly inexpensive to buy at feed stores, but ridiculously costly at garden centers and nurseries.
- Advantages: High source of nitrogen. Its nutrient profile is 12-0-0.
- Cons: Cost at some retailers. Can burn plants.
Bone Meal. Another by-product of the slaughterhouse industry, bone meal is one of the most common natural sources for phosphorus. It has a nutrient profile of 3-15-0.
- Advantages: High amount of phosphorus in a form readily available to plants.
- Cons: Cost. Like blood meal, you should look for bone meal at a feed store, rather than a garden center for the best prices. Also, the phosphorus in bone meal might not be available to plants in alkaline soil.
Fish Emulsion. Fish emulsion products have been around for a long time. These products are made from heat and acid-processed fish waste. They are usually fairly inexpensive and supply a good source of major and micronutrients. Fish emulsion is usually a liquid product, which makes it easy to use. One that is highly regarded is Protogrow ™.
- Advantages: Good source of micronutrients.
- Cons: Some have a foul, fishy odor.
Enzymatically Treated Liquid Fish Products. These products have been enzymatically processed, rather than heat or acid processed, for less odor and more nutrients. They have a nutrient profile of 4-2-2 and break down over one to four months.
- Advantages: No odor, more micronutrients.
- Cons: Cost.
Fish Meal and Fish Powder. These products are heat processed and ground into meal or powder. The powder is often combined with water to make a liquid product. Dried fish products are a great source of nutrients. Fish meal has a nutrient profile of 10-6-2, while fish powder has a profile of 12-0-1.
- Advantages: Fast-acting source of nutrients.
- Cons: Cost.