Organic Fertilizers Offer Many Advantages

Posted: January 26, 2015

Natural organic fertilizers are made from plants and animals – living things. They are the wastes and by-products of some sort of processing.

Alfalfa meal, cottonseed meal, blood meal, and manure are all examples of organic fertilizers. Other materials approved by the National Organic Standards Board for organic farmers are considered organic but are not made from plants or animals. These include inorganic minerals such as rock phosphate and greensand. There are also synthetic organic fertilizers, which are man-made and include urea and ureaform. Some fertilizers, such as sewage sludge, are called organic but are not approved for organic farming. Beware of products labeled “organic.” Always read the labels to determine their actual make-up--just because they are labeled “organic” does not mean that they have desirable components.

Synthetic fertilizers are rapidly absorbed by plant roots and help the plants grow, but they do nothing for the soil because they do not feed the soil critters (bacteria, fungi, earthworms, etc.). By not adding organic matter to the soil and using only chemical fertilizers, your soil will have fewer microorganisms and the structure will deteriorate and lose its water-holding capacity. The chemical fertilizers just wash away, requiring you to add more and more just to get that plant to grow. Obviously, none of this is good for the environment.

Organic fertilizers release their nutrients over a longer period of time than synthetic fertilizers, although not necessarily when the plant needs it for growth. They need those soil critters to break down the nutrients and make them available to the plants. The critters need warm, moist soil in which to do this. This is why adding organic matter is so important and what “feeding the soil” actually means. By doing this you are not only improving the soil structure and increasing its organic content, but you are increasing the activity of those microorganisms that are so important to healthy, fertile soil.

While you are building your soil’s fertility with organic matter, and after you have gotten the results of your soil test, you may want to use some organic fertilizers to give your plants a little boost. You can use commercial blends or make up your own, depending upon what you need. The NPK numbers (the percentages by weight of nitrogen, phosphate, and potash) should be low in any organic fertilizer. It is important to read and follow the package directions because you can overdo organic fertilizers as well as synthetic ones. In general, apply a dry fertilizer by broadcasting it over the area and then raking it into the top several inches of soil. It can also be added to planting holes in small amounts. Because of the low nutrient content of organics, most will not burn roots and are safe for transplants. They can also be used as a side dressing when raked into the top inch of soil. Liquid fertilizers can be used every 2-4 weeks or so and are most useful when the plants are stressed after transplanting or by weather conditions.

An excellent source for information about organic fertilizers is Rodale’s Ultimate Encyclopedia of Organic Gardening, published in 2009. The following is a partial listing of some of the options. Many are a good source of one of the Big Three, nitrogen, phosphate, or potash, and some have several other nutrients that plants need.

For nitrogen:

Cottonseed meal:

7-3-2 or 6-2-1. Byproduct of cotton manufacturing. Used for acid-loving plants such as azaleas. Could contain pesticide residues; look for organic source.

Blood meal:

11-0-0 or 12-2-0. Dried powdered blood from animal slaughterhouses. Releases the nitrogen fairly quickly, so use caution around tender roots. Some trace elements.

Fish emulsion:

4-1-1 with some sulfur and trace elements. Fish meal: 5-3-3 to 9-3-0. Nitrogen is released slowly. Strong fish odor goes away in a few days.

Corn gluten:

9-0-0. Byproduct of corn processing (cornstarch). Used as a weed inhibiter for lawns with limited success, but can also be used as a fertilizer. Apply to established gardens and lawns (remember, it inhibits seed germination). May attract rodents.

Soybean meal:

6-1-1 or 7.0-0.5-2.3. Nitrogen is released a little faster than fish or bone meal but not as fast as blood meal. This and cottonseed meal may come from genetically-modified plants.

For phosphorus (phosphate):

Bone meal:

1-11-0 but could be as high as 6-12-0. Also contains calcium. Mix thoroughly in the soil since phosphorus moves slowly through the soil profile.

Rock phosphate:

0-3-0. A mined mineral. High in phosphorus (30%) but only a small amount (3%) is available at a time. Used to increase the phosphate content of the soil over time. Also has calcium and several trace elements.

For potassium (potash):


0-1-7. A natural mined rock powder containing glauconite. Contains slow-release potassium and many other nutrients, including calcium, magnesium, iron, and phosphorus.

Kelp meal:

1.0-0.5-2.5. Ground, dried seaweed. A great soil conditioner that breaks down quickly. A source of potassium and a wide variety of other nutrients; also contains natural enzymes and hormones that promote plant growth.

Liquid seaweed:

0-0-1. Extract of kelp meal. Most effective applied as a foliar spray but can also be applied to the soil.


Alfalfa meal:

5-1-2, 3-0.5-3, or 2-1-2; contains other nutrients.


The best stuff for your garden! Compost is low in nutrients (1-1-1 for dry commercial compost) but they are released evenly throughout the growing season. Homemade compost and mushroom compost vary in nutrient content. The latter may contain pesticide residues, as well as having a high salt content and a pH near 8.0.

Some things you may have around the house/yard: coffee grounds add nitrogen and should be added to compost; shredded leaves and grass clippings add organic matter; eggshells add calcium and some trace minerals; sawdust adds organic matter and should be well rotted before using; and wood ashes add potash that is immediately available—use in small amounts to avoid raising soil pH too much.

To summarize: get a soil test every few years, build up your soil fertility with organic matter, choose organic fertilizers carefully, and add them to your garden according to package directions and soil test results.

Contact Information

Sarah Graham
  • Master Gardener, Allegheny County