The numbers on a bag of fertilizer tell you the percentage of nutrients contained in the bag. This is shown in percentage of weight by volume.

The law requires these nutrients always be listed in the same order. The first number represents the nitrogen (N) content. The second number represents phosphorus (P) and the third number represents potassium (K). These nutrients always will be listed in this order, N, P, K. If the fertilizer bag shows 5-20-10, it means that 5 percent of the weight of the bag is nitrogen, 20 percent is phosphorus and 10 percent is potassium.

By now you have noticed that those three numbers add up to 35, not 100. What is the remaining 65 percent? These are the inert ingredients. Inert ingredients vary by manufacturer, but may contain sand, vermiculite or other things that provide no nutritional value to plants and make the fertilizer easier to distribute.

What are the differences between “processed” or “chemical” fertilizers and “organic” fertilizers? The nitrogen in processed fertilizers is derived from petroleum and natural gas.

Phosphorus is derived from rock and undergoes treatment with sulfuric acid. Most potassium is recovered from underground deposits of soluble minerals.

Chemical fertilizers are highly concentrated. When applied to plants, chemical fertilizers give the plant a big shot of pure nutrient. However, they do nothing to feed the soil and the beneficial microorganisms in the soil.

Microorganisms break down plant material and other organisms found in soil into organic matter (humus), which is key to healthy soil structure, which is key to “feeding” plants. Used exclusively, chemical fertilizers can damage soil.

Basic organic fertilizers can consist of chicken, steer or goat manures. Certain animal manures have a higher concentration of nitrogen than others. Chicken manure has a higher nitrogen concentration than steer manure.

Other nitrogen organics include blood meal, fish meal and alfalfa meal. Phosphorus can be found in bone meal and rock phosphate. Potassium can be found in greensand and kelp meal. Organic fertilizers typically contain a combination of these things to provide a balanced fertilizer product. Organic fertilizers break down more slowly, giving plants a more consistent dose of nutrients over a longer period of time. Organic fertilizers also feed the microorganisms in the soil. Compost and other organic matter improve soil structure and provide nutrients to the microorganisms in the soil.

If you choose to use chemical fertilizers, consider composting vegetable scraps from your kitchen and healthy garden waste and use your compost along with the chemical fertilizer to feed your soil.

Organic fertilizers contain a fair amount of organic material, but adding additional compost will further improve soil structure.

The March 11 “Get It Growing” article “Understanding Garden Soil – Part I – The Components” and the March 18 article “Understanding Garden Soil – Part II – The Process” are available through the Sequim Gazette.

Judy English is a Washington State University-certified Clallam County Master Gardener.