Fertilizers - Knowledgebase Question

Cedarburg, Wi
Question by christinap_2
June 30, 2010
There are so many fertilizers available, how do you know which is best for your garden? What do the 3 numbers mean? What should you look for?

Answer from NGA
June 30, 2010


Here?s some basic info on fertilizer and nutrients that plants require. The 3 numbers on a fertilizer container refer to the percentage of N (nitrogen), P (phosphorus), and K (potassium) in the bag. There are different formulations for different purposes. In general terms, nitrogen produces lush green growth, phosphorus helps strengthen stems and produce flowers, and potassium keeps the root system healthy. If you're applying fertilizer to fruiting (e.g., tomatoes) or flowering plants, you're not as interested in the plant developing leaves as you are in it flowers and fruit, so you'd use a formulation lower in nitrogen and higher in phosphorus, such as Miracle-Gro's Plant Food at 15-30-15.

The other big difference among fertilizers is whether they are organic or inorganic. There are positives and negatives to both. Organic fertilizers are made from the remains of once living things, therefore they contain carbon. Inorganic fertilizers are man-made from mineral salts, and contain no carbon. (The definition of organic varies widely, but it usually contains carbon.) Miracle Gro is considered inorganic. At the smallest level that plant roots are taking up nutrients, there is no difference to the plant. However, organic fertilizers (such as fish emulsion, alfalfa meal, blood meal, bone meal) provide long-term benefits to the soil, by giving soil microorganisms and earthworms food. Organic matter also improves the structure and workability of soil over time. Organic fertilizers are less likely to burn plants because they have low concentrations of nutrients (e.g., 5-3-1). Organics are slower working, but remain available to the plants longer. They are usually more expensive.

Inorganic fertilizers are usually less expensive, work faster, and have higher nutrient concentrations (e.g., 15-30-15). However, they leach through the soil faster, and can burn plant roots if improperly applied. Because they contain no organic matter, they don't improve the soil over time. Because they are made of mineral salts, they can cause salt build-up over time. These are a few of the basic differences between the two types.

Organic sources of nutrients:

Nitrogen: alfalfa meal, blood meal, cottonseed meal, fish emulsion, seabird guano.
Phosphorus: bone meal, rock phosphate
Potassium: greensand, seaweed, kelp

There are fertilizers formulated for specific plants, such as roses, and they usually have specific NPK concentrations as well as micronutrients that help that plant type. They may be more expensive than buying a balanced fertilizer, such as 10-10-10 that can be used more generally. I hope this info helps!

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