What Do Fertilizer Numbers Mean?
Commercial fertilizer products are required to have a guaranteed analysis of their contents on the label. Part of that is the three numbers on the package, often separated by dashes. Those numbers indicate the amount of nitrogen (N), phosphorus (P), and potassium (K) as a percentage by weight in the bag or bottle.
Fertilizers contain varying amounts of each of these nutrients. A product with an NPK ratio of 12-6-3 contains 12% nitrogen, 6% phosphorus, and 3% potassium. The rest, in this case about 79% of the bag, is inert ingredients and, sometimes, trace amounts of other nutrients such as calcium, boron, or iron.
The Role of N-P-K
Of the 17 chemical elements needed for plant growth, N, P, and K are considered macronutrients. They're so named because plants use a lot of them, relatively speaking. If the soil is depleted of an element, plant growth and health are negatively affected. Plants absorb the three macronutrients via their roots (or water in the case of hydroponics).
Plants require nitrogen, phosphorus, and potassium to be able to make their own food. They combine carbon, hydrogen, and oxygen and use energy from the sun to convert those elements into sugars. Each macronutrient plays a different role.
Nitrogen is a key building block for growing foliage and stems. It is also a component of the chlorophyll molecule and is used in cell walls.
Phosphorus is needed for developing flowers, fruits, and root systems.
Potassium is responsible for root growth and supports fruit development.
Organic vs. Conventional Fertilizer
When shopping for fertilizer, you might notice that the numbers of organic fertilizers are often lower than those of synthetic fertilizers. For example, an organic all-purpose fertilizer bag may say 5-2-2, whereas a synthetic fertilizer may say 24-8-16. By nature, organic fertilizers are less concentrated than their conventional counterparts.
Organic fertilizers are based on natural sources such as blood meal, bone meal, alfalfa pellets, and composted manure. These fertilizers do more than add nutrients. They also add organic matter and structure to the soil, which helps with drainage and improves the nutrient-holding capacity of the soil. The release of nutrients to the soil is slower than with synthetic fertilizers. Another benefit of organic fertilizers is that they support microbial life in the soil, which will improve your garden's overall health and vigor.
How to Use Fertilizer Numbers
One way to use fertilizer numbers is to compare product value and price. For example, if product A is a 10-pound bag of 5-5-5 fertilizer and product B is a five-pound bag of 10-10-10 fertilizer, both bags contain the same amount of all three macronutrients. Product A has 5% of ten pounds (half a pound), and product B has 10% of five pounds (half a pound). Although both products offer the same amount of plant nutrients, organic fertilizers usually have a higher price tag than conventional non-organic fertilizers.
The other way to use fertilizer numbers is to find a fertilizer that meets specific plant needs or corrects a specific nutrient deficiency in the soil. For example, you might choose a product with a high first number (nitrogen) and lower second (P) and third (K) numbers if your soil test indicated a lack of nitrogen, but other nutrient levels were fine.
An NPK ratio of 10-5-5 indicates that the product contains twice as much nitrogen as phosphorus or potassium and is a good choice for encouraging green, leafy growth. Fertilizers with a lower nitrogen number and higher phosphorus numbers are used for young transplants, as phosphorus supports root growth and fruiting.
Frequently Asked Questions
What is the best fertilizer for a vegetable garden?
Vegetable gardens generally do well with a balanced fertilizer. Whether you use a synthetic or organic product, make sure it is slow-release (usually in granular form) so nutrients are added to the soil over a longer timespan and become available as the plants grow.
Do I need to fertilize my vegetable garden every year?
It depends on how intensely you are growing crops and the type of crops. If you fill every square foot of your garden with heavy feeders such as tomatoes, corn, or potatoes, you will need to fertilize it every year. But if you plant it only lightly and regularly add plenty of organic matter such as compost, you might not need fertilizer every year. Only a soil test, preferably done in a lab (contact your local Cooperative Extension for a test kit) will provide reliable details on exactly how much fertilizer and what type is needed.
