You want to make sure your plants are getting the nutrition they need to do you proud, but there are so many choices when it comes to selecting a fertilizer. How do you know what is really in the bag? There are certain rules that all fertilizer makers must follow when they label their products and understanding these rules can make comparing fertilizers much easier.
Here are the things you need to know about garden fertilizers and how they are labeled.
Most commercial fertilizers are prominently labeled with a three-part number on the front of the package. The three numbers will be separated by dashes and may look sometimes like "5-10-5", for example. This measurement refers to a percentage, by weight, of three major nutrients in the fertilizer product—nitrogen, phosphorus, and potassium, respectively. These are often abbreviated N-P-K, according to their elemental symbols from the periodic chart.
For example, if you purchase a 10-pound bag fertilizer labeled 5-10-5, it contains 5 percent nitrogen, 10 percent phosphorus, and 5 percent potassium by weight. The remaining 80 percent of the bag's weight is comprised of other minor nutrients or fillers.
Nitrogen: the First Number
The first number gives the percentage of nitrogen in the product. Nitrogen encourages foliage growth, among other benefits. A 5-10-5 fertilizer would contain 5 percent nitrogen by weight. So for every pound of fertilizer applied there is really only .05 pounds of nitrogen. In a 10-pound bag of 5-10-5 fertilizer, then, there is 0.5 pound of nitrogen. Fertilizers high in nitrogen are often used for grass or for other plants where green foliage growth is more important than flowering.
Phosphorous: the Second Number
The middle number refers to the percentage of phosphorous in the fertilizer product. Phosphorus contributes to many fundamental plant processes, such as rooting and setting flower buds. A 5-10-5 fertilizer contains 10 percent phosphorous by weight or .1 pounds of phosphorous per pound of product. A 10-pound bag, then, contains 1 lb. of phosphorus.
Potassium: the Third Number
The final number in the major ingredients listing gives the percentage of potassium in the product. Potassium contributes to the overall health and vigor of plants. Again, a 5-10-5 fertilizer contains 5 percent potassium by weight or .05 pounds of potassium per pound of product. In a 10-pound bag, there is .5 lb. of potassium.
Fertilizers that contain all three major nutrients are known as complete fertilizers. There are also specialized fertilizers which are called incomplete because they lack one or more major nutrients, such as a fertilizer labeled 0-20-20, which is lacking in nitrogen.
Another way to compare different complete fertilizers is by ratio rather than by weight. A fertilizer labeled 5-10-5 has a ratio of 1-2-1. This becomes important when you are looking for fertilizer to meet a specific need. A 1-2-1 ratio is often recommended for vegetables, which need plenty of phosphorous to set fruit. A 1-2-1 fertilizer could be labeled 5-10-5, 10-20-10, or any number with the same proportions.
In addition to the major nutrients that are usually noted on the front label, most fertilizers also include additional ingredients that are listed on a side or back label. This may include other nutrients like calcium, magnesium, iron, micronutrients, and even the percentage of organic matter and fillers. Although the minerals and micronutrients are less critical than the major nutrients, a good fertilizer product will include small amounts of other ingredients, as well.
Products labeled as organic fertilizers must specify which of the nutrients are organic, and they must be identified as either synthetic and/or natural, by percentage. For example, you might read "20 percent of nitrogen (6 percent synthetic, 14 percent organic)."
Strictly speaking, an "organic" material is anything that contains carbon atoms. However, in popular usage, we have come to expect that organic fertilizers, like organic food, are manufactured by natural processes and contains nothing synthetic. That tends to be the case with most commercial products, especially as consumers become more educated, but be sure to read the label before you make a purchase.
Tips for Using Fertilizers
- Having a soil test done before you start adding amendments will tell you what you actually need. If your soil pH is too high or too low, your plants will not be able to access some nutrients, even if they are present in the soil.
- There is no one-size-fits-all fertilizer. Fertilizer choice depends on the type of plant being grown and the soil it is being grown in.
- Avoid over-fertilizing your plants, especially lawns. Nutrients that aren't taken up by plants may run off into sewer systems and rivers, leading to serious pollution problems.
- Always follow the label instructions when using any garden product. Just because a little is good, it doesn't follow that a lot is better.
- You can get by with a smaller quantity of fertilizers that have high analysis numbers than those with lower numbers. In other words, five pounds of 10-20-10 would give you the same nutrient value as 10 pounds of 5-10-5.
- Organic fertilizers made from natural ingredients often have lower concentrations of the three major nutrients, so you will need to use larger amounts. However, they do contain many other nutrients that feed both the plant and the soil. If you are using synthetic fertilizer, you should supplement with some type of organic matter, such as compost or manure, to maintain soil health.
“A Guide to Understanding Fertilizers.” OSU Extension.
Quick Guide to Fertilizing Plants. UMN Extension.