I’m always thankful when I’m out and about at community events, because I am inevitably told that folks recognize me from the paper and enjoy reading my articles. My response nearly every time is to ask what topics they’d like me to write on. This week, someone suggested I write about “fertilizer math”.
Plants are living things that require essential nutrients in order to grow. Nutrients are provided through air and water (carbon, hydrogen, oxygen) and through the soil the plant is grown in. Fertilizer is any substance added to soil that increases the amount of nutrients it contains. Most fertilizers contain a variety of nutrients, but the three macro (major) nutrients are nitrogen (N), phosphorus (P), and potassium (K). As a very simple explanation, nitrogen causes vegetative growth, phosphorus is necessary for young plant and seedling growth, and potassium promotes reproduction (flowering and fruit). We refer to these nutrients as “NPK” when we talk about fertilizers.
In a bag of fertilizer, these nutrients exist in their chemical forms: nitrogen (N), phosphate (P2O5), and potash (K2O). The amount of each nutrient in the fertilizer formula is expressed as a percent, and is required by law to be accurate. If this sounds confusing, it’s really not – a 10-10-10 fertilizer has 10% nitrogen, 10% phosphate, and 10% potash. If that bag is 50lbs, you can multiply that 10% by 50 to get 5lbs of each nutrient in the bag, or a total of 15lbs NPK of nutrients. The remaining 35lbs is usually material designed to improve soil health or texture, or just filler.
Okay, so let’s talk about a few other things. First off, we should always apply fertilizer based on a current soil test. A soil test will tell you exactly what nutrients you need to add for optimal plant growth. Just so you know, nitrogen is not included in soil tests, because there are a lot of factors that influence the amount of nitrogen in the soil at any times. Recommendations for nitrogen are made based on current research. In this example, let’s say we soil tested our vegetable garden and our pH is within the recommended range (6.0-6.5), but we are low on NPK. Our soil test says we need to 30lbs of 10-10-10 fertilizer per 1,000 square feet of garden. This recommendation means we need to add 3lbs of N, P, and K to our 1,000 sq ft (30lbs x 10% N; 10% P, 10% K).
What happens if we can only purchase a 5-5-5 fertilizer? If we do the same rate – 30lbs of 5-5-5 fertilizer – we only add 1.5lbs of N, P, and K to our garden. We would need to provide twice as much as the recommendation – 60lbs of 5-5-5 to the same area to provide sufficient nutrition. Similarly, if we want to use a 15-15-15 fertilizer, we only need to use 20lbs of fertilizer instead of 30. UGA has a neat fertilizer calculator online (https://aesl.ces.uga.edu/soil/fertcalc/) that may be helpful for some of this math – or you can always just call your county agent.
What if your soil test says you need 12-4-8, but you can’t purchase that mixture? First off, you could use a 3-1-2 at twice the recommended rate (like we discussed above). The more challenging situation is when you can’t find a fertilizer that quite matches what your soil test tells you that you need. In that case, I usually recommend to go with a fertilizer that matches your nitrogen need (the first number), and has a similar phosphate (the second number) to nitrogen. For example, a 15-10-5 would be a better substitute than a 9-23-30. Fertilizer recommendations are guidelines, not absolute quantities, so use the closest fertilizer grade you can find.
I didn’t touch on organic vs inorganic this week- all of the above information pertains to inorganic fertilizers, but that’s an article for another time. If you need assistance with deciphering fertilizer math, please feel free to contact me at 706-359-3233 or uge3181@uga.edu.
