Which fertilizer and how much?

Published 6:47 pm Thursday, June 27, 2019

This was a great question! As usual, there are so many more questions that we have to ask before we can answer this simple question, though.

The beginning point here is the simple slogan, “Don’t guess, soil test!” There is no replacement for knowing what nutrients are in your soil. There are two pieces of knowledge a soil test will give us that are priceless in the horticultural world. The first is a measure of what nutrients and quantities in which they are present in our soil. The second, and albeit most important piece of knowledge a soil test will give you, is the pH of the soil.

The main idea behind fertilizing our plants is to supplement what is already in our soil not to provide 100% of our plants’ needs. In order to accomplish this, we need to know what is in soil, what is available to the plant and what the plant needs for optimum growth. When we take and send a soil sample to the North Carolina Department of Agriculture and Consumer Services they analyze the sample and prepare a report that gives us this information as well as recommendation.

There are also a few general “rule of thumb” guidelines that we can use in lieu of a soil test. However, this is a lot like gambling, in my book! I like ice cream, especially from the local service station down the road from my house. Nothing is better, after I mow the grass on a hot day, than a big cup of Moo Moo Hugs. However, if I slip down to the station for that ice cream too often, my waistline may begin to suffer the consequences! The same can be said for our plants. … No, they don’t like ice cream, they do in fact like nitrogen though. However, if we provide too much nitrogen to our plants, they put all of their energy into growing rather than producing flowers, fruit or vegetables. There is a point at which too much nitrogen, or any macronutrient for that matter, can actually become toxic and kill the plants.

We need to know the analysis of our fertilizer and what material it is derived from. The analysis is a measure of how much of a nutrient is in the fertilizer. Usually this is expressed in a number, such as 10-10-10. This is a measure of the big three macronutrients, in order, nitrogen (N), phosphorus (P) and potassium (K). These numbers are actually the percentage of that nutrient contained within the bag or container we are purchasing. If I purchase a 10-pound bag of 15-0-15 for example, 15% of that bag is N, 0% is P and 15% is K. This means that I have 1.5 pounds each of nitrogen and potassium but no phosphorus. In the instance of an organic product such as Fish Emulsion, it is sold as a concentrate. The analysis on the container is the analysis for the concentrate, not the dilute mixture. We want to always look at the label to see what the nutrients are derived from, as well. It is important to know whether or not that nutrient will have an acidifying or alkaline effect on the soil. If we have a low pH for instance, we do not want to add ammonium sulfate fertilizer because it has a rapid acidifying effect on the soil. We would want to add calcium nitrate instead to help change the pH of the soil closer to the proper range.

Now to address the second piece of knowledge you can gain from an NCDA soil report: your soil’s pH. The pH of the soil determines how available many nutrients are to the plants we are trying to grow. The pH is a measure of how basic or acidic your soil is. Most plants like slightly acidic soil to acidic soil for optimum growth. This is because, chemically speaking, many nutrients that a plant requires for optimum growth are only available within a narrow band or range of pH. If the pH is too high (basic) or too low (acidic) there are certain nutrients that are chemically bound to soil particles and therefore unavailable to the plant. So, if the pH is off, it may not matter how much or how little fertilizer you add! To make the pH rise, and therefore be more basic, we would add lime to the soil. To make the pH more acidic, elemental sulfur would be used to lower the pH to a range in which the plant will grow best.

It is so important to know what is in your soil and the only way is to get a soil test! Soil test kits are available at your local extension center, along with a brochure that outlines how to take a good sample. Results are tailored to your plants and emailed to you with recommendations. Often, this is the first step in the diagnostic process to determine what is wrong with a garden plant, house plant, ornamentals or even our shade trees. So, don’t guess, soil test!!

If you have a question to submit, please email Gene Fox at gene_fox@ncsu.edu. If you’re having trouble with growing in your home landscape, call the Extension office at 252-946-0111 to speak to an Extension Master Gardener volunteer, Mondays and Wednesdays between 10 a.m. and noon. Learn more on Facebook at the Blacklands Area Horticulture page or visit the Extension Office located at 155 Airport Road in Washington.

Gene Fox is the area horticulture agent for the N.C. State Cooperative Extension.