Published by permission: Scott Evans MAg Nebraska Extension Horticulturist
Scott Evans MAg
Have you ever noticed when you go to your favorite garden center that there are more choices of fertilizers to choose from than cereal options at the grocery store? How do you decide which one to buy and what do all those numbers mean?
Let's clear the air of a big misconception.
Fertilizer is not food. Plants make their own food and fertilizer helps them do it. When you look at the product label you will see three numbers, such as 9-3-6 or 10-10-10. These numbers represent the percentage of a specific nutrient. It is an industry standard that those numbers will always represent nitrogen, phosphorus, and potassium. They will always be in that order. Excluding oxygen, hydrogen, and carbon there are currently 15 or 16 essential elements that a plant needs. We are going to talk about the big three: N-P-K. What do they do?
A simplified way to remember what the big three do is: up, down, all around. Nitrogen helps with green vertical growth. Phosphorus helps with roots (and flowers). Potassium helps with overall plant health. Generally speaking, these are the three elements that the plant needs the most to be happy and healthy. Nitrogen is the most needed nutrient for plants. Nitrogen promotes green growth and helps with other biological processes. When a plant is lacking nitrogen symptoms of deficiencies will show up in older leaves first. We will also see poor new growth that will be lighter in color. Most species and older cultivars of violets naturally have a light green color to their leaves. Knowing what your plant should look like will help determine if the plant is lacking nitrogen. Too much nitrogen can promote rapid green growth, brittle leaves, and poor flower formation. When you look at the product label you will notice that nitrogen could have two or three different types. Most often we see ammoniacal, nitrate, and urea nitrogen listed. Typically, urea is not plant ready until converted to ammonium by bacteria. That bacteria may or may not be present in a potting mix. Using only one formulation of nitrogen can also affect the soil pH (acidity or alkalinity). It is best to use a product that has at least two formulations of nitrogen such as ammoniacal and nitrate. This will help reduce the chances of the soil pH swings.
Phosphorus is the second most needed nutrient. Phosphorus helps with root development, encourages flowers, and other biological functions. When a plant is deficient in phosphorus, we often see leaf tips on older leaves browning following by turning dark green with a purplish hue on the backside of the leaf or the petiole. This can be challenging to diagnose because many of our violet cultivars have a red back. When a plant has too much phosphorus, we see it exhibited through micronutrient deficiencies. Most fertilizers use the phosphate formulation of P2O5.
Potassium is the third most needed nutrient for plants. Potassium helps with the overall plant health. It will aid in the movement of water, nutrient flow, carbohydrates, and other biological processes. When a plant is lacking potassium we will see weak stems, yellowing of older leaf tissue (chlorosis) followed by localized death of tissue. It is rare to see potassium toxicity in the plant. However, if it does occur we will see micronutrient such as zinc, magnesium, and manganese become deficient. Most products will use the soluble potash formulation of K2O.
Soil pH and temperature of the growing space will also affect the availability and uptake of nutrients by the plants. Traditionally most plants, including violets, prefer a soil pH around 5.8 to 6.2 (Optimara African Violet Growers recommendation). This will allow all of the macro and micro nutrients to be available for uptake. Both warm and cool growing conditions can affect the availability of nutrients. Ideally it is best to keep the temperature between 20°C to 27°C (68°F to 82°F). Remember that grow lights will produce some heat and keeping a thermometer on your stands will allow you to monitor. New LED lights produce less heat so you may need to adjust your growing space accordingly.
Which fertilizer ratio do you choose from? Garden centers are flooded with endless options. An industry standard is the 3-1-2 or a 1-1-1 ratio. This provides the plants with the essential nutrients that a plant needs to be happy and healthy. We often forget that most fertilizers deliver the nutrients in a form of a salt. These salts can build up when using larger percentage numbers. Some newer natural based products bypass the salts. However, they use manures, meal-based ingredients and oils. Those come with a pronounced aroma which may or may not be desirable in a home setting.
Please remember that organic does not mean it is safer and everything is a chemical. When using any fertilizer remember that more is not better and always follow the labeled instructions.
More growers have switched over from tap water to reverse osmosis (RO) water. If this is your case you may want to choose a fertilizer that contains a portion of our micronutrients. The RO process does an excellent job removing impurities from the water and in some cases it will remove micronutrients. Look for a product that will also contain sulfur, iron, manganese, and zinc.
When deciding on what to use try to buy the smallest size available. This way if your plants react poorly to the product, you are not left with a Litres or Kilos of something you may never use again. If you decide to switch products try it first on a few plants for three months before introducing to the whole collection. This way you can monitor for any issues. Finally, if it isn't broke, don't fix it. If you found a product that you get great results with, there is no need to change.