Katherine Clarkson

Looking for a fertilizer can be particularly intimidating when you face an array of numbers such as 10-8-10 or 20-10-10. Choosing the correct fertilizer does not have to be complex if you learn what the numbers signify. I will explain what these numbers represent, why these three nutrients are essential, how N-P-K affects plants, and when to apply various proportions.

The three conspicuous numbers are plants’ primary macronutrients and are needed in enormous quantities. They are nitrogen (N), phosphorus (P), and potassium (K). Weight represents each of the numbers, because soil amendments are indicated in weight rather than a specific fertilizer.

For example, a 50-pound bag of 10-6-8 has 10% nitrogen, 6% phosphorus and 8% potassium. To determine the total pounds of nutrient in the bag, multiply the percent of that nutrient by the bag weight. The 50-pound bag of 10-6-8 will have 5 pounds of nitrogen, 3 pounds of phosphate, and 4 pounds of potassium.

Each of these nutrients has a unique task. Nitrogen is part of the chlorophyll cycle, which produces plants’ green color and aids in creating food through photosynthesis. Phosphorus plays a critical role in development and is a key component for storing energy. Potassium helps fight disease, produces seeds, fruit, and flowers. When plants are depleted of these nutrients poor plant health will occur. Nitrogen can be leached from heavy rains; phosphorus is often lost through erosion, and potassium deficiency results from a lack of soil oxygen.

When a plant does not have enough nitrogen plants will look yellowish and have very slow growth. Too much will result in an explosion of foliar growth, but at the expense of flower and fruit formation. Phosphorus focuses its energy on strong root development and helps the plant use other nutrients efficiently. The most common cause of too much phosphorus is from repeated use of manures or non-organic fertilizers. The deficiency often yields yellowing and withering plants. Potassium assists with the movement of water, nutrients and carbohydrates in a plant’s tissue. Too much potassium will affect the way the plant gets nutrients from the soil and too little will result in brown scorching and curling of the leaf tip. So how do you know what fertilizer to use for your plant’s specific needs?

The element percentages are offered in varying proportions to suit different plant needs. If you are looking to boost flower production, you want a mix like 15-30-15, which is high in flower-developing phosphorus. If you want to green up your lawn, choose a mix like 25-6-4, which is high in nitrogen. Many fertilizers are formulated for specific plants like roses, bulbs, or vegetables. Be sure to check the label for the N-P-K ratio, as you may be able to use a general fertilizer with close to the same nutrient percentages but at a lower price.

Knowing when to fertilize is as important as using the right fertilizer. Most perennials, annuals, vegetables, and lawns will reward you handsomely if fed with a balanced granular fertilizer in early spring.

Annuals like to be fed an additional three to four times during the growing season with a high-phosphorus fertilizer.

Trees and shrubs, especially those that flower, also like a dose of a balanced fertilizer in the spring and another in the fall. But remember late and light when fertilizing trees and shrubs in autumn.

Late fall is also a good time to fertilize bulbs, especially if you are planting them for the first time.

Roses have insatiable appetites. To keep them healthy and happy, feed them a fertilizer every seven days during their blooming season. It is also important to only feed well-established plants; fertilizing seeds or tiny seedlings will cause fertilizer burn.

You really don’t have to be a chemist to be an excellent gardener. However, awareness of how fertilizers impact your plants will improve your success rate. Remember to always follow package instructions for application rates and timing and, if in doubt, be conservative. If you have any questions or comments, please email me at katherineclarkson2@gmail.com.

(Katherine Clarkson is the president of the Park County Master Gardeners. She lives in Wapiti.)