Corner Iris Picture

Why Do We Use Fertilizers?

By Peggi Ishman
(Note: This information is for soil that has grown irises before and not virgin soil)

When you look at plants in your garden do you sometime think some are not thriving? Do you know what to give them for help? Haven?t we all found our plants in need of special care at some time? 3-10-3, 4-10-6, 10-10-10, 15-30-15. Have you wondered what those three numbers on a fertilizer bag mean? Here is help for you to decide what will work best to change your plants into thriving ones or to ensure your plants keep their health. Each fertilizer comes with information showing three numbers. These are nitrogen-phosphoruspotassium. So for example, one like Miracle Gro that shows 20-20-20 has equals parts of all three primary elements.

If your plants have yellow or light green leaves, or are stunted in their growth they need nitrogen. Nitrogen stimulates healthy grown of nice green leaves and good stems. This can be provided by adding in cottonseed meal, bone meal, manure, or fish emulsion. Manure can burn irises, so choosing another source for this element is wise. For example, if using bone meal it would be applied in 5 pounds per 100 square feet of garden area.

If the plant has red leaves or has cell retardation, or is lacking in utilizing other elements, it may need phosphorus. Phosphorus stimulates early formation and growth of roots. It also gives plants a rapid and vigorous growth. Sources for this are bone meal and rock phosphate. Super triple phosphate which is 0- 45-0 is a wonderful source applied in June after the bloom stalks are cut off and again in the fall. Always water this in well.

Potassium increases a plant?s vigor and helps make it disease resistant. Be careful because too much can cause the plants to have reduced absorption of magnesium or calcium. Potassium is the third number on a product label.

Other nutrients
Secondary elements like magnesium, sulfur, and calcium are included in many plant formulas. Before working with these elements it would be good to test the soil for the pH levels. So often nothing needs to be added.

Magnesium aids in photosynthesis. When we see chlorosis on old leaves, this can be a sign of lack of magnesium. Epsom salts can correct this. I have never needed to add magnesium to irises. Only to rose bushes.

Sulfur helps build proteins. A deficiency here would look like a lack of nitrogen. If the pH balance is too low sulfur burn can occur. Lack of calcium stops growing points in plants. Excess calcium can reduce the intake of potassium and magnesium. Sources for this are gypsum or oyster shells.

The final elements to consider are air and water. Carbon comes from air in carbon dioxide. It is the keystone of all organic substances. Oxygen for respiration comes from water and oxygen in the air. Hydrogen comes from water and is necessary for all plant functions.

Now if all this information seems more than enough, the things to know when it comes to our irises is they need sun, water, good amended soils, and fertilizer during the growing months when it is cool, and before they have bloomed. We all know that watering correctly, choosing a sunny location for the irises, and planting in well amended soils helps irises thrive.

Once irises bloom, we don=t need to fertilize again until October or November when we see new growth. Then we start using a 10-30-15 or a 20-20- 20 monthly. The exception being the application of 0-45-0 we use for giving them good root growth when we plant in the Fall, and just after they bloom in May or June.

If we want to use a boost of a 10-55-10 (sometimes called Bloombuster or Superbloom) as a spring wake-up call in mid-March, that will help bring on really nice-sized irises.