Fertilizing Houseplants
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Fertilizing Houseplants Why Fertilize? it is broken down by soil microbes or bacteria, Outdoor plants can get nutrients from the soil, but neither of which may be present in your planting houseplants depend on us to supply these nutrients. medium. A build up of excess urea can cause root Potting soil often contains fertilizer, but eventually burn. those nutrients get used up and must be replaced. Micronutrients When to Fertilize? Indoor plants also need small amounts of the Fertilizer should be applied only during seasons of micronutrients. Some houseplant fertilizers provide active plant growth. In New Hampshire, this is micronutrients, but many do not. generally late March through October. Potting soils with a mineral component may supply Fertilizer is not a cure-all. If a plant is suffering, it’s sufficient amounts of micronutrients. Ground more likely due to an environmental factor, such as limestone is often added to potting soil to supply too little light or overwatering, than a nutrient calcium and magnesium, for example. deficiency. Fast- or Slow-Release? Fertilizer is Not Plant Food Fertilizer comes in various forms (liquid, powdered, Despite the product labels, fertilizer is not “plant granular, and so on) and is either fast- or slow- food.” Plants make their own food through release. photosynthesis, but they require at least 16 essential elements to carry out this process. Pros and Cons of Fast- and Slow-Release Fertilizers Pros Cons Essential Elements (see Appendix for descriptions) Fast-release • Usually less • Must be applied often Macronutrients Micronutrients fertilizer expensive for plant to get • Liquids • Application rate continuous supply of From Air From Soil From Soil under your control nutrients and Water: or Fertilizer: or Fertilizer: • Wettable powders • Higher burn potential 1. Carbon 4. Nitrogen 10. Iron Slow-release • Can be applied • Usually more 2. Hydrogen 5. Phosphorus 11. Copper fertilizer infrequently, for expensive 3. Oxygen 6. Potassium 12. Manganese • Granules example, every • Application rate not 4 months 7. Calcium 13. Molybdenum (prills) under your control • Lower burn 8. Magnesium 14. Zinc • Spikes potential 9. Sulfur 15. Boron • Tablets 16. Chlorine Organic or Not? Macronutrients Presuming you won’t be eating your houseplants, Some houseplants have specific nutrient needs, but and that what you feed them won’t make its way most will do fine with an all-purpose, balanced into the environment, using organic fertilizer for fertilizer — one with roughly the same percentage of indoor plants may not be a priority. (N) Nitrogen, (K) Phosphorous, and (P) Potassium. Flowering plants respond best to fertilizers higher in Organic options include fish and seaweed fertilizers. phosphorous. Indoors, you will want something that does not smell or attract pests. By their nature, organic Be aware that nitrogen in fertilizer is derived from fertilizers generally contain some or all of the needed different sources. Urea nitrogen is a common form, micronutrients. Another benefit of organic fertilizers for the reason that it is relatively inexpensive to is that they do not contain soluble salts. Problems produce. But this form of nitrogen can pose a related to soluble salts are covered later in this problem for houseplants, because it is unusable until handout. Amherst Garden Club — Horticulture Hint — For Club Use Only: Do Not Disseminate 1 of 4 Fertilizing Houseplants Which Brand? Certain brands dominate the market. The following chart compares the nutrient list for some well-known brands that are widely available, as well as a few lesser-known brands that may be more complete but are harder to find. Osmocote (by Scotts) Miracle Gro (by Scotts) Jack’s Classic Neptune’s Harvest Slow release Slow release Slow release Fast release Fast release Fast release Nitrogen* 19% Nitrogen* 15% Nitrogen* 6% Nitrogen* 8% Nitrogen* 15% Nitrogen* 2% Phosphate 6% Phosphate 8% Phosphate 12% Phosphate 7% Phosphate 30% Phosphate 4% Potash 12% Potash 11% Potash 6% Potash 6% Potash 15% Potash 1% Calcium — Calcium — Calcium — Calcium — Calcium — Calcium 0.75% Magnesium — Magnesium 1.45% Magnesium 0.5% Magnesium — Magnesium — Magnesium 0.04% Sulphur — Sulphur 2.4% Sulfur 2% Sulphur — Sulphur — Sulfur 0.17% Iron — Iron 0.175% Iron 0.15% Iron 0.1% Iron 0.10% Iron 0.0026% Copper — Copper — Copper 0.05% Copper — Copper 0.05% Copper <0.001% Manganese — Manganese 0.05% Manganese 0.05% Manganese — Manganese 0.05% Manganese <0.001% Molybdenum — Molybdenum 0.014% Molybdenum — Molybdenum — Molybdenum <0.001% Molybdenum <0.001% Zinc — Zinc — Zinc 0.05% Zinc — Zinc 0.05% Zinc <0.001% Boron — Boron 0.02% Boron 0.02% Boron — Boron 0.02% Boron <0.001% Chlorine — Chlorine — Chlorine — Chlorine — Chlorine — Chlorine — * No urea nitrogen * No urea nitrogen * 0.75% urea nitrogen * 5.6% urea nitrogen *10.63% urea nitrogen * No urea nitrogen Schultz Bonide Jobe’s Dyna-Gro Fast release Fast release Slow release Slow release Fast release Nitrogen* 10% Nitrogen* 10% Nitrogen* 13% Nitrogen .007% Nitrogen* 7% Cobalt 0.0015% Phosphate 15% Phosphate 10% Phosphate 4% Phosphate .015% Phosphate 9% Sodium 0.1% Potash 10% Potash 10% Potash 5% Potash .010% Potash 5% Nickel 0.001% Calcium — Calcium — Calcium — Calcium — Calcium 2% Magnesium — Magnesium — Magnesium — Magnesium — Magnesium 0.5% Sulphur — Sulphur — Sulphur — Sulphur — Sulphur 0.05% Iron 0.10% Iron — Iron — Iron — Iron 0.1% Copper — Copper — Copper — Copper — Copper 0.05% Manganese 0.05% Manganese — Manganese — Manganese — Manganese 0.05% Molybdenum — Molybdenum — Molybdenum — Molybdenum — Molybdenum <0.001% Zinc 0.05% Zinc — Zinc — Zinc — Zinc 0.05% Boron — Boron — Boron — Boron — Boron 0.02% Chlorine — Chlorine — Chlorine — Chlorine — Chlorine 0.1% * 8.2% urea nitrogen * 7% urea nitrogen * No urea nitrogen * No urea nitrogen * No urea nitrogen Amherst Garden Club — Horticulture Hint — For Club Use Only: Do Not Disseminate 2 of 4 Fertilizing Houseplants How Slow-Release Granules Work How much salt is making its way into your pots and Osmocote indoor plant food is as an example of a whether or not the level is harmful is difficult to granular, slow-release fertilizer. Each granule holds calculate. If your plants don’t show signs of soluble powdered fertilizer contained by a porous resin salt injury, then it may not be a concern. coating. Signs of Soluble Salt Build Up When water comes into contact with the granule, the Signs of soluble salt build-up include: coating swells and the pores enlarge. Water mixes • Reduced growth with the powdered fertilizer, and small amounts of dissolved fertilizer are released back through the • Brown leaf tips pores. • Dropping of lower leaves • Yellowish-whitish crust on top of the soil Preventing Soluble Salt Injury The best way to prevent soluble salt injury is to stop the salts from building up. Once all the powdered fertilizer has been used up, 1. Water Correctly the resin coating remains behind. Water the pot until water drains from the bottom to flush out the excess salts. About Soluble Salts Synthetic fertilizers are salts, much like familiar table 2. Empty Excess Water in the Saucer salt, except that they also contain plant nutrients. As If you leave the drained water in the saucer, the salts a by-product of fertilizing, soluble salts can you just flushed will get reabsorbed by the plant. accumulate. In high enough concentrations, these salts can injure and even kill plants. 3. Leach Every 4 to 6 Months Leaching involves pouring a lot of water through the Outdoors these salts get flushed away naturally, but plant and letting it drain out. The amount of water in a pot there is nowhere for them to go. used for leaching should equal twice the volume of the pot. Periodic leaching more thoroughly flushes How Fertilizer Salts Work residual salts. The salt in fertilizer draws water toward it. The water then mixes with the fertilizer and disperses the Leach a plant before you fertilize, so that you don’t nutrients throughout the soil. wash away the fertilizer. If a crust of salts has formed on top of the soil, remove the crust before you begin If tender plant roots are close to where the fertilizer to leach. is placed, water will be drawn from these roots. As more fertilizer is applied, more water is drawn away. 4. Consider Using Organic Fertilizer If soil moisture is limited, plant roots can burn or Organic fertilizers do not produce soluble salts. dehydrate to a point where they cannot recover. 5. Consider Using Distilled Water Other Sources of Salts If your well water contains significant concentrations Salts may also be present in your potting medium of salt or other undesirable elements, such as and well water. And if your home has a water Fluroide, you might consider watering your plants softener, this may be another source of salt. with distilled water. Amherst Garden Club — Horticulture Hint — For Club Use Only: Do Not Disseminate 3 of 4 Fertilizing Houseplants Sources division. Required for uptake of nitrogen readily lost from soil but may be and other minerals. Leached from soil by unavailable. Deficiency: die back of • UNHCE Master Gardener Handbook watering. Immobile-requires a constant shoot tips; terminal leaves develop • UConn “Fertilizing Houseplants” supply for growth. Deficiency: stunting brown spots. Excess is toxic. • NC State University “Fertilizing of new growth in stems, flowers, roots; Houseplants” black spots on leaves and fruit; yellow (Fe)-Iron: Enzyme functions, catalyst for • Clemson University “Indoor Plants” leaf margins. synthesis of chlorophyll, essential for • Oklahoma Cooperative Extension new growth. Deficiency: pale leaves, “Houseplant Care” (Mg)-Magnesium: Critical component of yellowing of leaves and veins. Leached • NDSU Extension Service chlorophyll, needed for functioning of by water and held in lower parts of soil. “Houseplants” enzymes for carbohydrates, sugars and High pH soils may have iron present but fats, fruit and nut formation, germination unavailable to plants.