Outdoor plants obtain the nutrients required for growth from the natibe soil they are planted in. Houseplants rely on us to supply these nutrients through the application of fertilizers. Underfertilized houseplants often exhibit symptoms of slow growth, weak stems, pale leaves and reduced flowering. Below are basic Guidelines for fertilizing housplants.

Types of Fertilizers

Houseplant fertilizers come in a number of formulations. Some are wettable powders or concentrated liquids that are diluted with water then applied to the potted plant. Time release fertilizers are available as coated pellets or as spikes. A few are sold premixed and applied directly to the potting soil.

The label on a fertilizer package states the guaranteed analysis or grade. These are the three numbers listed on the package, such as 15-30-15, or 20-20-20. They refer to the percentage of nitrogen (first number, N), available phosphorus(second number, P) and water soluble potassium (third number, K) contained in the fertilizer. Plants require large amounts of these three nutrients and they are often referred to as primary nutrients. Nitrogen promotes green, leafy growth. Phosphorus encourages flowering and root growth, and potassium is necessary for stem strength and stress tolerance. Secondary nutrients, or micronutrients, are required in slightly lesser quantities and include calcium, magnesium and sulfur. Ground limestone is sometimes added to potting mixes to supply calcium and magnesium.

The fertilizer label will also state from what sources these nutrients are derived; chemical and/or organic. If synthetic or chemical fertilizers contain trace elements, these too will be listed on the label. Trace elements or micronutrients are needed by plants in very small amounts and include iron, boron, zinc, copper, molybdenum, chlorine and manganese. It is usually safe to assume that organically derived fertilizers, because of their nature, contain some or all of the necessary micronutrients. Potting mixes that have a mineral soil component may also supply sufficient amounts of micronutrients. If you are not using an organic houseplant fertilizer, consider purchasing a chemical fertilizer that does include micronutrients since they are typically not present in soilless potting mixes.

Choosing a Fertilizer For Houseplants

Which is the best houseplant fertilizer? This will depend on the types of plants being grown, cultural conditions and your schedule. In general, foliage houseplants appreciate fertilizers high in nitrogen (first number) while flowering plants respond best to those with higher phosphorus (second number) analysis. There are plenty of specialty houseplant fertilizers out there but do examine their labels. Often the difference is more in the packaging than in the amounts or proportions of nutrients supplied.

Purchase a water-soluble powder or liquid concentrate if you will be fertilizing on a weekly, biweekly or monthly basis. If there will be long intervals between fertilizer applications, select time release formulations in either pelleted or spike forms. These can be applied at intervals from 2 to 9 months and will provide houseplants with a steady supply of nutrients.

Frequently houseplant lovers amass quite a collection of different plant species. Sometimes plants have specific fertility requirements but usually an all-purpose, balanced fertilizer could be applied to all plants. For example, a fertilizer where the three numbers on the package are equivalent or just about equal, such as a 20-20-20 or a 10-8-7, are usually sufficient.