Fertilizing House Plants: A Primer

Fertilizing houseplants doesn't have to be hard: Just do it by the numbers. All plants require nitrogen, phosphorus and potassium to grow, also known by their chemical symbols: N-P-K, and all fertilizers list their chemical compositions in that order (the "K" stands for Kalium "potassium " in medieval Latin). Without these your plant is dead.

How much of each depends on what you are growing. Foliage plants and cacti need larger amounts of nitrogen. For the best flowering plants, try the fertilizers that go heavier on the phosphorus and the potassium. When applying these, less is more. Never follow the manufacture's directions or even worse increase them. The suggested application is based on ideal conditions rarely found in the home or garden and sick or ailing plants should not be fertilized at all. Ideally, you will want to use ¼ of the suggested amount and feed twice as frequently or follow the manufacturer's suggestions for frequency, but cut the amount in half.

This is because fertilizers are salts. Overdose on them and you could burn the plant alive. Furthermore, chemical fertilizer run-off from lawns and agricultural use is a major contaminant of water supplies and can led to a terrible disease known as "blue baby." In this condition caused by nitrogen contaminated water, children turn blue from lack of oxygen as the nitrogen interferes with the body's ability to transport oxygen. The excess nutrients also can cause algae blooms that ruin recreational waterways and fisheries.

To avoid these problems, many avid gardeners swear by composting. Maintaining compost heaps is a science in itself, but for the home gardener, the best way to reap the benefits of a compost heap with none of the hassle is to regularly add coffee grinds and egg shells to your potted plants. Coffee grinds are acidic and rich in Nitrogen while egg shells are alkaline and abound in Potassium and Phosphorus.

Organic elements such as these are unlikely to burn the plant and help chemical fertilizers work more effectively by providing trace elements and humus. Coffee grounds do retain water, however, so you will want to reduce watering when you use a lot of these. Don't use coffee grinds with cacti either, since these plants require drainage and a more alkaline environment. Finely powdered egg shells is better there.

One caution, if you are starting seedlings, use chemical fertilizers and sterilized soil-less mix only. Until the plants are well-established, they are susceptible to blights that can rot them at their root-lines overnight. The resultant plants will look as if they have been pinched in the middle of the stem and will fall over.

PH another essential number. This number gives the acidity or alkalinity of the soil. Lower numbers mean more acid, while higher numbers are alkaline. Neutral PH is 7. The PH will affect your plant's growth and even its appearance. Hydrangea bushes are known to be bluer in more acidic soils. As a rule of thumb, plants that grow in wet climates usually prefer lower PH numbers, while cacti and other succulents like it drier.

Rhododendrons, azaleas, blueberries, conifers and hydrangeas are examples of plants commonly fed with a special food that acidifies the soil. This acidity is necessary for iron absorption and to produce the bluest blooms in hydrangeas. For these plants, a commercial product that contains iron and acidifying chemicals such as Miracid from Miracle Grow is the way to go. You will also want to add lots of coffee grounds to indoor plants and use acidic mulches around your outdoor plants.

For plants that require more neutral conditions, liming may be necessary. Lime can be used to correct extremely acidic soils to a more neutral range, but is not necessary indoors. Egg shells can be composed and added to your outdoor plants as well as indoor plants and will decrease acidity to a small degree.

To sum up, success with feeding lies on these fundamental principals: Provide the correct mix of NPK, keep the soil at the appropriate PH for what you are growing, add compost to indoor and outdoor gardens to improve soil quality, and use less than the recommended amounts of fertilizers for plants that are not being grown commercially in climate-controlled greenhouses. Don't fertilize at all if the plant is growing slowly due to disease, infestation, poor lighting conditions, or improper watering.