Poinsettias are flowering plants with bright red bracts (flowers). Fertilizing poinsettias is a lot like wrapping a package; there are numerous ways to do the same thing that all get the job done. There are many different factors that affect fertilization, such as the amount of different nutrients in the fertilizer, the rate at which it is applied, the amount of leaching and the type of soil the poinsettia is growing in.
Common fertilizers use a three-number system to detail the percentages present of three different nutrients: nitrogen, phosphate and potassium. According to the University of Massachusetts at Amherst, a few common fertilizers for poinsettias are15-0-15, 15-16-17 and 20-10-20. Poinsettias should be fertilized on a regular basis up until about two weeks before they are to be sold. Reducing the fertilizer at this time allows the plant to live longer post-harvest and prevents soluble salts in the soil from injuring the bracts.
Calcium deficiency in poinsettias can lead to leaf-edge burn, weak stems and bract necrosis. A calcium deficiency can be overcome by using a high-calcium fertilizer mixed with a high-nitrate fertilizer or by adding calcium supplements to the existing fertilizer regimen. Calcium deficiency is not solely caused by a lack of calcium in the soil, but also by inadequate water flow to the roots.
According to the University of Massachusetts, a lack of phosphorus in the growing medium can result in small bracts and an overall reduction in the size of the plant. Phosphorous deficiency can be caused by prolonged use of a low phosphorous fertilizer or by growing in a medium that is low in phosphorous. This can be corrected by adding phosphorous supplements or a high-phosphorous fertilizer to the medium. On the other hand, excess phosphorous can lead to excessive growth, which will weaken the plant overall. Excess phosphorous can also lead to bract necrosis and can pollute groundwater.
A deficiency in molybdenum can lead to necrosis and interveinal chlorosis on recently mature leaves and middle-aged leaves. To prevent molybdenum deficiency, maintain the proper pH level in the growing medium--it should be between 5.8 and 6.2. Using a liquid application of 1ppm molybdenum can greatly assist in preventing molybdenum deficiency, according to the University of Massachusetts.
Poinsettias can be fertilized with compost, whether it is homemade or purchased. Natural fertilizer can boost all-around nutrient levels in the soil and help prevent most deficiencies in poinsettias. The downside to using homemade compost is the relative inability to increase a single nutrient level, such as a potassium deficiency.