October 2012 Newsletter (vol.11) >> Avoid Poinsettia Nutrition Problems
The Starting Point
The starting point for all fertility programs is appropriate pH and decent water quality. If you have major problems with either, it won’t matter what fertility programs you establish, nutritional problems are bound to follow. In an ideal world, your root medium pH will be in the range of 5.5-6.8. At lower pH levels, calcium, magnesium, and molybdenum may become deficient. At higher pH levels, some micronutrients may become deficient. It is easiest to maintain a root medium pH in this range if the pH of your water source falls between 5.5-7.0. If the alkalinity of your water (it’s capacity to neutralize acid) is higher than 2.0 meq (100ppm calcium carbonate equivalent), you may need to look towards downward adjustments in your lime use or try acid-injection.
You should know your water quality at all times. For poinsettia production, check your water quality report and make sure that lithium is <2ppm and that fluoride is <5ppm. These elements may be associated with bract edge burn in the crop. High levels of Boron (B) (e.g. >0.5ppm) have been linked to marginal necrosis of leaves.
Typical Fertilization Programs
We typically think of poinsettias as a crop which requires high levels of fertility. A fertilizer, which provides N-P-K at 200-300ppm nitrogen (N) and potassium (K) at each irrigation, is a common starting point for a poinsettia fertilization program for crops, which are NOT sub irrigated. If you sub irrigate a starting point would be concentrations half as high.
To help prevent ammonium toxicity problems, no more than 40% of your total N should be from an ammonium or urea source (check your fertilizer labels). This means that you would avoid a fertilizer like 20-20-20 which provides about 60% N as ammonium-N. You may want to apply even less ammonium-N later in the season when temperatures fall and the nitrifying bacteria in your root medium which convert ammonium-N to nitrate-N, are less active.
The Dark-leafed Cultivars
Constant liquid fertilization (CLF) with no higher than 200 ppm nitrogen (N) and potassium (K), along with phosphorus (P) is recommended for the dark leafed cultivars. The typical high concentrations (e.g. 300 ppm N and higher) may result in reduced crop quality by contribution to the development of brittle stems and increased root loss which, of course, makes the crop more susceptible to root rot disease pathogens. If you grow both light and dark-leafed cultivars, a different fertility program is recommended with dark-leafed cultivars receiving two thirds of the fertilizer rate applied to the light-leafed cultivars. One approach to accomplish this strategy would be to use the same fertilizer concentration for all cultivars, but every third watering apply only clear water to the dark-leafed cultivars.
Calcium and Magnesium
Both of these nutrients can cause nutritional problems during poinsettia production. Apply adequate calcium (Ca) in your fertilization program that will maintain a 2:1 Ca: Magnesium (Mg) ratio. One recommendation is to apply 80 ppm CA and 40 ppm Mg at each watering. Later in the season, you may want to apply weekly sprays of 400 ppm Ca from calcium chloride to help avoid bract necrosis. Magnesium can be supplied as Epsom sales (1-3 lbs. Epsom sales/100 gal of water) every few weeks during production if Mg is not present in your everyday fertilization program. This brings us back to the water quality issue. “Hardness” is a measure of Ca + Mg. If you have “hard” water, then you have some Ca and/or Mg in you water source and these levels should be accounted for in your fertilization program, not ignored.
Poinsettias are rather unique in that they have a high requirement for the micronutrient molybdenum (Mo). To meet this need, you can apply 0.1 ppm Mo in your CLF program (dissolve 3 oz Sodium or ammonium molybdate in a gallon of water. Use 1.5 fl oz of this stock solution/100 gal of final strength fertilizer solution) or you may make periodic application of higher concentrations.
Another strategy to provide Mo would be to apply a foliar spray which contains molybdate (1-2 oz of either ammonium or sodium molybdate/100 gal of water + spreader-sticker). Two to three spray application of Mo could be made during production as preventative treatment.
Careful of forming precipitates
Remember that you cannot necessarily dump all of these fertilizers in the fertilizer proportioner stock tank. In concentrated solutions phosphate and sulfate will chemically react with CA, Mg, and some micronutrients to form insoluble compounds that will sink to the bottom of your stock tank. When this happens, these nutrients are not being supplied to your plant, either. Make separate applications of Ca and Mg fertilizers, apart from your N-P-K fertilizer or use a multiple head injector which would allow you to keep fertilizer that will precipitate separate from each other. You might try one of the Excel water-soluble fertilizers which can be combined with calcium nitrate and magnesium nitrate in one tank.