December 2002

Grower 101: Top 10 Misconceptions of Plant Nutrient Management By Kimberly Willliams

Plant nutrient management requires a thorough understandingof the relationships between irrigation practices, fertilization programs androot medium selection. Everything is connected. Even a minor change in any ofthese cultural practices impacts plant nutrition; recognizing and manipulatingthese relationships is the key to successful plant nutrient management.Following are some top misconceptions, or inaccurate assumptions, of plantnutrient management that can lead to problems during production.

“What worked last year will work the same thisyear.” You’ve hired new members of your watering crew. Youpurchased a different root medium that was more affordable. You are now addingwater into your mix before sending it through the flat-filler. It was a dry spring,and the level of your well is low. And I won’t even mention that theweather, and thus the production environment, is never the same from year toyear. All of these changes, and hundreds of others, will impact plant nutritionand require that attention be paid to nutrient management. The work of a groweris never done!

“All I need to worry about in my fertilizer program isN-P-K.” Actually, there are no fewer than 14 essential nutrients (17 ifwe count carbon, hydrogen and oxygen, which plants accumulate from gases in theatmosphere and water). The first step of nutrient management is to know wherethe plant is getting each of these 14 nutrients. For example, sulfur isvariably available in water sources across the United States. If it’s notin your water, fertilizer or root medium, it is important to plan for itssupply to your crop. How do you know if it’s in your fertilizerformulation? Just read the bag.

Certain crops are subject to more unique deficiencies, suchas poinsettias’ unusual requirement for molybdenum. So it is important torecognize the specific needs of a crop when developing its fertility program.

The flipside of the coin is the inadvertent over-applicationof one or more of the essential nutrients because you are not aware of all ofthe sources supplying the nutrient during production. For example, boron loadsare high in some water sources, especially in the Southwestern United Statesand Chicagoland waterways. If a grower is not aware that boron is beingprovided in his water source and uses a fertilizer formulation containing boronas well as a manufactured root medium amended with micronutrients pre-plant,boron toxicity can occur.

“The pH of my water determines the pH of mymix.” The pH of the root medium is so important because it controls theavailability of nutrients for uptake by the plant by controlling whether or notthey are soluble, or dissolved in water in the root medium. But it’s notthe pH of the water source that dramatically impacts the pH of the mix; it’sthe alkalinity of the water source. An unbuffered water pH will quickly changeto the pH of the mix; however, high alkalinity must be neutralized, otherwisethe pH of the mix will inevitably increase during production.

“All forms of nitrogen are equal.” There are twoforms of nitrogen that the plant absorbs — one is a cation (ammonium, orNH4+; urea, or CO(NH2)2, can be considered to behave like ammonium in the rootmedium) and the other is an anion (nitrate, or NO3-). Because this is the onlyessential nutrient that the plant can absorb as either a cation or an anion andbecause the plant absorbs more nitrogen than any other fertilizer nutrient (80percent of the total anions and cations taken up by the plant are nitrogen),the form of nitrogen that is supplied as fertilizer is an important contributorto “pH drift,” or the slow change in root medium pH over time.

Ammonium stimulates large leaves and long internodes,whereas nitrate stimulates the compact growth of smaller leaves and shorterinternodes. Nitrogen applied as fertilizer can be a valuable growth managementtool, though it is not as important as the concentration of nitrogen applied incontrolling growth. Another consideration when determining the nitrogen form toapply is that high percentages of ammonium may contribute to “ammoniumtoxicity,” especially during cool production seasons. Physiologicallyspeaking, this occurs because most plant species are unable to store theammonium-nitrogen form (it must be assimilated in the roots), but they canstore nitrate-nitrogen. Thus, when the supply of ammonium in the root mediumexceeds demand, plants are subject to potential ammonium toxicity.

“If 20-20-20 is best for my garden store customers,it’s fine for production.” There are two reasons why I do notadvocate the use of 20-20-20 for general production: It has too muchammonium-nitrogen (60 percent, and the maximum should be around 40 percent);and it has too much phosphorus. Both factors contribute to soft vegetativegrowth and the development of reproductive growth — great for the homegardener, but pushing lush growth is not generally an ideal productionstrategy.

“Calcium behaves like all of the other cations.”Physiologically speaking, calcium is special. As a component of cell walls (thecement between them, actually), it is difficult for the plant to move it aroundthrough those very cells. Calcium moves with some difficulty to the growingpoints of plants, where it is needed to lay down new cells, in water movedthrough the plant as driven by the transpiration stream. So you guessed it: Iftranspiration is minimized due to, say, cool temperatures, cloudy weatherand/or high humidity, calcium deficiency can become a problem.

“My irrigation practices do not impact cropnutrition.” Actually, irrigation practices are directly tied to theplant’s ability to absorb nutrients as Á well as the health of theroot system. There are three phases in a root medium: solid, liquid and gas. Insituations of over-watering, the gas phase is flooded with water and oxygendeficiency becomes a problem. In situations of under-watering,”connections” between soil particles and the roots are missing, somoving nutrients to the roots is not possible. The plant thrives somewhere inbetween.

Iron deficiency is most often encountered in high pH media,with iron solubility at least between a pH of 7.4-8.5. A disorder known as”lime-induced chlorosis” is aggravated under conditions of wet,poorly drained media; a chemical reaction resulting in the formation ofbicarbonate from lime is promoted by the accumulation of carbon dioxide thatoccurs in over-watered media.

“If the plant has yellow leaves, it’s anutritional problem.” Not so fast. Temperatures outside of the optimalrange for a species can contribute to physiological disorders. Good examplesare foliage “whiting” in ivy geraniums as the temperature exceedsoptimum and the cupping, stunting and heavy zonation that occurs in zonalgeraniums under cool-temperature production.

And then there are chlorotic leaves that develop during the occurrenceof diseases such as Pythium, Alternaria and Xanthomonas. Yellow foliage thatevolves during the progression of root rot pathogens is induced bydysfunctional roots, and thus decreases capacity for nutrient absorption,caused by the disease.

Chlorosis can also occur from over-applications of someplant growth regulators, including Cycocel and Florel.

“Nutritional disorders look distinctly different fromeach other.” Don’t we wish! Examples of three different nutrientproblems resulting in the same symptom of marginal necrosis of the foliage arepotassium deficiency, chlorine toxicity and boron toxicity, among other things.The only way to sort it all out is to run a tissue analysis to go hand-in-handwith the visual symptomology.

“Phosphorus fertilizer does not leach from soillessroot media.” This misconception is based upon the fact that phosphorusdoes not leach readily from most soils. Field soils consisting of clays,especially 1:1 types like kaolinite and/or available aluminum, will fix phosphatesand remove them from the solution. Peat- and bark-based soilless media containpredominantly organic components that lack the ability to retain thismacronutrient. Even the inorganic components of soilless media such asexpanded, horticultural-grade vermiculite and perlite, and 2:1 calcined clayslike arcillite, retain minimal phosphorus. So that triple super-phosphate thatyou mixed in pre-plant? It’s gone after a couple of weeks and youprobably cannot count on it to meet the phosphorous requirement for a croppingcycle longer than one month.

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This article was reprinted with permission from SoutheasternFloriculture.