Essential Plant Nutrients - February 27, 2008
Jeff Schalau, Associate Agent, Agriculture & Natural Resources, Arizona Cooperative Extension, Yavapai County
Plants need certain chemical compounds to grow and reproduce. Two of these compounds are water are carbon dioxide. Compounds are molecules composed of two or more elements. The compound water is composed of two units (atoms) of hydrogen and one unit (atom) of oxygen. There are 18 essential plant nutrients (elements). They are considered essential because they are needed for plant growth and reproduction.
The 18 nutrients can be divided up into mineral and non-mineral. Mineral nutrients enter plants primarily through the soil and non-mineral nutrients may enter plants through either the soil or atmosphere. Essential plant nutrients can also be divided into macro and micronutrients: those needed in relatively large quantities vs. those needed in relatively small quantities.
Non-mineral nutrients are in the compounds required (reactants) for photosynthesis. These are carbon (C), hydrogen (H), and oxygen (O). While added carbon dioxide could be considered a “fertilizer”, it is not practical for most gardeners to increase the amount available to plants. Water, supplied via irrigation or rainfall, provides the hydrogen (and electrons) required to convert solar energy (light) into chemical energy (sugars).
All other essential plants nutrients are mineral nutrients. These are dissolved in water and most commonly enter the plant when absorbed by plant roots. One or more of these nutrients are usually in short supply and will therefore limit plant growth. Macronutrients are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). The one or two letter symbols in parentheses are the universal chemical symbols for each nutrient.
Don't be afraid: chemistry is what makes up our world and allows life as we know it. N, P, and K are often referred to as primary nutrients and are the most common elements found in commercial fertilizers. Ca, Mg, and S are referred to as secondary nutrients and are also found in fertilizers and soil amendments.
Micronutrients include iron (Fe), manganese (Mn), zinc (Zn), boron (B), copper (Cu), chloride (Cl), molybdenum (Mo), nickel (Ni), and cobalt (Co). These are required in relatively minute quantities and are usually sufficient in the natural environment. Zinc and iron are two exceptions in north central Arizona. This is primarily due to our soil alkalinity making them less available to plants and that some plants require these two elements in relatively higher quantities. Pecans are regularly fertilized with zinc and apples may also occasionally show signs of zinc deficiency. Some plants (Photinia, fruit trees, and others) display iron chlorosis during spring when growth occurs faster than iron can be extracted from cold, alkaline soils. This is easily remedied by a foliar application of iron chelate.
When I was in college, there were 16 essential nutrients. Nickel and cobalt have been added to the essential nutrient list since then. One of the problems in identifying essentiality is that plants require these elements in such small quantities that it is difficult to keep them out of the plant growth medium. In my college plant physiology course, we grew bean plants in liquid culture (water plus nutrients) with all elements except one. This allowed us to observe nutrient deficiency symptoms as they developed. These nutrients can be oversupplied and cause toxicity. There can be too much of a good thing.
Sodium (Na), vanadium (V) and silicon (Si) are sometimes designated as beneficial plant nutrients. They are not required by all plants but appear to benefit certain groups of plants or plants growing under specific conditions. Sodium and vanadium benefit some plants although they can complete their life cycle without it. Silicon is found in plant cell walls and appears to produce tougher cells. This increases the resistance of these plants to piercing and sucking insects and decreases the spread of fungal diseases.
Plant nutrition is important to all gardeners, especially those growing crops hydroponically (without soil) where all nutrients must be provided in a nutrient solution. If you compost your yard and garden waste, you are also recycling many essential plant nutrients and reapplying them when you use it to amend the soil.
The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at email@example.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: February 21, 2008
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