Understanding Plant Fertilizers

August 1, 2009

Fertilizers supply plants with a steady supply of nutrients.  They are the equivalent of the nutrition that we humans get from our foods and supplements.  Fortunately, plant nutrition is not as complicated as human’s nutrition and a little knowledge can help keep your plants actively growing and producing.  Natural gardeners will use organic fertilizers made from the remains of by-products and compost but most fertilizer is purchased as an inorganic(chemical) supplement for your plants.  Knowing what your plants require and understanding the fertilizer product label will help you decide what product to purchase. 

 Understanding the Label
Fertilizers are generally a composite of three nutrients.  Nitrogen (N), Phosphorus (P), and Potassium (K).  Other filler components, secondary components and/or micronutrients may also be included and will be listed on the label.  When reading a fertilizer label, the blend combination of nitrogen, phosphorus, potassium will be listed in that order i.e. 10-10-10, for example.  A complete fertilizer will contain a combination of all three nutrients.  A simple fertilizer will include only one or two of the nutrients.  A nitrogen only fertilizer example is 21-0-0.  A fertilizer with phosphorus and potassium example is 0-10-10 and will not include any nitrogen.  A general-purpose or all-purpose fertilizer will contain nearly equal or equal amounts of all three nutrients and is intended to meet most plant’s requirements during the growing season.  A high nitrogen blend helps keep lawns green and growing quickly.  A high phosphorus blend is used to promote flowering and production.  There are also specific fertilizers blended for specific flowers and their maximum performance. 

The Three Nutrients
Nitrogen (N) is used to synthesize the proteins, chlorophyll and enzymes that plant cells require.  Nitrogen is generally the most needed nutrient in a garden and is easily leached (washed) out of the soil.  Nitrogen will generally need to be added to the soil throughout the growing season as a result of this leaching.  Excessive nitrogen can make plants overly lush and leafy.  This can create a beautiful plant but can also attract unwanted pests that can harm the plant.  A deficiency of nitrogen will be evident in yellow or smaller that normal leaves and the plant may appear stunted or dwarfed.  In some plants the leaves may turn red or purple.     

Phosphorus (P) promotes strong root growth, flowering and fruiting by transferring energy from one point of the plant to another.  It works best when placed close to the root system of the plant so application to the soil before planting is preferable to applying to the surface afterward.  A deficiency of phosphorus will be evident in leaves with small, scorched, purplish or blue-green leaves.  The leaves may also fall off early.  An excess of phosphorus will interfere with the plants absorption of other nutrients and create a multitude of problems.

Potassium (K) is used by the plant to regulate the synthesis of proteins and starches that make plants strong and sturdy and to increase resistance to diseases, heat and cold.  Like phosphorus, it works best next to the root system and so is best to apply before planting and not to the surface of a growing plant.  Also like phosphorus, excess potassium will interfere with the absorption of other micronutrients.  A deficiency of potassium is evident in the tips and edges of leaves that become yellow and scorched looking with brownish-purple spotting underneath.

Deficiencies of Secondary and Micronutrients
A quick preview of deficiencies of the secondary and micronutrients is also helpful.  A shortage of calcium will turn leaves dark from the base outward and then die.  A shortage of iron will show in leaves with yellow between the veins which remain green or slightly yellow. (Iron deficiency may likely be a result of alkaline soil preventing absorption of the iron.) A shortage of magnesium will show in leaf centers that turn reddish or yellow and dead spots will appear between the veins.  A shortage of manganese will show in the upper leaves turning yellow in the center between the veins but with no sign of red.  A shortage of sulfur will show in leaves with veins that are lighter in color than the tissue in between the veins.  Treatment of these shortages may require help from your nursery or a plant expert.

How to Apply Fertilizers
When planting a bed, incorporate the fertilizer evenly throughout the soil.  This is the best time to apply phosphorus and potassium.  Dry fertilizers such as powders, granules or pellets are the easiest to apply and can be of the controlled-release type.

During the growing season, a liquid fertilizer can be applied with an injector device, through an irrigation system, through a watering can or a hose-end sprayer.  Liquid fertilizers need to be reapplied often as their nutrients leach rapidly. A dry fertilizer can be applied as a side dressing if scratched into the soil a few inches away from the base of the plant and moistened.  Rinse any leaves to prevent burning from the fertilizer.

 With the proper nutrition, plants will grow, produce and reward you for your love and attention to them.  The information provided here is only a general review of plant nutrition, but will hopefully give you have enough information to purchase the fertilizer your plants need.

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