The Homeowners Column

The Homeowners Column

Fertilizer 101

March 15, 2007

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

Take two pounds of poultry manure and call me in the morning. Plants can't exactly tell us what's wrong when they look sick. They can, however, give us hints when they have nutrient deficiencies.

Plants need at least 16 different nutrients. Carbon, hydrogen, and oxygen are needed in large quantities, but plants usually get these in sufficient amounts from air and water. Plants also need nitrogen, phosphorus and potassium in relatively large quantities. Nitrogen promotes leaf and stem growth. Phosphorus is involved in flowering, fruiting, root development, and seed germination. Potassium promotes disease resistance, winter hardiness, and root development.

The micronutrients of sulfur, magnesium, calcium, iron, copper, manganese, boron, zinc, molybdenum, and chlorine are just as important but are needed in smaller quantities. Plants require differing amounts of all the nutrients. The amount of plant available nutrients can depend on soil pH (acid or alkaline).

Symptoms of common nutrient deficiencies:

  • Nitrogen - leaves small, and pale; new growth spindly; old leaves turn yellow and drop.
  • Phosphorus - stunted growth; leaves appear scorched or reddish.
  • Potassium - stunted growth; margins of older leaves may turn yellow then scorch and die; leaf edges may roll.
  • Calcium – center of flower buds turn black; new leaves turn yellow; terminal buds die; leaves become distorted with a hooked tip.
  • Iron – leaves turn yellow between veins; leaf margins scorch. Iron deficiency commonly appears in acid-loving plants such as blueberries, rhododendrons and azaleas because iron is less available in high pH (alkaline) soils.

Unfortunately it's not as simple as a quick diagnosis of symptoms then immediately conclude it's a nutrient deficiency. Environmental problems, diseases, and pests can cause similar symptoms. Just think of how many different things give us headaches – bad food, bad manners, bad hair days. A soil test may help to determine deficiencies. Rule out environmental conditions such as too much water or too little water before assuming a nutrient deficiency.

Fertilizers (often incorrectly called plant food) can correct nutrient deficiencies. Plants make their own food through photosynthesis. Fertilizers contain plant nutrients. All fertilizers have a guaranteed analysis such as 10-20-10 listed as three numbers on the label. The analysis on the fertilizer container shows the percentage of each nutrient by weight or ppm.

The first number is nitrogen. Second number is phosphorous (phosphoric acid). Third number is potassium (potash). Amount varies according to fertilizer purpose.

Fertilizers are inorganic or organic. Inorganic fertilizers do not contain carbon. Some are naturally occurring minerals such as muriate of potash, rock phosphate, etc. and some may be synthesized by people such as ammonium nitrate (33-0-0) or ammonium sulfate (21-0-0). Some inorganic fertilizers may also have herbicides in them such as "Preen and Green" and "Weed-n-Feed".

Organic fertilizers are derived from living organisms. This would include items such as dried blood meal, cottonseed meal, Zoo-Doo, fish emulsion, manures or seaweed extracts.

Inorganic and organic have advantages and disadvantages.

Inorganic fertilizer:

  • inexpensive when considering just the amount of nutrients per weight.
  • quickly released which may be good for quick fix but can burn roots.
  • already in plant available form.
  • does little to improve the soil. Think of inorganic fertilizers as vitamins.

Organic fertilizer:

  • can improve soil structure.
  • foster beneficial soil organism growth.
  • is expensive when considering just amount of nutrients it provides.
  • is often cheap and readily available.
  • releases nutrients slowly so burning is not as likely except with fresh manures.

Not everything needs to be fertilized. To prevent nutrient deficiencies and encourage good plant health concentrate on feeding the soil with compost or other organic matter rather then feeding the plants. Plants need a balance meal too.

Sat, March 17, 9am-4pm and Sun, March 18, 1pm-3pm. Rare Himalayan Blue Poppies at the University of Illinois Plant Biology Greenhouses at 1201 South Dorner Drive in Urbana. www.life.uiuc.edu/plantbio/greenhouse

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