Arizona Gardening: Fertilizers go strictly by the numbers - East Valley Tribune: Get Out

Arizona Gardening: Fertilizers go strictly by the numbers

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Posted: Thursday, July 3, 2008 10:31 pm | Updated: 11:28 pm, Fri Oct 7, 2011.

When I go into nurseries and plant centers I see rose food, citrus food and many others. Please give me some tips on which fertilizer I should use on which plants.

Q: When I go into nurseries and plant centers I see rose food, citrus food and many others. Please give me some tips on which fertilizer I should use on which plants.

A: Fertilizers are often packaged for the specific needs of those plants — citrus food, palm tree food, rose food, etc. The word “food” is a marketing tool — the contents are not food, but nutrients that plants use to manufacture their own food, sugar and carbohydrates, which are combined with plant nutrients to produce enzymes, vitamins and other compounds essential for plant growth.

Have you noticed the three numbers on fertilizer bags, like 16-20-0, 16-8-8 or 15-15-15? Those numbers represent nitrogen, phosphorous and potassium. They are always in that order and represent the percentage by weight of the contents.

There actually are 18 chemical nutrients that are needed for good plant nutrition, but not all needed in equal amounts. If plants lack any of these elements, they show signs of nutrient deficiency. Nitrogen, phosphorous, potassium, calcium, magnesium and sulfur are considered fertilizer macronutrients because plants require them in the largest quantity for maximum growth. Nitrogen, phosphorous and potassium are the primary nutrients that are most likely to be short these amounts. Calcium, magnesium and sulfur are secondary macronutrients.

A fertilizer is called a complete when it contains nitrogen, phosphorus and potassium. An incomplete fertilizer will be missing at least one of the major components, like 16-20-0 ammonium phosphate, 21-0-0 ammonium sulfate or 0-45-0 triple super phosphate. Types of fertilizer include those that dissolve in water. Granular and slow-release dissolve slowly and release the nutrients over a longer period of time.

So which one do you use? The one that has nutrients the plant needs.

Just remember it this way:

Nitrogen promotes quick leaf and plant growth. Use it on grass and established citrus.

Phosphorus promotes strong roots and flowers. Use it on vegetables and flowers.

Potash imparts disease resistance and winter hardiness. This is just an added bonus; our desert soils generally have enough potash.

The next time you are shopping for fertilizer, compare the ratios of different brands and pick the one that is the cheapest and supplies the nutrients for citrus or roses. It doesn’t have to be exact. Plants don’t read labels!

CONTACT WRITER: john@johnchapman.com

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