Gardening with Soule: Fertilizer for the heat - Tucson Local Media: News

Gardening with Soule: Fertilizer for the heat

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Posted: Wednesday, July 3, 2013 4:00 am

In the past I have written about fertilizer in July in these Explorer pages because ample summer rains wash can nutrients out of the soil, and leave landscape plants lacking. The rains have not started yet, so hold off on fertilizer. Encouraging growth by fertilizing your landscape plants will just make them thirstier for water you will have to provide.

Vegetable gardens and container plants are a different issue. They could use some fertilizer, but which one?   There are a large number of fertilizers on the market, with a bewildering array of claims, instructions, and recommendations.  The key thing to remember when buying fertilizer is that learning to read fertilizer labels is just like learning to read human food labels.  Instead of listing the calories and carbohydrates that humans need to eat for life, fertilizer labels list the elements that plants need so that they can manufacture their own food.

There are sixteen essential elements, or minerals, that plants need for growth and maintenance.  Carbon, hydrogen, and oxygen are taken in from the air and water.  Nitrogen, phosphorous, potassium, calcium, sulfur, magnesium, iron, manganese, zinc, boron, copper, molybdenum, and chlorine are taken in from the soil. 

Most of the sixteen plant nutrients are found in sufficient amounts in desert soils, and in potting soils as well. The exceptions are nitrogen, phosphorous, and potassium.  

All fertilizer labels will display three numbers in a ratio to one another.  The three numbers are always in the order of nitrogen, phosphorous, and potassium.  If you remember your chemical symbols, that is N : P : K.  

Nitrogen is used to make chlorophyll molecules, and proteins for growth and energy transport.  Thus, nitrogen stimulates vegetative growth. Deficiency symptoms are pale green or yellow-green leaves and dwarf or stunted plants.  Too much nitrogen can result in no flowers.  

Phosphorous is very necessary for maximum flower and fruit formation. Failure to flower or set fruit along with stunted leaves with purple or red discoloration are signs of deficiency.  

Potassium is needed for general growth and development, including the thickness of plant cell walls.  This in turn is a factor in how well a plant resists the stresses of heat, cold, drought, and disease.  Deficiencies are indicated by weak stems, and yellowing and browning or leaves at the tips.   

Armed with this knowledge, look again at the three numbers presented on the fertilizer box. If you have plants growing primarily for their lush leaves, like palms, look for a fertilizer high in the first number of the ratio of three, nitrogen.  If you are growing a plant so you can enjoy tasty fruits like tomatoes or corn (or beautiful flowers), a fertilizer high in phosphorous, the middle number of the ratio of three, is what you should select. Potassium is useful for all plants, but is especially useful for tasty root crops growing now, like jicama and yams.  If your plants flower periodically, like bougainvillea, then a good mix with high numbers of the three essential elements is ideal.  

Always read and follow label directions for any chemical compound, and this includes fertilizer.  Too much fertilizer can kill the plants you want to help.  If you are in doubt, err on the side of caution and apply half-strength fertilizer twice.  Wait two weeks in between applications so plants have a chance to respond.  

(Editor’s Note: Jacqueline is an award-winning garden writer. She can be reached at gardeningwithsoule.com.)

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