Fertilizing Your Garden and Landscape

Want to keep your green plants growing and your flowers and vegetables blooming and producing? If so, you'll have to fertilize them from time to time.

Most plants need to be fertilized. Even the desert plants you have in your landscape will preform better with some extra nutrients. Fertilizing on a regular basis is a must for flowers, fruits, vegetables herbs and lawns.

Our desert soil is rich in trace elements also referred to as micro nutrients. That's good, because these are very important for plant growth. Manganese (Mn), zinc (Zn), copper (Cu), boron (B), iron (Fe), Molybdenum (Mo), and Chlorine (Cl) all contribute to healthy growth.

We also have adequate supplies of certain macro nutrients, needed in larger quantities by plants. These include: Potassium (K), Magnesium (Mg), Calcium (Ca), and Sulfur (S).

What desert soils lack is nitrogen and phosphorous, both very important macro nutrients. Nitrogen promotes leaf growth and lengthening of stems. Phosphorous promotes the formation of flowers and fruits, hastens plant maturity, and promotes root growth.

Organic matter such as animal manures, sewage sludge or various composted materials are a good source of plant nutrients, including nitrogen and phosphorous. These are particularly useful when preparing the soil for planting. They can be mixed into the soil, as high as 50% by volume, for vegetable and flower gardens. In fact, straight compost can be used for growing in raised beds and containers.

Commercial fertilizers can also be used to obtain all essential nutrients. They are less expensive than organic sources of nutrients, but have the disadvantage of quicker nutrient release.

The most popular forms of commercial fertilizer here in Tucson are ammonium phosphate (16-20-0) and ammonium sulfate (21-0-0). The series of numbers indicate the percentage of nitrogen - phosphorous - and potassium, in that order. Ammonium phosphate contains both nitrogen and phosphorous and is typically used to fertilize fruits, vegetables, and flowers. Ammonium sulfate, containing nitrogen and sulfur, is useful for landscape plantings of trees, shrubs, and ground covers. Both are quick release fertilizers.

Fertilizers are also available in slow release forms. They have the advantage of feeding over a longer period of time. This better simulates nature's method of supplying plant nutrients.

If a fertilizer has slow release nutrients it will usually be advertised on the label, as this is a selling point. Although more expensive, the nutrients can be better utilized by plants. A high quality fertilizer will have from 30 to 50 percent of it's nitrogen in slow release form. The only draw back to such fertilizers is that most come with a full compliment of nutrients. In most cases all you really need is nitrogen and phosphorous.

Regardless of which fertilizer you purchase, it's important to know how much to use and how often it should be applied.

The rate or amount of fertilizer application is always based on nitrogen, as it is the nutrient most often lacking. Landscape plantings should be fertilized at the rate of one pound of nitrogen for every 1,000 square feet of landscape area. To figure out how much fertilizer gives you one pound on nitrogen, simply divide the percentage of nitrogen listed on the fertilizer bag into 100. For example; ammonium sulfate has approximately 20% nitrogen, so 100 divided by 20 equals 5. Therefore you would apply 5 pounds of ammonium sulfate to each 1,000 square feet of landscape area.

Remember that tree and shrub roots grow out well beyond the edge of the branches. In fact, their roots can grow out one and one half to four times farther than the distance from the trunk to the outer branches. In most landscapes that means that roots from various plants are intermixed throughout the landscape. Fertilizer can therefore be evenly distributed over the total planting area, rather than just around individual plants.

Vegetable and flower beds should be fertilized at the rate of one and one half pound of nitrogen per 1,000 square feet. If your using ammonium phosphate, which contains 16% nitrogen, this requires using approximately ten pounds of fertilizer per 1,000 square feet, or 1 pound per 100 square feet.

On new landscape plantings fertilize after the plants have been in about four to six weeks. For the first three years, fertilize in March and again in October. Once trees and shrubs are established after three years, cut back to one application per year in March. Native desert plants do not require fertilization, but will establish and grow more rapidly with it.

Fertilizer should be mixed into the soil at the time of planting vegetable and flower beds. Flowers should continue to be fertilized on a monthly basis. Vegetables probably won't need monthly applications but may require additional applications on an as-needed basis. Whether fertilizing established flowers or vegetables, be careful not to get fertilizer granules on the leaves or stems. If some fertilizer gets on the plant, just hose it off with water.

Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on April 20, 1997
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