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Fertilizers – Revitalizing the Landscape

By: Tim Sloan and Joe Feather, Woodlake Outdoor


Everyone agrees that plants, especially turf grasses, benefit from the regular application of supplemental fertilizers. Understanding when to supplement and when not to supplement plant fertilizers is a growing challenge.

Start at the Beginning
Attractive trees and shrubs are important components of all landscaped properties. Planned maintenance and care are essential for keeping them healthy, vigorous, and attractive. Fertilizer, however, cannot be expected to overcome problems caused by the use of un-adapted varieties, improper planting techniques, poor soil drainage, soil compaction, or incorrect watering practices. Woodlake Outdoor begins with the basics of zone-specific varieties planted into correctly prepared soils. Correct any preexisting landscaping issues, then move forward with a balanced fertilization plan.

Complementary Planting
Landscaped areas that incorporate turf grasses have a reduced need of fertilizer. Often times a good turf maintenance program may actually eliminate the need for supplemental fertilization for trees and other woody plants in the turf. Turf requires approximately 5-7 pounds of nitrogen per 1000 square feet, which the trees will also uptake. Additional fertilizer would simply be a waste of money and might result in nutrient imbalances or pollution of local water supplies.

In areas where a tree or shrub root growth is restricted by streets, curbs, or other structural features, additional fertilizers may be needed. The same holds true for shrubs and vines serving as screens or borders. Frequently these areas can be forgotten or neglected in the normal turf fertilization program.

Deficiency Signs
With some variation between species, plants generally indicate the need for fertilization in similar manners. Signs include a displaying pale green or yellow leaves, mottled leaves, lack of terminal growth, dead branches, stunted leaves, or early loss of leaves.

Nutrient deficiency in trees can show up as chlorosis, where the leaf is light green but the veins are a dark green. Deficiencies can also show in the deformation of new leaves or in leaf curling. General tree vigor can be determined by comparing the length of twig growth during the past three to four years. Young trees should have at least 9 to 12 inches of terminal growth per year. Large mature trees usually average 6 to 9 inches of growth. Shrub vigor can be determined the same way.

Soil Analysis
Fertilizer recommendations should be based on a professional soil analysis. Such analysis allows the application of fertilizers in amounts and ratios that minimize nutrient waste or damage from over/under fertilizing. Without a soil analysis, the general fertilizer recommendation for most trees and shrubs is a 1 to 2 pounds of actual nitrogen per 1,000 square feet per year.

Fertilizer Science
Fertilizers are purchased according to their “analysis.” A fertilizer’s analysis is the percentage of the three major plant nutrients contained – nitrogen (N), phosphorus (P) and potassium (K) in the amount purchased. Details of the analysis are shown on the bag or container listed as three numbers (i.e., 16-4-8). The first number indicates the percentage of nitrogen (N), the second gives the percentage of phosphorus as phosphoric acid (P2O5), and the third is the percentage of potassium as potash (K2O). For example – A 50-pound bag of a 16-4-8 fertilizer contains 8 pounds of N, 2 pounds of P2O5, and 4 pounds of K2O. The specific NPK fertilizer ratio for Texas soils is 3:1:2 or 4:1:2 ratio.

Nitrogen is volatile and therefore application is needed more regularly. Phosphorus and potassium are normally plentiful in Texas soils, in addition to being less volatile. The only way to know accurately what analysis is needed is through soil analysis. Because phosphorus is very insoluble, it tends to remain in the soil for extended periods of time. Regular applications of a “complete fertilizer” (a material that contains all 3 nutrients NPK) can result in phosphorus build-ups in the soil. This accumulation can inhibit the plant from receiving other essential nutrients, and poses an increased potential for environmental contamination. For these reasons, it is now recommended to use a No and/or Low phosphorous fertilizer. These include ammonium nitrate (33-0-0), ammonium sulfate (21-0-0) and urea (46-0-0). Although urea has the highest N content and is typically a low-cost nitrogen source, it does need to be mixed with other fertilizers to avoid damaging plant material.

Plants require 13 nutrients including the three primary macronutrients (NPK). These secondary nutrients are; calcium (Ca), magnesium (Mg), and sulfur (S). There are usually enough of these nutrients in the soil, so additional fertilization is not always needed. The micronutrients used in small quantities are; boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). These nutrients are used in significant amounts by most growing plants, so they must be replaced periodically to sustain productivity.

Organic and Synthetic
By definition, organic fertilizers are materials derived from plant or animal matter or residues. Examples would include blood meal, bone meal, compost, manure, seaweed, worm castings.

Synthetic fertilizers, on the other hand, are manmade compounds using natural components. Examples are ammonium nitrate, ammonium phosphate, superphosphate, and potassium sulfate.

Plants cannot distinguish between an organic or synthetic fertilizer – the nutrients are processed in exactly the same way, however, there are other differences.

Organic fertilizers tend to have substantially less nitrogen in their makeup, resulting in the need to apply more pounds per square foot. The structure of organic fertilizers also makes them slower to take effect, and require multiple applications (6 – 8 applications per year) resulting in higher costs.

Synthetic fertilizers accurately address and amend specifically what the soil needs. Used in conjunction with a soils test, these fertilizers can precisely be formulated, eliminating over-fertilizing and with little to no waste.

Plants require more than just nutrients, however, to thrive. They do need organic matter to support the bacteria and other microbial life within the soil, essential to plant health. These tiny organisms are responsible for breaking down organic matter into stable amendments for soil quality and fertility. Synthetic fertilizers do not encourage microbial soil life, therefore the addition of a composted bark mulch into the landscape plan will help sustain and fortify this soil life. And healthy soil creates a healthy landscape.

Strategic Timing
The key to effective fertilizer is balance and consistency. In general, the best time to apply fertilizer is in the late winter before spring growth begins. The maximum growth response to the fertilizer is obtained if the fertilizer is available to the roots at or slightly before the start of spring growth.

Do not apply fertilizers from August until late fall. Late summer fertilizing can stimulate new growth, making plants more susceptible to winter injury.

To keep landscapes healthy and vibrant, a consistent, balanced fertilization plan is crucial. This will ensure the longevity and beauty of the property season after season.

At Woodlake Outdoor, our goal is to continuously enhance and increase the value of the properties we care for as they mature. As a premier maintenance provider, we deliver extensive Texas-based landscape management and horticultural expertise.