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The source material for these articles are excerpts from various university 
research papers, private industry, and web sites that pertain to the subject matter.

Fertilizing Trees & Shrubs

With years of research and reports, we still are unable to unanimously agree on the importance of tree and shrub fertilization. When it comes to applying fertilizers, there is a lack of consensus among landscape and tree professionals whether to fertilize: what are the benefits and needs of the trees, shrubs and soil.

Fertilizers are applied as preventive medicine, supplementing mineral elements in an effort to ward off marauding insects and disease pests. Also they are applied as a rescue treatment for plants besieged by any number of biotic and abiotic stresses. Still others adopt a policy of non-intervention, avoiding the application of fertilizers altogether.

Forest trees are able to thrive, usually without additional fertilizer. In their natural setting, trees benefit from the accumulation of layers of leaf litter and natural organic matter on the forest floor. Nitrogen and nutrient levels do fluctuate just after leaf fall, both in the autumn and early spring. The soil microbes during increased organic decomposition temporarily tie up soil nitrogen. With the rains in fall and early spring, this also acts as a boost to the nitrogen depletion by volatitilization and leaching. Although nitrogen levels are low in most natural forest Eco-Systems, trees slow their growth rate to sustain acceptable green and healthy looking foliage over all. (See Nitrogen article) light application of a high nitrogen fertilizer (21-0-0 ammonium sulfate or 36-6-6 miracle grow) will help supplement for Mother Nature at these low nitrogen intervals.

A balanced availability of nutrients is one of the most important factors affecting tree vitality.

An unbalanced nutrient status can predispose a tree to disease or insect damage. The resistance of trees against insects has been found to be dependent on protective compounds, the concentrations of which are dependent upon tree nutrition.

Fertilizer cannot remedy poor growth resulting from causes other that nutrient deficiencies such as soil compaction, poor drainage, restricted rooting areas, asphalt or landscape asphyxiation, mechanical injury, or turf competition. The only valid reason to fertilize landscape plants is to correct nutrient deficiencies.

Urban soils are strongly affected by construction development, human activities and by various landscape practices. These impacts adversely affect tree growth, vigor, longevity and general overall appearance. Top soils of high organic content and microbial activity is routinely removed during construction and the sub-soils become severely compacted. The result is a hard, nearly impenetrable, poorly aerated, nutrient-poor root environment with reduced water holding capacity and with restricted water and air penetration. The soils that are replaced for the landscaping process are usually void of organic material and microbial life. In simple terms landscapers are just adding even greater woes that are near to impossible to remedy. This will take a barrage of fertilizer and organic applications and soil management to restore the soils with microbial life and conditions conducive to providing healthy tree and shrub development.

We must preserve balance without impairing growth.
We must encourage growth without destroying balance.
W. R. Nixon

Trees and shrubbery can be mulched to help restore microbial life and moisture availability. A moderate fertilization program with light and repetitive applications will in time restore or at least somewhat simulates a forest type eco system. For our Hill Country type soils, approximately four applications, Dec., Feb., May, and July, using ammonium sulfate 21-0-0 or a Miracle Gro 36-6-6 is sufficient. Additional Iron supplement such as Iron Plus may also have to be applied at some point during the year. (See article Iron Chlorosis). Apply according to manufacturer recommendations. However, a light application and more frequent applications provide optimal benefits.

Over fertilizing trees, encouraging rapid succulent growth can promote insect and disease attacks. Studies have shown that resistance to certain insects and diseases decreases in rapidly growing trees and such trees are more nutritionally suitable to some pests. In simple words, the faster the growth, the less defense ability, there is. When conditions restrict growth, more energy can be diverted for defense. Trees under moderate stress produce higher levels of defensive (pest inhibiting) chemicals.

Trees naturally grow in response to their environmental conditions, adjusting root growth and mass to provide adequate nutrients. In deficient soils, roots increase mass and area and the canopy is in ratio to those particular conditions. In nutrient rich soils, the root mass will occupy a smaller volume area and the canopy would respond with greater mass volume relative to the available nutrients.

Trees do not obtain energy directly from mineral nutrients in the soil. Sunlight energy along with carbon dioxide from the air, water and oxygen from the soil converts chemical energy (sugar) during photosynthesis. During Transpiration, glucose is broken down to release stored biochemical processes. Mineral elements are the basic building blocks for new growth and cellular function. Mineral elements, usually positively charged particles or ions from organic matter decomposition and fertilization are dissolved in the water and absorbed by the roots as ions The negative charged particles are anions, and it is this positive and negative property of each ion that affects its behavior in the soil. (See article Cation-Exchange Capacity), and (See article Trees are Vital)

As trees age, we see distinct changes in their ability to respond to stress. Young trees have a high ratio of photosynthetic (leaves) to non-photosynthetic (woody) tissue. Consequently for their size, they produce a relatively high amount of energy. Healthy young trees produce enough energy for growth and abundant storage. They tolerate environmental changes and maintain treatments. In contrast, mature trees have greater demands on their energy supply. They utilize some energy for growth, but a larger percentage is used just to maintain the massive amounts of existing woody tissues in the trunk, branches and roots. Additional energy is needed to seal wounds that occur from wind breakage, insect and disease attacks and for the development of reproductive structures. Because of their tighter energy budget, mature, healthy trees are in a delicate balance with the environment. The key to preserving this balance, and their health, is to maintain environmental stability around mature trees.

Comprehensive tree care programs should always be considered long term, beginning before planting and continue throughout the life of the tree. Practices should be geared to mesh with natural changes and development and to be modified to meet the trees ability to withstand and cope with change.

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Contact: cell: 830.257.8871
                     Jim Rediker - Nurseryman -  Arborist  - TDA Certified

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