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Fertilizer Components

Edited by Len Phillips

Fertilizer consists of elements or nutrients that are easily released from the soil for the plant to absorb.

MACRONUTRIENTS

Three nutrients known as macronutrients are the primary ingredients in most fertilizers: Nitrogen (N), Phosphorus (P), and Potassium (K). While they are only three of the 13 elements known to be essential for tree growth, a fertilizer is called a complete fertilizer when it is comprised of varying percentages of these three elements.

Nitrogen

Nitrogen is the main element to consider in choosing a fertilizer. Nitrogen encourages above ground vegetative growth. It is the fundamental element in amino acids and proteins, the building blocks of life, and is a major component of DNA and RNA. Deficiencies in nitrogen can be one of the reasons for yellowing of mature leaves and stunted growth. Nitrogen is often in short supply because watering and rainfall flush it from the soil. Nitrogen is also the most volatile and transient of all the elements used by a tree.

A growing plant needs a continuous supply of nitrogen in a usable form. Air is a source of nitrogen along with soluble sources of nitrogen in the soil and fertilizer. Although it is estimated that of all the nitrogen fertilizer that is put in the ground, the tree utilizes only .001%. The rest is sucked up by other organisms, volatilized, leached away into ground water, or converted into nitrogen compounds by soil organisms. Excessive nitrogen in the soil and water is not good for the environment.

On many fertilizer bags are the letters W.I.N. (Water Insoluble Nitrogen). The higher the percentage of W.I.N., the safer and more longer lasting the nitrogen will be available in the soil.

Phosphorus

Phosphorus is important for seedling development, cell building, disease resistance, and root growth. A lack of phosphorus shows up in the lower leaves as a lack of chlorophyll, blotches of deep green, or a reddish color in the leaves. Unfortunately, cold temperature, high light intensity, insect damage, and drought can also induce such foliar reddening. Deficient trees may have weak roots, delayed maturity, and low flower and fruiting. Adding phosphate to trees increases color as well as vigor and stimulates growth. Overfertilizing with phosphorus results in leaf chlorosis and is detrimental to mycorrhizal health. Excess phosphate will also find its way into waterways and induce algal blooms and eutrophication. To control excess phosphorus, maintain organic soil and mulch on landscapes. This provides a slow release of phosphorus and other needed macro and micronutrients over time.

Phosphorus is released from organic matter and soil minerals. Phosphorus usually enters plants as phosphate. The plant uses about 10% to 20% of the phosphorus applied by fertilizer. It does not move within the soil except by cultivation. Research has indicated that phosphorus is affected by the moisture supply. Good watering practices and cool weather help maximize the available phosphorus.

Phosphorus is usually made available to trees through the breakdown of organic matter by fungi in the soil. Urban sites, where there is little organic material, will usually be deficient in available phosphorous, even though it is a major component in soils. Phosphorous is used in the transfer of energy and is the adhesive component of DNA and RNA.

Potassium

Potassium or potash assists plants in forming starches and proteins and helps trees resist disease. Potassium can be unavailable to plants in mineral form, sandy soils are particularly susceptible to leaching of potassium salts. Trees use potassium in large amounts in the production of sugar, respiration, transpiration, and synthesis of proteins. It is released slowly from the soil and may be deficient in trees during peak growth periods. Deficiencies result in slow growth and scorched leaves, particularly at the tip and along the margins. By providing potassium to a tree, you increase insect and disease resistance.

The total potassium content of U.S. soils generally increases from east to west and from south to north in the eastern half of the U.S. Many trees use as much potassium as they do nitrogen, which is three to four times the amount of phosphorus used. Because potassium may be leached from the soil, soil testing should be made to determine the plant's needs for the long term.

SECONDARY NUTRIENTS

Many fertilizers contain secondary nutrients or microelements. Many microelements are used by trees in similar quantities to macroelements, but are generally found in more available forms.

Calcium is common in most soils and is ranked third behind nitrogen and potassium in use by plants. It is used mainly in cell walls and interactions with potassium and magnesium.

Magnesium is abundant in most soils. Magnesium is a key ingredient in chlorophyll, is active in enzymes, interacts with phosphorus, and is most abundant at the meristem. When magnesium is lacking, leaves turn light green with streaks or bands and the leaf margins appear up-turned. Tip necrosis is also a sign while lower limbs may be deep green. In the Southeast and in warmer coastal areas, palm trees are particularly prone to magnesium deficiencies.

Lime is the principal source of calcium and magnesium in most areas. Lime can be added to the soil in several forms including agricultural lime, dolomite, and marl. In areas where the soil is neutral to alkaline, there is normally enough calcium and magnesium naturally available in the soil for a plant's needs.

Sulfur is just as important as nitrogen for the making of new protoplasm for plant cells. Sulfur is found in organic matter and in the atmosphere in the form of sulfur dioxide. It is vital in many amino acids and proteins used in plant growth, and it is a vital portion of enzymes. Sulfur chlorosis appears interveinally with stunted new growth. Excessive sulfur causes pH imbalances that can reduce the availability of other microelements.

Micronutrients

Micronutrients are needed by trees in very small amounts, but they are nevertheless essential to plants. Many commercial fertilizers contain micronutrients as part of the formulation. However, in most soils micronutrients are seldom in short supply. Specific micronutrients may need to be added to the soil if deficiency symptoms occur.

The diagnosis of micronutrient deficiencies is tricky, because many environmental (e.g., drought) and structural (e.g., girdling root) problems can also produce chlorotic leaves. Micronutrient deficiencies on urban trees are often due to a high pH (7.8-8.2 on soils with significant lime content) that makes the elements unavailable to plants.

Iron deficiency is the most common cause of chlorosis or yellowing in trees. Iron is needed for chlorophyll synthesis, enzyme activity, and other energy transport reactions. Iron is fixed in plant tissues and will not move. Some species like Pin Oak require more iron than others and needles of conifers lacking in iron turn yellow-green. Iron must be administered before new leaves appear in the spring to be effective.

Manganese is a microelement that falls into a category similar to iron. It is needed for chlorophyll and activates certain enzyme reactions. Manganese interrelates with iron and is immobile. Manganese deficiency often appears as whitish in color on new growth. Since both iron and manganese are closely connected, a misdiagnosis can result. A mildly chlorotic plant as a result of manganese deficiency may become severely chlorotic if iron is used to correct the problem. Therefore, it is necessary to treat with both elements in this situation. A plant tissue analysis can determine which element is missing, but results are often unreliable. Using both iron and manganese in a 3:1 ratio is justified when treating these problems.

Zinc is found in most soils but is commonly needed along the U.S. Gulf Coast and in sandy soils. Some plants, particularly pecans and conifers, benefit from high levels of zinc. Zinc serves as a catalyst along with copper and manganese and is essential for photosynthesis. Zinc chlorosis appears uniform, sometimes with necrotic spots and small leaves that are narrow and pointed. Terminal meristems produce rosettes of leaves that die back. Fruit is small and highly colored.

Copper is utilized in enzyme reactions. Although this element is almost never found to be deficient, it has been demonstrated that the addition of small amounts of copper affects a tree's metabolism. This results in greater tree vitality and vigor.

Molybdenum is important for nitrate utilization by plants.

Boron and Chlorine are only needed in extremely small amounts. Boron is applied in calcium usage and assists with sugar movement, while chlorine is used to regulate osmotic pressures and ionic chemical exchange.

Sodium is needed by certain plants for maintaining osmotic pressure and photosynthesis.

Cobalt is needed by microorganisms within certain plants for nitrogen fixation but is not used in the plant system itself.

If a plant is lacking an essential nutrient, its growth will suffer, it will be more easily stressed, it may be more likely to be attacked and damaged by pathogens, etc, with different effects on growth and color depending on whatÂs lacking.

Sources

Chalker-Scott, Dr. Linda, "Phosphorous", WNLAÂs B&B newsletter, September 2000

Dong, S., L. Cheng, C. Scagel, and L. Fuchigami, "Nitrogen Absorption, Translocation, And Distribution", Tree Physiology 2002. 22:1305­1310