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Extension > Garden > SULIS > Maintenance > Sustainable Woody Plant Maintenance > Fertilizer Basics for Trees and Shrubs

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Fertilizer Basics for Trees and Shrubs

The goal of any fertilization program is to provide nutrients needed by plants to sustain healthy growth and as defense against insects, weeds, and diseases. There are seventeen nutrients, including macronutrients (required in relatively large amounts) and micronutrients (required in relatively small amounts) that are required by plants. Nitrogen (N), phosphorus (P) and potassium (K) are most critical with nitrogen being required in the largest amounts. N, P and K are identified in their percentages by weight in the analysis on the fertilizer label.

Fertilizer Before applying fertilizer, it is a good idea to have your soil tested to see what you are starting with. For most accurate results, you should send a soil sample into a qualified lab. A basic soil test will provide you with information on the texture of the soil (sand, silt, clay), the pH, the amount of organic matter, and the levels of phosphorous and potassium. You can also have soil tested for other nutrients or for heavy metals.

The soil testing lab will provide you with the results of your test along with recommendations for fertilization based on your soil test.

Note: All chemicals should be applied according the manufacturer's recommendations on the label. Remember: the label is the law.

Plant Nitrogen (N) in plants is critical to amino acids, proteins, and chlorophyll. It is required in very large amounts and is very mobile in the soil, meaning it is more likely to "wash out" with rain. Therefore, nitrogen is the nutrient that requires replenishment most often. Plants deficient in nitrogen will have characteristic yellowing (chlorosis) of the leaves starting with the older leaves. Chlorosis will start at the leaf tip and progress down the leaf midrib, and growth will slow due to reduced protein synthesis and photosynthetic enzyme activity. Nitrogen deficiency may also mimic herbicide damage. Over-fertilizing with N can also cause a decline in plant health exhibited in excessive shoot and leaf growth, reduced root growth, low plant energy reserves, and increased susceptibility to environmental stresses and some diseases. Because it is a mobile nutrient, excess nitrogen can leach through the soil and into the ground water. From there, it is carried to drinking water sources in the form of nitrates, and into lakes and streams where it reduced water quality for aquatic wildlife.

Bark Phosphorus (P) plays a significant role in respiration - the production of energy-rich compounds in plant cells called ATP and ADP. It is also important to the development of nucleic acids, enzymes, and phospholipids. Phosphorus promotes early plant vigor and stimulates early root growth. A phosphorus deficiency is noted by a dark, greenish-purple color change to foliage, especially in the leaf margins. Contrary to nitrogen, phosphorus moves very little in the soil, binding tightly to soil particles. Because of this, soils that are high in phosphorous will sustain plant growth without additional applications of phosphorus fertilizers. Leaves and other organic yard wastes are the primary sources of excess phosphorus in surface waters which causes algae bloom. If left on hard surfaces such as driveways, sidewalks, and gutters, phosphorus can leach from these wastes into the storm sewer system and eventually into surface waters. Therefore, it is important to rake, remove and compost grass clippings, fallen leaves, and other yard waste from hard surface areas.

Potassium (K) is important in enzyme activation, transpiration, osmosis, and the opening and closing of the stomates on plant foliage. Potassium deficiency symptoms include mottled or chlorotic leaves with small necrotic spots at the leaf tips and margins. Chlorosis starts at the lower leaf margins and progresses upward. Plants with potassium deficiency may also have poor drought tolerance and weak stems. Potassium is held on the surfaces of soil particles and moves little in most soils; however, it can gradually move out of the root zone in very sandy soils. Soils naturally high in potassium do not require supplemental fertilization.

Micronutrients are usually available in sufficient amounts to support good plant growth. However, on occasion, micronutrient deficiencies do occur. For example, chlorosis (yellowing) of foliage might be an indication of an iron deficiency. Before applying micronutrients, however, be sure to do a soil test to determine the pH. Excessively high or low pH levels often interfere with a plant's ability to take up nutrients. For example, a high pH may cause trees such as pin oaks and white pines to be unable to take up enough iron, even though there is more than enough iron present in the soil. Adding iron fertilizer will not help the problem.

There is no reason to apply micronutrients to your soil unless you have reason to suspect you have a problem. If a plant is not performing well, you need to do some investigation into what the problem might be. First, rule out other more likely possibilities, such as compacted soil, sunlight availability, soil moisture, etc. Then, if the symptoms are consistent with a deficiency of some micronutrient, you could have your soil tested for that micronutrient and/or apply that micronutrient and see if the plant's performance improves.

For more on soil pH:
Modifying Soil pH
Iron Chlorosis

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