Fertilization is a very important component of plant health care in the landscape. Fertilization is necessary to supplement naturally occurring essential mineral elements in the soil to maintain an optimum supply for plant growth. Soil analysis (testing), combined with observations of plant growth, are the keys for the home gardener to develop the most effective nutrition program for the landscape. The mineral elements critical for optimum growth and development of landscape plants must be present in the soil and plant at proper levels.
The objective of this factsheet is to help the gardening public make informed decisions regarding the nutrition of their landscape plants. Included is a brief review of soil analysis, soils, pH, essential elements, fertilizers and fertilizer rates, timing, and methods of application.
Soil Analysis
Prior to planting, one of the first priorities is to have the soil tested, simply because it is much easier to correct nutrition imbalances at this stage. Additional soil tests every 2 to 3 years are highly recommended to monitor the fertilizer program and prevent mineral element deficiencies that could result in abnormalities or a decrease of optimum plant growth.
Samples should be taken from a minimum of 6 to 8 sites per area (tree and shrub beds, vegetable garden, annual beds, etc.). The samples should be combined and thoroughly mixed to provide uniformity. Dry the soil sample at room temperature and place it in a self-mailer available from Ohio State University Extension offices in most counties or private soil testing laboratories listed in the classified section of the telephone directory.
Results from the testing laboratory will include corrective recommendations for soil pH, phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg). Nitrate nitrogen (NO3N) and soluble salts (EC, electrical conductivity) are not tested regularly by most laboratories; however, these tests can be requested.
Soil Properties
The physical and chemical properties of soils significantly influence the growth of landscape plants. Fertilizer applications are dependent on organic matter, soil texture (size of soil particles), and drainage.
Organic matter in soil may be a slow-release source of nutrients, may contribute to desirable soil structure (arrangement of soil particles), and increases total water available to crops. Organic matter increases the water-holding capacity of sandy loam soils while increasing aeration of silt and clay loam soils. As organic matter decomposes into humus, it becomes colloidal in nature and cation exchange occurs (positively charged ions, such as calcium and magnesium, are adsorbed on to negatively charged particles). Incorporation of sphagnum peat moss, composted municipal sludge, composted yard waste, pine bark chips, among other sources, is recommended at planting if tests indicate less than five percent organic matter in the soil.
Soil texture is determined by the relative amount of sand, silt, and clay in the soil. Common soil textural classes are sandy loam, silt loam, and clay loam. The surface area of soil particles is important and varies with the size of these soil particles. Clay particles have 100 times the surface area as the same volume of sand particles; therefore, clay that is negatively charged has a greater capacity to attract positively charged soil nutrients. Sandy loam soils must be fertilized more often than clay loam soils because of their lower capacity to attract and hold (adsorb) positively charged mineral elements.
As stated above, clay has a negative charge that can be measured to indicate the exchange capacity for cations such as Ca++, Mg++, K+, and others. This is called cation exchange capacity (CEC) and its determination is included in many soil test results. The CEC is an indication of the soil's capacity to provide nutrients for plant use, and is a measure of nutrient leaching potential.
Soil drainage is critical to survival and growth of most landscape plants, especially evergreen trees and shrubs. When the rate of water movement through soil is restricted by fine-textured clay soils, sub-soil, hard pan, or other material difficult to penetrate, a saturated zone may develop in the root zone of plants. Spaces in the soil normally containing air are filled with water, resulting in saturated soil. Wet soils cause more problems to landscape crops than any other single cause. When drainage is poor, roots are injured from the lack of oxygen, fertilizer uptake is limited, and plant growth is reduced. Soil moisture problems can be solved by installing surface and/or internal drainage.
Adjusting Soil pH
Mineral soil pH values between 6.0 and 7.0 result in the greatest number of mineral elements to be available for uptake by plants. Several plants such as certain conifers, most broadleaf evergreens, maples, oaks, sourgum, and sweetgum should be grown in acidic soils with a pH from 5.5 to 6.0. Other plants such as viburnum, hydrangea, and lilac grow best at neutral (7.0) to slightly alkaline soil pH values. In most situations, mineral element deficiencies can be avoided by proper soil pH management.
When the pH of a mineral soil drops below 4.5, aluminum (Al), iron (Fe), and manganese (Mn) are very soluble. When this occurs, these elements are absorbed in large quantities and may become toxic to certain plants, while nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and/or magnesium (Mg) may become limiting for plant growth.
As the soil pH increases, ions of Al, Fe, and Mn precipitate (settle out of the soil solution) and the availability of these elements decreases to a point where nutrients may become deficient for normal plant growth.
It becomes evident that a soil pH of 6.0 to 7.0 is generally desirable, although slight adjustments are needed for specific plants. A soil test will indicate the amount of lime needed to increase the pH of acidic soils or the amount of sulfur needed to lower pH of alkaline soils.
Essential Elements
Nine essential elements required in relatively large amounts for plant growth are called macronutrients or major elements. Included are nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, carbon, hydrogen, and oxygen. The last three are readily available in air and water. Seven other essential elements required in small amounts by plants are called micronutrients or minor elements and include iron, manganese, zinc, boron, molybdenum, copper, and chlorine.
If an insufficient amount of any of these 16 essential elements is lacking or in excess, plants will not grow properly. More or less distinct symptoms occur for individual nutrient element deficiencies or excesses because each element has its own role in the growth and development of the plant. Once a deficiency or toxicity symptom is visible, plant growth has been and will continue to be reduced until corrected.
SOLVER CHEM
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