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Aug 092012

Fertilizing gardens and landscapes is important to maintain healthy growth and acceptable appearance. Under natural forest conditions, the annual decomposition of leaves, needles and twigs provide a fresh resource of minerals for plants to use. Landscapes usually do not have this nutrient source and are in need of additional minerals since landscape debris is routinely hauled away. There is a plethora of products available to fertilize your plants. It is important to understand basic plant nutrition and fertilizer application principles in order to meet garden fertilizer needs.

Sixteen chemical elements are known to be important to a plant’s growth and survival. Three of these, carbon (C), hydrogen (H) and oxygen (O) are acquired by plants in sufficient quantities from air and water. The other 13 mineral nutrients are acquired by plant roots, which absorb soil minerals dissolved in water. These 13 required mineral nutrients are divided into two groups: macronutrients and micronutrients.

The primary macronutrients are nitrogen (N), phosphorus (P), and potassium (K). Primary macronutrients usually are the ones to be depleted from the soil first because plants use large amounts for growth and survival. Expectations are that some amount of these three nutrients will be needed in the garden every year. The secondary macronutrients are calcium (Ca), magnesium (Mg, and sulfur (S). Fertilization with these nutrients is not always needed. Micronutrients are needed in only very small quantities. The micronutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn). In the high pH soils of southern Idaho, levels of S, Fe, Zn and Mn are often deficient.

Calculating fertilizer application amounts can be a daunting task for the novice gardener. Before fertilizing, you must first determine how much of which nutrient(s) are needed. Determining the amount to apply can be made using historical recommendations found in many garden publications, or using the results of a soil test. The most reliable way is a soil test.  Your local count Extension educator can provide instructions for taking a soil sample.

Next, nutrient content or grade of the fertilizer must be determined. This information is found on the fertilizer package in the form of three numbers. For example, if the fertilizer grade is listed as 10-10-5, the fertilizer contains 10% nitrogen (first number), 10 percent phosphorus (second number), and 5% potassium. If there is a fourth number, it is the percentage of sulfur. The numbers on a fertilizer package are always in the same order, nitrogen-phosphorus-potassium.

The final piece of information that is needed is the land area to be fertilized. Once all of these factors are known, refer to the bulletin Using Soil Tests Results for Garden Fertilization to determine the amount of fertilizer material to apply. For information about fertilizing vegetables, refer to the Soil Preparation for Garden Vegetable section in this web site.

Organic materials are available that can take the place of inorganic fertilizers in the garden. Common forms include blood meal, bone meal, cottonseed meal, sewage sludge, composts, and manures. These fertilizers are usually relatively low in nutrient content compared with conventional formulations and sometimes relatively large quantities need to be applied. Especially when purchased pre-packaged, organic materials can be more expensive than inorganic fertilizers. Colorado State University has published a great discussion on organic fertilizers.

Fertilizing in a landscape is complicated by the fact that different plants have different nutrient requirements. For instance, a lawn uses high amounts of nitrogen while trees generally need very little nitrogen, especially in late summer and fall when applications may induce new growth, which may result in winter cold injury. Managing fertility on other types of plants in Idaho is described in these University of Idaho on-line publications:

By accessing the University of Idaho Resources for Idaho catalog, production and fertilization guides for many additional garden plants can be purchased. Peruse the list at: http://www.cals.uidaho.edu/edComm/catalog.asp?category1=Gardening

Sometimes adding materials to the soil may be beneficial in certain geographical areas while detrimental in other areas. For example, addition of wood ashes and lime make acidic soils more alkaline (higher pH). Consequently, these may be good amendments for northern Idaho’s soils, but not for southern Idaho’s calcareous, alkaline (high in lime) soils.

There are other excellent fertilizer guides on-line. These include:

  • A general discussion about fertilizing a home garden is found in the bulletin, Fertilizing Gardens, published by University of Idaho.
  • A good publication, Fertilizing Your Garden, by Oregon State University that provides information on interpreting soil test results, determining fertilizer needs, and calculating application amounts.
 August 9, 2012