   Evaluating Fertilizers - March 4, 2009 Jeff Schalau, Associate Agent, Agriculture & Natural Resources University of Arizona Cooperative Extension, Yavapai County Plants require adequate amounts of essential nutrients (18 are known) to grow and reproduce. In wildland ecosystems, plants have adaptations and partnerships with other organisms that help them satisfy their nutrient requirements. In our gardens, orchards, and landscapes, we often plant exotic plants that are not well adapted to our desert climate and soils. These plants usually have higher nutrient requirements and require fertilization to produce the fruits and flowers that we value. The nutrients nitrogen (N), phosphorous (P), and potassium (K) are considered primary macronutrients. They are called macronutrients because plants require them in relatively large quantities. Nitrogen is a constituent of all amino acids. Amino acids are building blocks that create proteins. Nitrogen is also a component of nucleic acids, DNA and RNA, the genetic "blueprints" of life itself. These internal functions make nitrogen important for vegetative growth (leaves and shoots). Healthy, functional leaves are responsible for photosynthesis: the production, transportation and accumulation of sugars in the plant. When nitrogen is deficient, plants respond to nitrogen fertilization by growing faster and producing lots of green leaves. Phosphorus helps to promote root growth and improves the quality of flowers, fruits, and seeds (the crops we value). Phosphorus is found in some proteins, coenzymes, nucleic acids, and used to transfer energy within plants. Phosphorus deficient plants usually respond to phosphorus fertilization by growing better root systems and increasing crop yields. Potassium, commonly called potash, is important because it helps regulate photosynthesis, makes plants hardy, and helps them to withstand the stress of drought and fight off disease. Potassium is not normally a limiting factor in our area, but is also a major component of “winterizing” fertilizers. Calcium, magnesium, and sulfur are considered secondary macronutrients. In our area, these three nutrients are usually present in sufficient quantities. However, the addition of elemental sulfur (sulfur all by itself) can help to temporarily acidify our typically alkaline soils. Micronutrients are required in very minute quantities and include iron, zinc, copper, molybdenum, manganese, boron, chlorine, nickel, and cobalt. In most cases, it is not necessary to provide additional micronutrients. However, iron deficiency can occur in certain situations (alkaline soils and certain plant species). Zinc fertilizer is used in the production of pecans. Fertilizers are prominently labeled with the percentage by weight of nitrogen (N), phosphate (P), and potash (K). If the fertilizer contains all three of these nutrients (N-P-K), then it is considered a complete fertilizer. If the fertilizer contains one or two of these nutrients, then it is considered an incomplete fertilizer. Fertilizers may also contain secondary macronutrients and micronutrients as indicated on the label. Fertilizers are sold as granules, pellets, liquids, time-released formulations, and pre-measured amounts (stakes or tablets). Some gardeners prefer to grow their fruits and vegetables organically. Organic fertilizers are derived from various plant and animal products. Remember, organic fertilizers will not provide plant available nutrients until soil temperatures are warm enough to activate soil microbes. Soil microbes further decompose these materials to release the plant available nutrients. Conversely, organic fertilizers have the advantages of being released over time and contributing organic matter in addition to many other essential plant nutrients. Gardeners can compare fertilizer prices by calculating the cost of a particular nutrient per pound. This is done by calculating the fertilizer cost per pound, then dividing it by the decimal percentage of the nutrient in question. For example, a 10 lb bag of ammonium sulfate (21-0-0) may cost $5.99. To calculate the cost per pound of nitrogen, divide $5.99 by 10 to calculate a cost per pound of fertilizer: $0.60. Then divide $0.60 by 0.21 to calculate the cost per pound of nitrogen: $2.86. Each gardener evaluates fertilizer based on costs, benefits, convenience, personal preference, etc. There is no perfect fertilizer for every gardener. This is why we see such a wide range of products available. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |