   Warm Season Grasses Need Summer Fertilization - June 7, 2000 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County All plants need nitrogen to survive. Nitrogen is used by plants for photosynthesis (the process of converting light energy into chemical energy), to build proteins, and during reproduction. In native landscapes, plants are well adapted to their surroundings and native nitrogen supplies are usually in balance plant needs. Since many plants we grow are non-native, they need supplemental nitrogen. Nitrogen availability is especially crucial when we grow vegetable crops and fruit trees. The most common symptoms of nitrogen deficiency are: light green foliage, older leaves yellowing and/or dropping, slow growth, or stunted plants. Supplemental nitrogen can be provided through inorganic (synthetic) or organic fertilizers. Inorganic fertilizers are produced in large batches by mixing and reacting chemicals such as anhydrous ammonia and nitric acid. Some common inorganic nitrogen fertilizers are ammonium nitrate, ammonium sulfate, urea, and potassium nitrate. These are relatively inexpensive and give the fastest plant growth response. They are also subject to loss from the soil through downward leaching by water and volatilization to the atmosphere. The inexperienced gardener can also "burn" plants by applying excessive amounts. Organic fertilizers are manures or organic by-products from plant or animal sources. They are more expensive but tend to be released slowly and therefore are not subject to losses through leaching and volatilization. Common organic fertilizers are cow and chicken manure, bat guano, cottonseed meal, alfalfa meal, blood meal, hoof and horn meal, and fish emulsion. They tend to be more expensive than synthetic fertilizers. Plants can take nitrogen from the soil in two forms: ammonium or nitrate. Chemical fertilizers provide these two forms directly to the plant. Organic fertilizers contain more complex compounds that must be broken down by organisms (bacteria, fungi, protozoa) in the soil to release ammonium and nitrate. Plants cannot differentiate between nitrogen that came from a chemical vs. that from an organic source. Organic fertilizers have the added advantages of adding organic matter to the soil, nitrogen availability over time, and increasing soil biological activity. All products labeled as fertilizers must also be labeled with the amounts of nitrogen (N), phosphorus (P), and potassium (K). This is call the guaranteed analysis. When purchasing a fertilizer, check the analysis on the package to determine the percent nitrogen in the fertilizer. The nitrogen percent is always the first of the three numbers on the package. For example, ammonium sulfate analysis is 21-0-0. This means that 21% of the contents of the package is plant available nitrogen. The University of Arizona Cooperative Extension has recommended nitrogen applications for most crops and fruit trees as well as turf and ornamentals. These recommendations are usually in pounds of nitrogen so that the gardener can use the nitrogen fertilizer they prefer. At this time of year, many gardeners are (or should be) fertilizing their warm season grass lawns (bermudagrass, zoysia, buffalograss, and other grasses that are dormant in the fall, and spring). To correctly apply a nitrogen fertilizer, some calculations must be done. For the math challenged, I will try to provide a cookbook method. The fertilizer recommendation for June, July, and August is 3/4 pound of nitrogen per 1,000 square feet of turf. First you must measure your turf area. A 40 x 75 foot area has 3,000 square feet. Next, calculate the amount of nitrogen required for the whole lawn. A 3,000 square foot lawn is 3 times larger than 1,000 square feet, so multiply 3 times 0.75 (3/4) pounds of nitrogen. The resulting number, 2.25 pounds of nitrogen, is the amount of real nitrogen to be applied. We still need to convert pounds of nitrogen to pounds of fertilizer. As stated above, ammonium sulfate is readily available, inexpensive, and 21% nitrogen by weight. Divide the pounds of nitrogen required by the lawn (2.25) by the decimal equivalent of the nitrogen percent in the fertilizer (0.21). 2.25 divided by 0.21 equals 10.7. This tells us that 10.7 pounds of ammonium sulfate are needed to fertilize your 3,000 square foot lawn in June, July, and August to keep it green and lush. Be sure to irrigate your lawn well after fertilization. Remember, this assumes that you can apply that amount of fertilizer evenly over the entire lawn. I recommend using a cyclone type spreader rather than one that simply drops the fertilizer out of the bottom. Do not try to apply all of your summer nitrogen in one application. You will be mowing the grass three times per week. For comparison sake, let's calculate the amount of steer manure, an organic fertilizer, that would be needed by the same 3,000 square foot lawn. Steer manure is about 1% nitrogen. Divide 2.25 by 0.01 and you learn that it takes 225 pounds of steer manure to fertilize this lawn each month during the summer. Your neighbors will love you for it. The University of Arizona Cooperative Extension has publications and information on composting and cover crops. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. The Yavapai County Cooperative Extension web site can be accessed at http://ag.arizona.edu/yavapai/. |