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How to Convert an Inorganic Fertilizer Recommendation to an Organic One Revised by Julia Gaskin, David Kissel, Glen Harris and George Boyhan Original manuscript by Wayne McLaurin, retired Horticulture Professor, and Water Reeves, retired Horticulture Educator Successful production of any crop begins with the soil. Organic Matter A fertile, biologically active soil provides plants with Organic matter consists of a wide variety of carbon- most of the nutrients needed for good growth. Fertil- containing compounds in the soil. It is created from izers can supplement or renew these nutrients, but they plant debris, roots, microbes and other organisms that should be added only when a soil test indicates the live in the soil. Organic matter provides energy and a levels of available nutrients in the soil are inadequate food source for biological activity. Many nutrients are for proper plant growth and high yields. held in organic matter until soil microorganisms de- compose the materials and release them for plant use. Whether you are growing annuals or perennials, veg- This is an important point, because although organic etables or flowers, most crops have a few short months growers add fertilizer in an organic form, the nutrients to grow and develop flowers and fruits. The soil must have to be converted to an inorganic form before they provide a steady, uninterrupted supply of readily avail- are available for plant use. For example, nitrogen in an able nutrients for maximum plant growth. Fertilizer organic fertilizer can be in the form of a protein (the form, particle size, solubility, the amount applied and organic form) that must be converted to ammonium the potential uptake are important factors in providing and/or nitrate before it can be taken up by plants. fertility for successful growth. Organic matter also helps attract and hold plant nu- Many farmers and gardeners use natural minerals and trients, reducing the amount lost through leaching. It organic fertilizers rather than synthetic ones to build improves the soil structure so that air reaches plant their soil. If you use organic materials as all or part of roots and also aids in retaining soil moisture. Because your fertilization program, this publication will help organic matter has such a strong influence on the you calculate the proper amount to use from the rec- chemical, biological and physical properties of the ommendations provided by a soil test. soil, building and maintaining soil organic matter is central to organic production. Certified Organic growers use fertilizers that meet standards in the National List by the USDA National Organic Program (www.ams.usda.gov/nop). The Organic Materials Review Institute (OMRI – www. OMRI.org), a private organization, evaluates and endorses products that meet these standards. If you are a Certified Organic grower, you should always check with your certifier before using new products. Georgia soil with high organic matter content. Fertilizer Labels – What They Mean other questions about soil sampling, contact your local Georgia law requires fertilizer producers to display the county Extension office for more information. guaranteed analysis or grade on a label on the fertilizer container. A fertilizer grade or analysis that appears on Soil test results do not include nitrogen because the the bag gives the percentages of nitrogen (N), phos- amount of plant-available nitrogen in soils can change quickly due to unpredictable weather conditions phate (P2O5) and potash (K2O) in the material. A 5-10- 15 grade fertilizer contains 5 percent N, 10 percent such as heavy rainfall. Consequently, tests of plant- available nitrogen taken weeks ahead of planting are P2O5 and 15 percent K2O. A 50-pound bag of 5-10-15 fertilizer contains 2.5 pounds of N (50 x 0.05 = 2.5), 5 not reliable and do not correlate well with crop yields. Instead, inorganic nitrogen fertilizer recommenda- pounds of P2O5 (50 x 0.10 = 5) and 7.5 pounds of K2O (50 x 0.15 = 7.5). tions are based on many research trials of crop yield response to nitrogen fertilizer rates. These trials and The fertilizer ratio is the ratio of the percentages of N, the resulting recommendations do not account for nitrogen available from a previous cover crop because P2O5 and K2O in the fertilizer mixture based on the nu- trient present in the smallest percentage. Examples of nitrogen from cover crops varies from season to sea- a 1-1-1 ratio fertilizer are 10-10-10 and 8-8-8. These son. Nitrogen fertilizer rates should be reduced by the fertilizers have equal amounts of nitrogen, phosphate amount of nitrogen available from a cover crop. This and potash. An example of a fertilizer with a 1-2-3 is called nitrogen credit. A quick estimate of nitrogen ratio is 5-10-15. This fertilizer would have twice as credits from a cover crop can be made using the cal- much phosphate and three times as much potash as culations found on pages 22-23 of “Managing Cover nitrogen. Crops Profitably” (see Reference section). Fertilizer Recommendations pH Although different crops have different nutrient needs, An underlying cause of poor fertility in Georgia is in general, crops need major nutrients in an approxi- acidic soil. Soil pH strongly influences plant growth, nutrient availability and microorganism activity in the mate ratio of 4-1-2 (N-P2O5-K2O). Because soils differ in their ability to supply nutrients and because the soil. It is important to keep soil pH in the proper range to obtain the best yields and high-quality growth. A proportion of N, P2O5 and K2O in any given organic fertilizer does not usually match the proportions a crop pH that is too low or too high can cause nutrients to needs, it is rare to be able to supply all plant nutrient become unavailable to plants. needs from only one organic material. Consequently, most organic fertilizers are used in combinations. The best pH range for most plant growth is 6.0 to 7.0. Table 1 lists commonly available organic fertilizers There are some exceptions, including Irish potatoes, blueberries and rhododendrons, which grow well at and the usual proportions of N, P2O5 and K2O. pHs of approximately 5.5, less than 5.0 and around There are no one-size-fits-all fertilizer recommenda- 5.5, respectively. tions. All fertilizer recommendations should take into consideration soil pH, residual nutrients and Most soils in Georgia are naturally acidic. Limestone inherent soil fertility as well as the needs of the crop that contains calcium and magnesium carbonates, to be grown. Fertilizer recommendations based on which increase soil pH, must be applied to keep the soil analyses are the best chance for getting the right soil pH in the proper range. A soil test is essential for amount of fertilizer without over- or under-fertilizing determining how much limestone should be applied. and result in the most efficient use of lime and fertil- This type of testing should be conducted at least every izer materials. This efficiency can occur only when two years. valid soil sampling procedures are used to collect the samples submitted for analyses. To be beneficial, a soil Calcium does not move quickly down through the soil sample must reliably represent the field, lawn, garden profile. If lime is recommended for vegetable crop or “management unit” from which it is taken. Infor- production, in most cases, limestone should be broad- mation on how to take a representative soil sample is cast and thoroughly incorporated to a depth of 6 to 8 referenced in the back of this publication. If you have inches before planting to neutralize the soil acidity in 2 UGA Extension Circular 853 • How to Convert an Inorganic Fertilizer Recommendation to an Organic One the root zone. For farmers using no-till, lime can be Calculating Organic Fertilizer Needs surface applied regularly to maintain pH. For best re- Soil test reports give fertilizer recommendations in sults, limestone should be applied two to three months three ways. The first, generally used by larger com- before seeding or transplanting. However, liming can mercial farms, gives the nutrients needed for a par- still be beneficial if applied at least one month before ticular crop in pounds per acre (lbs/ac). The second, seeding or transplanting. generally used for smaller farms or larger gardens, gives a particular fertilizer grade or combination of There are two types of limestone. One is composed grades that should be used per 1,000 square feet (sq. primarily of calcium materials and is referred to as cal- ft.). The third gives fertilizer grades per 100 feet of citic limestone. The second, known as dolomitic lime- row. Examples of how to make conversions in each of stone, contains both calcium and magnesium. Your these cases are included below. soil test will indicate which limestone is most suitable for your situation. If plant-available magnesium levels Because a combination of organic fertilizers is usually in the soil are low, dolomitic limestone is preferred. needed, the conversion process has several steps. In general, start with the most complex organic fertilizer, Environmental Effects on such as compost and animal manures (e.g., poultry Organic Nutrient Uptake litter). Many organic growers use these as a fertilizer Soil Temperature – Early spring in Georgia is cool base. These fertilizers will contain amounts of all three and soil temperatures rise slowly to the point where major nutrients – N, P and K – as well as micronutri- microorganisms are active. Until the soil warms suf- ents; however, the amount of nutrients in a given ani- ficiently and the organic fertilizer materials are broken mal manure or compost is variable, so these materials down into their usable form, these fertilizers may not should be analyzed.

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