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. A fertile, biologically active soil provides plants with most of the nutrients needed for good growth. Fertilizers can supplement or renew these nutrients, but they should be added only when a soil test indicates the levels of available nutrients in the soil are inadequate for proper plant growth and high yields.

Whether you are growing annuals or perennials, vegetables or flowers, most crops have a few short months to grow and develop flowers and fruits. The soil must provide a steady, uninterrupted supply of readily available nutrients for maximum plant growth. Fertilizer form, particle size, solubility, the amount applied and the potential uptake are important factors in providing fertility for successful growth.

Many farmers and gardeners use natural minerals and organic fertilizers rather than synthetic ones to build their soil. If you use organic materials as all or part of your fertilization program, this publication will help you calculate the proper amount to use from the recommendations provided by a soil test.

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.

Organic Matter

Georgia soil with high organic matter content.

Organic matter consists of a wide variety of carboncontaining compounds in the soil. It is created from plant debris, roots, microbes and other organisms that live in the soil. Organic matter provides energy and a food source for biological activity. Many nutrients are held in organic matter until soil microorganisms decompose the materials and release them for plant use. This is an important point, because although organic growers add fertilizer in an organic form, the nutrients have to be converted to an inorganic form before they are available for plant use. For example, nitrogen in an organic fertilizer can be in the form of a protein (the organic form) that must be converted to ammonium and/or nitrate before it can be taken up by plants.

Organic matter also helps attract and hold plant nutrients, reducing the amount lost through leaching. It improves the soil structure so that air reaches plant roots and also aids in retaining soil moisture. Because organic matter has such a strong influence on the chemical, biological and physical properties of the soil, building and maintaining soil organic matter is central to organic production.

Fertilizer Labels — What They Mean

Georgia law requires fertilizer producers to display the guaranteed analysis or grade on a label on the fertilizer container. A fertilizer grade or analysis that appears on the bag gives the percentages of nitrogen (N), phosphate (P₂O₅) and potash (K₂O) in the material. A 5-10- 15 grade fertilizer contains 5 percent N, 10 percent 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 pounds of P₂O₅ (50 x 0.10 = 5) and 7.5 pounds of K₂O (50 x 0.15 = 7.5).

The fertilizer ratio is the ratio of the percentages of N, P₂O₅ and K₂O in the fertilizer mixture based on the nutrient present in the smallest percentage. Examples of a 1-1-1 ratio fertilizer are 10-10-10 and 8-8-8. These fertilizers have equal amounts of nitrogen, phosphate and potash. An example of a fertilizer with a 1-2-3 ratio is 5-10-15. This fertilizer would have twice as much phosphate and three times as much potash as nitrogen.

Fertilizer Recommendations

Although different crops have different nutrient needs, in general, crops need major nutrients in an approximate ratio of 4-1-2 (N-P₂O₅-K₂O). Because soils differ in their ability to supply nutrients and because the proportion of N, P₂O₅ and K₂O in any given organic fertilizer does not usually match the proportions a crop needs, it is rare to be able to supply all plant nutrient needs from only one organic material. Consequently, most organic fertilizers are used in combinations. Table 1 lists commonly available organic fertilizers and the usual proportions of N, P₂O₅ and K₂O.

There are no one-size-fits-all fertilizer recommendations. All fertilizer recommendations should take into consideration soil pH, residual nutrients and inherent soil fertility as well as the needs of the crop to be grown. Fertilizer recommendations based on soil analyses are the best chance for getting the right amount of fertilizer without over- or under-fertilizing and result in the most efficient use of lime and fertilizer materials. This efficiency can occur only when valid soil sampling procedures are used to collect the samples submitted for analyses. To be beneficial, a soil sample must reliably represent the field, lawn, garden or “management unit” from which it is taken. Information on how to take a representative soil sample is referenced in the back of this publication. If you have other questions about soil sampling, contact your local county Extension office for more information.

Soil test results do not include nitrogen because the amount of plant-available nitrogen in soils can change quickly due to unpredictable weather conditions such as heavy rainfall. Consequently, tests of plantavailable nitrogen taken weeks ahead of planting are not reliable and do not correlate well with crop yields. Instead, inorganic nitrogen fertilizer recommendations are based on many research trials of crop yield response to nitrogen fertilizer rates. These trials and the resulting recommendations do not account for nitrogen available from a previous cover crop because nitrogen from cover crops varies from season to season. Nitrogen fertilizer rates should be reduced by the amount of nitrogen available from a cover crop. This is called nitrogen credit. A quick estimate of nitrogen credits from a cover crop can be made using the calculations found on pages 22-23 of “Managing Cover Crops Profitably” (see Reference section).

pH

An underlying cause of poor fertility in Georgia is acidic soil. Soil pH strongly influences plant growth, nutrient availability and microorganism activity in the soil. It is important to keep soil pH in the proper range to obtain the best yields and high-quality growth. A pH that is too low or too high can cause nutrients to become unavailable to plants.

The best pH range for most plant growth is 6.0 to 7.0. There are some exceptions, including Irish potatoes, blueberries and rhododendrons, which grow well at pHs of approximately 5.5, less than 5.0 and around 5.5, respectively.

Most soils in Georgia are naturally acidic. Limestone that contains calcium and magnesium carbonates, which increase soil pH, must be applied to keep the soil pH in the proper range. A soil test is essential for determining how much limestone should be applied. This type of testing should be conducted at least every two years.

Calcium does not move quickly down through the soil profile. If lime is recommended for vegetable crop production, in most cases, limestone should be broadcast and thoroughly incorporated to a depth of 6 to 8 inches before planting to neutralize the soil acidity in the root zone. For farmers using no-till, lime can be surface applied regularly to maintain pH. For best results, limestone should be applied two to three months before seeding or transplanting. However, liming can still be beneficial if applied at least one month before seeding or transplanting.

There are two types of limestone. One is composed primarily of calcium materials and is referred to as calcitic limestone. The second, known as dolomitic limestone, contains both calcium and magnesium. Your soil test will indicate which limestone is most suitable for your situation. If plant-available magnesium levels in the soil are low, dolomitic limestone is preferred.

Environmental Effects on Organic Nutrient Uptake

Soil Temperature

Early spring in Georgia is cool and soil temperatures rise slowly to the point where microorganisms are active. Until the soil warms sufficiently and the organic fertilizer materials are broken down into their useable form, these fertilizers may not successfully stimulate plant growth.

Soil Moisture

In addition to warmer temperatures, soil microorganisms need a moist soil to grow and thrive. If rainfall is not adequate, crops may need to be irrigated for good nutrient release.

Calculating Organic Fertilizer Needs

Soil test reports give fertilizer recommendations in three ways. The first, generally used by larger commercial farms, gives the nutrients needed for a particular crop in pounds per acre (lbs/ac). The second, generally used for smaller farms or larger gardens, gives a particular fertilizer grade or combination of grades that should be used per 1,000 square feet (sq. ft.). The third gives fertilizer grades per 100 feet of row. Examples of how to make conversions in each of these cases are included below.

Because a combination of organic fertilizers is usually needed, the conversion process has several steps. In general, start with the most complex organic fertilizer, such as compost and animal manures (e.g., poultry litter). Many organic growers use these as a fertilizer base. These fertilizers will contain amounts of all three major nutrients — N, P and K — as well as micronutrients; however, the amount of nutrients in a given animal manure or compost is variable, so these materials should be analyzed. The amount of moisture in animal manures and composts can greatly affect the amount of nutrients applied. The University of Georgia reports nutrient content based on the moisture in the sample as it was received; consequently, these numbers do not have to be corrected for moisture content.

If you don?t have the animal manures and composts tested, approximate values for N, P₂O₅ and K₂O are listed in Table 1. When using these materials as a fertilizer base, calculate how much N, P₂O₅ and K₂O are supplied by these materials, then supplement nutrients from other sources as needed for a particular crop.

Examples 1 and 2 describe a method to balance a crop's nutrient needs with fertilizers and compost. This method will help prevent nutrient imbalances in the soil. You may need to try several different combinations of fertilizers or amendments to find the best combination. You should also compare costs of various combinations.

Another way of converting the inorganic fertilizer recommendations to organic ones is to look for organic fertilizer that contributes most of one nutrient. You can then calculate the amount of each fertilizer you need to meet the crop's needs. Example 3 shows you how to use this approach.

NOTE: Wood ash has long been used as a source of K₂O; however, it should be used sparingly. Overapplication can raise the pH above the recommended range for crops and can create problems due to high salt concentrations. If you use wood ash, it is recommended that no more than 10 to 12 lbs be used per 1,000 sq. ft. per year, or about 1 lb per 100 ft. of row. An analysis of the wood ash will help you know how much to apply.

Example 1: Conversion for Farms on an Acre Basis

Farmer Jolene receives a soil test report for Plot 1 that indicates the soil organic matter is 1.5%, the pH is 6.0, the soil test P is medium and the soil test K is low. She will be growing peppers in this section next spring. The soil test fertilizer recommendations call for: 150 lbs/acre of N, 80 lbs/acre of P₂O₅ and 120 lbs/acre of K₂O. She usually applies 1 ton of poultry litter compost (3-4-3) over her 1-acre plot and tills it in to build organic matter.

These calculations indicate that much of the N and all of the K₂O needs for the pepper crop can be met by applying the usual 2,000 lbs of compost plus 1,200 lbs of the organic 8-5-5 on Jolene?s 1-acre plot. Notice that with this combination of fertilizers, P₂O₅ is overapplied. Because Jolene?s soil test P is in the medium range and all the compost P may not be immediately available, this is not an immediate problem. But, if she continues to use this combination, she will end up with high levels of phosphorus in her soils. In some cases this can cause environmental problems. For true sustainability, she should try to better match the crop needs with the applied P₂O₅. By our calculations, Jolene is still 48 lbs of N short. She would need to use an N-only fertilizer like blood meal to make up this difference.

Example 2: Conversion on a 1,000 sq. ft. Basis

This is the same scenario as above with Farmer Jolene, except she is working with a 1,000 sq. ft. plot. She usually puts out 50 lbs of compost as her base soil amendment.

In this case, Farmer Jolene is overapplying both P₂O₅ and K₂O. Because her soil test P₂O₅ is medium and K₂O is low, the overapplication will not be detrimental at this point. The overapplication of K₂O will help build the soil test K into a medium or high range.

Example 3: Working with Fertilizer Grades on 1,000 sq. ft. Basis

Gardener Joe has received his soil test report for his 1,000-sq.-ft. garden. The soil test report indicates the pH is 5.5 and recommends 20 lbs of lime to correct the soil pH. It also recommends 20 lbs of 5-10-15 plus 1 lb of 34-0-0 per 1,000 sq. ft.

These calculations indicate Farmer Joe can meet his garden?s nutrient needs by applying 11 lbs of blood meal, 18.2 lbs of steamed bone meal and 13.6 lbs of Sulfate of Potash Magnesia.

Example 4: Organic Fertilizer for 100 Feet of Row

Farmer Jack?s soil test results recommend 7 lbs of 5-10-15 plus 0.5 lbs of ammonium nitrate per 100 linear feet of garden row.

Farmer Jack needs to apply an additional 0.5 lbs K₂O per 100 ft. of row. He can use Sulfate of potash Magnesia, as in Example 2.

Summary

These examples can help you convert inorganic fertilizer recommendations to organic ones. If you need further help, your local county agent is a good resource. How you use these inorganic fertilizer recommendations may depend on whether the field is in a soil building or soil maintenance stage. New and transitioning growers often have fields that need more soil organic matter and available nutrients because the soils are moving to a new biological and chemical equilibrium. If you are in a soil building stage, following the fertilizer recommendations will help you improve your soil fertility and quality.

Fields are likely in the soil building stage if:

• soil organic matter is below 1.5% in the Coastal Plain or 2.5% above the fall line, and/or
• soil test indices for P, K and other nutrients are in the low or medium range.

Many organic growers reduce fertilizer use by 10 to 20% in the soil maintenance stage. The percent reduction depends on site-specific conditions such as the amount of soil organic matter buildup and nutrients available as well as yield goals.

References and Further Information

Soil Testing for Home Lawns, Gardens and Wildlife Food Plots. University of Georgia Extension Circular 896. http://extension.uga.edu/publications/detail.cfm?number=C896

Managing Cover Crops Profitably. http://www.sare. org/Learning-Center/Books/Managing-Cover-Crops- Profitably-3rd-Edition

Manures for Organic Crop Production (ATTRA) https://attra.ncat.org/attra-pub/manures.html#fieldapp

Organic Fertilizer Calculator (Oregon State University) http://smallfarms.oregonstate.edu/sites/default/ files/em8936-e_med_res_0.pdf

For those who do not want to figure out the equivalent weights, here is an approximation of amounts of ingredients to use to attain the correct amounts of organic fertilizers called for in the soil test for 1,000 sq. ft. You should not rely on these approximations without doing soil testing to confirm nutrient needs.

Status and Revision History
Published on Nov 01, 2000
Published on Feb 26, 2009
Published with Major Revisions on Sep 28, 2011
Published with Full Review on Sep 15, 2014