Most nutrients needed by plants are supplied solely by soil. Insufficient supply of any of these nutrients may limit plant growth. In natural conditions, nutrients are recycled from plants to soil to meet plant needs. However, agricultural crops may require more nutrients than natural vegetation.
Significant amounts of nutrients are also removed in harvested crops. Optimal crop growth and profitability may require fertilization with inorganic fertilizers, animal manures, green manures, or legume management. This publication concentrates on commonly used inorganic fertilizers important in improving plant growth.
When managing fertilizers, stick to the four Rs: Use theright amountof theright fertilizerat theright placeat theright time.
The four Rs begin with soil testing. Soil tests assess the current nutrient status of the soil and indicate whether these levels are sufficient for crop production. If adequate amounts of nutrients are present in the soil, theright amountto apply is none.
If the laboratory results show response to added fertilizers is likely, there will be a rate recommendation. This is theright amount. Theright timeandright placedepend on site-specific agronomic factors accounting for crop biology and growth stage, and current environmental conditions. Follow the best management practice appropriate for your situation.
Conventions, Conversions, and Definitions
Fertilizer is labeled as nitrogen (N), phosphate (P2O5), and potash (K2O), respectively N-P-K, which are the oxide forms for elemental phosphorus (P) and potassium (K). In some cases, nutrients may be expressed either way. These are the simple conversions between the oxide and elemental forms:
Phosphorus
P × 2.3 = P2O5
P2O5× 0.44 = P
Potassium
K × 1.2 = K2O
K2O × 0.83 = K
Fertilizer recommendations by the Mississippi State University Extension Service Soil Testing Laboratory are listed as pounds of either phosphate or potash per acre.
Fertilizer grade or analysis is the weight percent of available nitrogen (N), phosphate (P2O5), and potash (K2O) in the material, expressed in the order N-P2O5-K2O. For example, 10-20-10 indicates the material is 10 percent N, 20 percent P2O5, and 10 percent K2O by weight.
Fertilizer ratio is the ratio of the weight percents of N-P2O5-K2O and is calculated by dividing the three numbers by the smallest of the three. Again using 10-20-10 fertilizer as an example, the ratio is 10/10-20/10-10/10 = 1-2-1.
If soil test-based recommendations call for certain amounts of plant food, you calculate the total fertilizer needed based on the grade of product.
A given weight of two fertilizers with different analyses (grades) has different amounts of actual plant food. One hundred pounds of a 10-30-10 fertilizer contains 10 pounds of N, 30 pounds of P2O5, and 10 pounds of K2O. One hundred pounds of a 7-21-7 fertilizer has 7 pounds of N, 21 pounds of P2O5, and 7 pounds of K2O. These fertilizers have the same nutrient ratio (1-3-1) but are different grades (10-30-10 versus 7-21-7), so different total amounts of fertilizer will have equal amounts of plant food. Application rates will be higher on a lower grade of product than a higher grade to supply the same amount of plant food.
Straight materials are the basic materials used in fertilizer manufacture. Many can be applied directly such as anhydrous ammonia, urea, urea-ammonium nitrate solutions, triple superphosphate, ammonium phosphates, and muriate of potash (potassium chloride).
Compound fertilizers are chemical or physical mixtures of the straight materials.
Considerations in Using Fertilizers
If the soil test-based recommendations include supplemental fertilizer, several factors must be considered to select theright source. These include physical and chemical properties, environmental stewardship, and economics.
Fertilizer Formulations
Many different physical and chemical forms of commercial fertilizer are available (seeTable1). Forms include solids, liquids, and gases. Each form has its own uses and limitations to consider when selecting the best material for the job.
Granulated fertilizermaterials are solid, hom*ogenous mixtures of fertilizer generally produced by combining raw materials such as anhydrous ammonia, phosphoric acid, and potassium chloride. Granulated materials are N-P or N-P-K grades of fertilizer. Each uniform-sized fertilizer particle contains the nutrients listed in the grade; each particle in a 10-20-10 granulated fertilizer contains 10 percent nitrogen, 20 percent phosphate, and 10 percent potash. The chief advantage of granulated materials is this uniform nutrient distribution. They are not separated in handling or spreading, and the applied nutrients are potentially available to plant roots. Granulated fertilizers are usually very simple to handle, with little tendency to cake or dust.
Blended fertilizersare mixtures of dry fertilizer materials. The ingredients of a blended fertilizer can be straight materials, such as urea or potassium chloride, or granulated compound fertilizer materials mixed together, or a combination of the two.
In blended fertilizers, the individual particles remain separate in the mixture, and the nutrients also may physically separate. This can be less problematic if materials are the same size. Properly made blends are generally as effective as other compound fertilizers. Blends have the advantage of allowing a very wide range of fertilizer grades to be mixed. This allows fertilizers to meet a soil-test recommendation.
Blends are often used as starter fertilizers, but urea and diammonium phosphate should not be used as starter fertilizers placed close to seeds because both materials produce free ammonia, which hinders seed germination and seedling growth.
Fluid fertilizersare used widely in Mississippi. These may be either straight materials, such as nitrogen solutions, or compound fertilizers of various grades. The major advantage is ease of handling. The disadvantages are that only relatively low analyses are possible, especially when the material contains potassium, and the cost per unit of nutrients is generally higher.
Fluid fertilizers are either clear solutions or suspensions. In clear solutions, nutrients are completely dissolved in water. Phosphorus in these materials is highly water soluble. Clear solutions are equal in agronomic effectiveness to other types of fertilizers, when equal amounts of plant food are compared.
Suspension fertilizers are fluids in which solubility of the components has been exceeded; clay is added to keep the very fine, undissolved fertilizer particles from settling out. The advantages are that they can be handled as a fluid and can be formulated at much higher analyses than clear solutions. These formulations may contain analyses as high as dry materials. Suspensions require constant agitation, even in storage, and suspension fertilizer cannot be used as a carrier for certain chemicals. As with clear solutions, the agronomic effectiveness of suspensions is equal to other types of fertilizer materials when equal nutrient levels are applied.
Fertilizer grade or analysis is always referred to on a weight percent basis, not on a volume (gallon) basis. Thus, to determine the actual plant nutritive value, you must know the weight per gallon of the material. Most fluids weigh between 10 and 12 pounds per gallon. More detailed information on fluid fertilizers is available in MSU ExtensionPublication 1466 Fluid Fertilizers.
Gaseous fertilizerrequires special considerations for handling and use. Anhydrous ammonia is high-analysis nitrogen gaseous fertilizer used directly in crop production. It is also the precursor material for the manufacture of most common nitrogen-containing fertilizers.
N | P2O5 | K2O | Form | |
|---|---|---|---|---|
Ammonium nitrate | 33.5–34 | Solid | ||
Ammonium polyphosphate – a | 10 | 34 | Liquid | |
Ammonium polyphosphate – b | 11 | 37 | Liquid | |
Ammonium sulfate | 21 | Solid | ||
Ammonium thiosulfate | 12 | Liquid | ||
Anhydrous ammonia | 82 | Gas | ||
Aqua ammonia | 20 | Liquid | ||
Calcium nitrate | 16 | Solid | ||
Diammonium phosphate | 18 | 46 | Solid | |
Monoammonium phosphate | 11 | 48–55 | Solid | |
Muriate of potash (potassium chloride) | 60–62 | Solid | ||
Ordinary superphosphate | 20 | Solid | ||
Potassium nitrate | 13 | 44 | Solid | |
Potassium sulfate | 50 | Solid | ||
Potassium-magnesium sulfate | 20 | Solid | ||
Sodium nitrate | 16 | Solid | ||
Triple superphosphate | 46 | Solid | ||
Urea | 45–46 | Solid | ||
Urea-ammonium nitrate | 28–32 | Liquid |
Once in the soil, anhydrous ammonia behaves similarly to any other ammonium-based source, but because anhydrous ammonia is stored as a compressed liquid, special handling methods and safety precautions are required. Anhydrous ammonia expands immediately into a gas when released. Thus, it must be injected into the soil to prevent the gas from escaping. Anhydrous ammonia can cause serious chemical burns and asphyxiation if it escapes, so follow safety precautions.
Fertilizer Properties
The specific chemical properties of fertilizers are complex and varied; this publication provides only an introduction. The fertilizer properties to consider are solubility, particle size, soil pH, chemical form, and soluble salts.
Solubilityindicates how readily nutrients are dissolved in the soil water and taken up by plants. Since nitrogen and potassium in fertilizers are essentially completely soluble in water, their solubility is not an issue for common fertilizer sources.
Phosphorus must be dissolved in water to be taken up by plants. The water solubility of available phosphorus can vary from 0 to 100 percent. Generally, the higher the water solubility, the more effective the phosphorus source is for short-season, fast-growing crops, for crops with restricted root systems, for starter fertilizers, and for situations where less than optimal rates of phosphorus are applied to low-fertility soils. Water solubility of the available phosphorus is less important in other applications. Thus, phosphorus soluble in neutral ammonium citrate (which includes water-soluble phosphorus) is counted as available phosphorus on the fertilizer label.
Fortunately, most common phosphorus sources (triple superphosphate and the ammonium phosphates) contain highly water-soluble forms of phosphorus. There is no apparent difference in agronomic effectiveness whether a highly water-soluble phosphorus source is applied as a fluid fertilizer or as a dry fertilizer. Note, however, that raw rock phosphate has very low water solubility, so it is very slow to react.
The particle sizeof fertilizers is important for both agronomic and handling reasons. In the field, particle size is most important for sparingly soluble materials such as rock phosphate. These must be very finely ground to promote solubility by increasing the surface area that reacts with water. For most soluble fertilizers, particle size is important in determining ease of handling of the materials. Very fine materials, which become dusty and cake up, are difficult to handle; granular materials do not have these problems and are easier to handle. While there is no official standard particle size, many fertilizers pass through a No. 6 (coarse) screen but not a No. 18 (finer) screen. Particle size is an important consideration for materials in blended products. Differently sized materials segregate as the fertilizer is handled and spread. The most important factor in stable, high-quality blended fertilizers is compatible particle sizes.
Soil pHcan be changed by fertilizers. The most important reaction is the microbial oxidation of ammonium nitrogen to nitrate nitrogen. This occurs regardless of the source of ammonium nitrogen (fertilizer, manure, or organic residues). There is a short-lived soil pH reduction in the vicinity of anhydrous ammonia injection slits.
The acidity of a fertilizer is usually given as the amount of pure limestone that would be required to offset the acidity produced by the reaction of the fertilizer. Equivalent acidities can be used to compare materials, but the actual amount of limestone required to neutralize the acidity from the fertilizer is probably greater than shown inTable2.
Material | Equivalent acidity (lb CaCO3per lb of N) |
|---|---|
Anhydrous ammonia | 1.8 |
Urea | 1.8 |
Ammonium nitrate | 1.8 |
Manure | 1.8 |
Diammonium phosphate (DAP) | 3.5 |
Ammonium sulphate | 5.3 |
Monoammonium phosphate (MAP) | 5.3 |
Another temporary pH change occurs with the superphosphate materials. The initial reaction lowers the pH around the fertilizer particle, but the residual effect of the superphosphates is very little change in soil pH. The common potassium materials are neutral salts that have little effect on the soil pH.
Chemical formsof the nutrient in the fertilizer are critical for agronomic crops only in specific situations. There is generally little practical difference, for example, between an ammonium and a nitrate nitrogen source in bulk situations. However, if leaching or denitrification are potential issues, then the ammonium form is preferred as it attaches to the soil cation exchange sites. Nitrate will stay in soil solution longer.
A consideration between orthophosphates and polyphosphates is whether insoluble micronutrients are added to a liquid fertilizer. In this situation, polyphosphates are preferred. Potassium sulfate should be considered if the crop (tobacco, for example) is sensitive to soil chloride.
High concentrations ofsoluble saltsin soil solution injure or kill plants or prevent seed germination. Normally, fertilizers that are uniformly distributed at recommended rates do not cause soluble salt levels high enough to damage plants. Concentrated applications of fertilizer or manure in contact with, or in a band near, plants can damage a germinating seed or growing plant. A salt index is used to estimate potential injury from different fertilizers. It is a relative scale to compare fertilizers for special placement, such as for drilling with the seed, banding at high rates, or pop-up treatments.Table3shows the salt index for several common fertilizer materials.
Material | Salt index |
|---|---|
Nitrogen (N) | |
Ammonium sulfate | 54 |
Ammonium nitrate | 49 |
Urea | 27 |
Anhydrous ammonia | 10 |
Phosphate (P2O5) | |
Triple superphosphate | 4 |
Monoammonium phosphate (MAP) | 7 |
Diammonium phosphate (DAP) | 8 |
Potash (K2O) | |
Potassium chloride | 32 |
Potassium sulfate | 14 |
Environmental Responsibility
Nutrient management using the four Rs (theright amountof theright fertilizerat theright timein theright place) should minimize detrimental environmental effects while maximizing agronomic and economic benefits. Proper nutrient management planning evaluates all potential nutrient sources, soil test levels, crop management needs, and environmental risk factors.
Implementing the four Rs includes these best management practices:
- Regular soil tests. The Mississippi State University Extension Service recommends soil testing at least every 3 years. Sample at the same time of year for year-to-year comparisons.
- Use realistic yield goals to determinenitrogen application rates if they are part of the recommendations.
- Select the most suitable nitrogen fertilizerfor the crop, application method, and climatic conditions.
- Use the proper application technique forthe situation.
- Maintain and calibrate applicationequipment.
- Avoidapplication to surfacewaters.
- Timenutrient applications appropriately for most agronomic benefit and minimalenvironmental impact.
- Control soil erosion because manynutrients move when soil particles move.
- Properly control waterflow.Slow waterdown when appropriate with conservation practices, or speed water movement whenappropriate.
- Use cover crops.
- Maintain residueon the soil surface.
More information on best management practice selection and implementation is available from your local MSU Extension office, local offices of the Natural Resources Conservation Service, and theMSU REACH program. More resources are listed at the end of this publication.
Economics
Fertilizer is a significant investment of money and time. Fertilizers work best when provided to growing plants when they need them using appropriate technology and careful decisions. The final decision about which fertilizer to use should be based on economics. Compare materials on the price per pound of actual plant food. Use this formula:
(Price per ton of fertilizer) / (2000 × plant food content as decimal value) = per pound of plant food
For example: Urea costs $385 per ton. Each ton of urea is 45 percent nitrogen, so each ton contains 900 pounds of nitrogen (2000 × 0.45). Dividing $385 by 900 shows the cost is $0.43 per pound of nitrogen in the fertilizer.
The maximum return per dollar invested in fertilizer is achieved from the first increment of needed nutrients applied to a deficient soil or crop. The maximum profit depends on site-specific factors such as weather or pest challenges. Using unnecessary amounts of fertilizer as insurance drains profitability.
For More Information
More information on managing nutrients with either inorganic or organic sources, soil testing, nutrient management planning, best management practices, and landscape environmental stewardship is availableonline.
Look for these publications:
Publication 2647Nutrient Management Guidelines for Agronomic Crops Grown in Mississippi
Information Sheet 346Soil Testing for the Farmer
Information Sheet 1614Soil and Broiler Litter Testing Basics
Publication 3050Natural Resource Conservation in Agriculture
Information Sheet 1620Useful Nutrient Management Planning Data
Information Sheet 1853Nutrient Management Planning Basics
Publication 1466Fluid Fertilizers
Information Sheet 767Nitrogen in Mississippi Soils
Information Sheet 871Phosphorus in Mississippi Soils
Information Sheet 894Potassium in Mississippi Soils
Information Sheet 1038Micronutrients in Crop Production
Information Sheet 1039Secondary Plant Nutrients: Calcium, Magnesium, and Sulfur
Publication 2500(POD-08-20)
ByLarry Oldham, PhD, Extension Professor, andKeri Jones, PhD, Laboratory Coordinator, Plant and Soil Sciences.
Copyright 2020 by Mississippi State University. All rights reserved. This publication may be copied and distributed without alteration for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.
Produced by Agricultural Communications.
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Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. GARY B. JACKSON, Director
