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Inorganic Fertilizers For Crop Production

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Publication Number: P2500
Updated: May 4, 2017
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Thirteen of the nutrients plants need are supplied solely by soil. Insufficient supply of any of these nutrients may limit plant growth.

Under natural conditions, nutrients are recycled from plants to soil to meet plant needs. This balance often differs with agricultural crops, because crops demand more nutrients than natural vegetation.

Significant amounts of nutrients are also removed in harvested crops. Because of these factors, supplemental nutrients may be added to the soil to ensure optimal crop growth and profitability. These supplements may include fertilizers, animal manures, green manures, and legumes. This publication concentrates on the properties of commonly used inorganic fertilizers important in improving plant growth.

Soil testing should be the basis for any fertilizer application. A good soil testing program indicates the current fertility status of the soil and provides sound guidelines for managing fertility to achieve optimal yields. Good records of soil test results and fertilizer, lime, and manure applications are also very important to proper nutrient management.

 

Important Conventions, Conversions, and Definitions

Nutrients are expressed on fertilizer labels as nitrogen (N), phosphate (P2O5), and potash (K2O), respectively (N-P-K). This is called the oxide form for elemental phosphorus (P) and potassium (K). In some cases, nutrients may be expressed in either form. Following are simple conversions between the oxide and elemental forms:

Phosphorus

P × 2.3 = P2O5

P2O5 × .44 = P

 

Potassium

K × 1.2 = K2O

K2O × .83 = K

Fertilizer recommendations by the Mississippi State University Extension Service Soil Testing Laboratory are made in terms of pounds of phosphate or potash per acre.

Fertilizer grade or analysis is the weight percent of available nitrogen (N), phosphate (P2O5), and potash (K2O) in the fertilizer, usually expressed as N-P2O5-K2O. For example, 10-20-10 indicates 10 percent N, 20 percent P2O5, and 10 percent K2O by weight.

Fertilizer ratio is the ratio of weight percents of N-P2O5-K2O and is determined by dividing the three numbers by the smallest of the three.

Again using 10-20-10 as an example, the ratio is 10/10-20/10-10/10 = 1-2-1.

After soil tests indicate the need for a certain amount of plant food, you must determine the total amount of fertilizer needed, based on the grade of product. A given weight of two fertilizers with different analyses or grade contain different amounts of actual plant food. For example, 100 pounds of a 10-30-10 fertilizer contains 10 pounds of N, 30 pounds of P2O5, and 10 pounds of K2O, but 100 pounds of a 7-21-7 fertilizer contains 7 pounds of N, 21 pounds of P2O5, and 7 pounds of K2O. Both of these fertilizers have the same nutrient ratio (1-3-1) but different grades (10-30-10 versus 7-21-7). Different total amounts of fertilizer are required to provide equal amounts of plant food. Application rates must be determined on the basis of the plant food needed.

Straight materials are the basic materials used in fertilizer manufacture. Many of these materials can be applied directly. Examples include anhydrous ammonia, urea, urea-ammonium nitrate solutions, triple superphosphate, ammonium phosphates, and muriate of potash (potassium chloride).

Compound fertilizers are made by chemically or physically combining the straight materials.

 

Considerations in Using Fertilizers

Once soil test results say you need supplemental fertilizer, you must consider several important factors in selecting the right one. These factors include physical and chemical properties, environmental stewardship, economics, and dealer service.

Table 1. Common inorganic fertilizers used for agronomic crop production in Mississippi.

Not including granulated fertilizers (such as 13-13-13 or 7-21-7).

 

N

P2O5

K2O

Form

Ammonium nitrate

33.5–34

0

0

Solid

Ammonium polyphosphate – a

10

34

0

Liquid

Ammonium polyphosphate – b

11

37

0

Liquid

Ammonium sulfate

21

0

0

Solid

Ammonium thiosulfate

12

0

0

Liquid

Anhydrous ammonia

82

0

0

Gas

Aqua ammonia

20

0

0

Liquid

Calcium nitrate

16

0

0

Solid

Diammonium phosphate

18

46

0

Solid

Monoammonium phosphate

11

48–55

0

Solid

Muriate of potash (potassium chloride)

0

0

60–62

Solid

Ordinary superphosphate

0

20

0

Solid

Potassium nitrate

13

0

44

Solid

Potassium sulfate

0

0

50

Solid

Potassium-magnesium sulfate

0

0

20

Solid

Sodium nitrate

16

0

0

Solid

Triple superphosphate

0

46

0

Solid

Urea

45–46

0

0

Solid

Urea-ammonium nitrate

28–32

0

0

Liquid

Fertilizer Formulations

Many different physical and chemical forms of commercial fertilizer are available (see Table 1). Fertilizer materials can be solids, liquids, or gases. Each physical form has its own uses and limitations, which provide the basis for selecting the best material for the job.

Granulated fertilizer materials are solid, homogenous mixtures of fertilizer materials 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 all of the nutrients in the grade. For example, each particle in a 10-20-10 granulated fertilizer theoretically contains 10 percent nitrogen, 20 percent phosphate, and 10 percent potash. The main advantage of granulated materials is this uniform distribution of nutrients. The nutrients are not separated in handling or spreading, and plant roots absorb a complete set of the applied nutrients. Granulated fertilizers generally have good handling properties, with little tendency to cake or dust.

Blended fertilizers are simple physical mixtures of dry fertilizer materials. The ingredients of a blended fertilizer can be straight materials, such as urea or potassium chloride; they can be granulated compound fertilizer materials mixed together; or they can be a combination of the two. In blended fertilizers, the individual particles remain separate in the mixture, and the nutrients can separate. This problem can be reduced 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. This makes it possible to match a fertilizer exactly to 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 can hinder seed germination and seedling growth.

Fluid fertilizers are used widely in Mississippi. Fluids can be either straight materials, such as nitrogen solutions, or compound fertilizers of various grades. Fluid fertilizers are categorized into two groups: clear solutions and suspensions.

In clear solutions, nutrients are completely dissolved in water. The major advantage is ease of handling. In addition, the phosphorus in these materials is highly water soluble. 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. 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 and clay has been added to keep the very fine, undissolved fertilizer particles from settling out. The major advantage is that they can be handled as a fluid. Another advantage is that they can be formulated at much higher analyses than clear solutions. These formulations may contain analyses as high as dry materials. The major disadvantages are that suspensions require constant agitation, even in storage, and suspension fertilizer cannot be used as a carrier for certain chemicals. As in the case of clear solutions, the agronomic effectiveness of suspensions is equal to other types of fertilizer materials when equal amounts of plant food are compared.

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.

Gaseous fertilizer requires some special considerations in handling and use. Anhydrous ammonia is a high-analysis nitrogen gaseous fertilizer used both in the manufacture of all other common nitrogen-containing fertilizers and in direct applications to the soil. Once applied, anhydrous ammonia behaves similarly to any other ammonium nitrogen source. But special handling methods and safety precautions are required, because anhydrous ammonia is stored as a compressed liquid. When expansion occurs during application to the soil, it immediately becomes a gas. Thus, it must be injected into the soil to prevent the gas from escaping.

Some hazards are involved in handling anhydrous ammonia. Since the material can cause serious chemical burns and asphyxiation, proper safety precautions are necessary. Anhydrous ammonia is an excellent nitrogen fertilizer, but you must handle it properly.

Organic materials commonly used as fertilizers have many varied properties, so you must individually evaluate the physical properties of these materials. Since the specific chemical properties of fertilizers also are very complex and varied, a detailed discussion of all their chemical properties is not possible here. But you will need to consider several important chemical properties in selecting a fertilizer material. These properties are solubility, particle size, soil pH, chemical form, and soluble salts.

 

Fertilizer Properties

Solubility indicates how readily nutrients are dissolved in the soil water and taken up by plants. Since the nitrogen and potassium in fertilizers are essentially completely soluble in water, their solubility is not a major consideration for the common fertilizer sources. Only phosphorus that is soluble in neutral ammonium citrate (this includes water-soluble phosphorus) is counted as available phosphorus on the fertilizer label.

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. 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 when a highly water-soluble phosphorus source is applied as a fluid fertilizer or as a dry fertilizer. Materials such as raw rock phosphate have very low water solubility.

Particle size of a fertilizer material can be important for both agronomic and handling reasons. In agronomic applications, particle size is most important for sparingly soluble materials such as rock phosphate. These materials must be very finely ground to ensure sufficient solubility. For most soluble fertilizers, particle size is not critical for agronomic purposes but is very important in determining ease of handling of the materials. Very fine materials, which often become dusty and can cake, are difficult to handle; granular materials are sized to avoid these problems and to promote handling convenience. While there is no standard for particle size, most fertilizers are sized to pass through a No. 6 (coarse) screen but be retained on a No. 18 screen (finer). Particle size is most critical for materials used in blended products. Materials of different sizes tend to segregate as the fertilizer is handled and spread. Particle size has been identified as the most important factor in producing a stable, high-quality blended fertilizer.

Soil pH can be changed by the reaction of fertilizer materials. The most important such 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). The acidity of a fertilizer is usually given by convention as the amount of pure limestone that would be required to offset the acidity produced by the reaction of the fertilizer.

Table 2. Equivalent acidity of some fertilizer materials.

Material

Equivalent acidity
(lb CaCO3 per 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

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 in Table 2. Many of these materials greatly, but temporarily, increase the soil pH. Another example of this temporary pH change is the reaction of the superphosphate materials. The initial reaction is a drastic lowering of the pH around the fertilizer particle, but the residual effect of the superphosphates changes the soil pH very little. The common potassium materials are neutral salts that have no effect on the soil pH.

Chemical forms of the nutrient itself are critical for agronomic crops only in special situations. There is generally little practical difference, for example, between an ammonium and a nitrate nitrogen source (if leaching or denitrification are serious potential problems, then the ammonium form is preferred) or between orthophosphates and polyphosphates (unless insoluble micronutrients are added to a liquid fertilizer, in which case the polyphosphates are preferred) or between potassium chloride and potassium sulfate (some crops such as tobacco are sensitive to chloride, in which case the sulfate is preferred).

Soluble salts, at high concentrations in soil solution, can injure or kill plants or prevent germination of seeds. Under normal conditions, fertilizers uniformly distributed at recommended rates do not cause soluble salt levels high enough to damage plants. But a concentrated application of fertilizer or manure placed in contact with the seed or in a band near the germinating seed or growing plant can cause damage. An estimate of potential salt injury from different fertilizers is given as the salt index for that material. The salt index is a relative scale useful for comparing materials for special placement (such as for drilling with the seed, banding at high rates, and for pop-up treatments) when a low salt index is preferred. Table 3 shows the salt index for several common fertilizer materials.

Table 3. Salt indices of various fertilizers, assuming equal weights of the nutrient.

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

Fertilizers are significant investments of money and time that require diligent management; they work best when provided to growing plants when they need them by using appropriate technology and careful decisions. ­­The goal is fertilizer management that will minimize detrimental environmental effects while maximizing agronomic and economic benefits. Nutrient Management Planning evaluates all potential nutrient sources, soil test levels, crop management needs, and environmental risk factors.

Nutrient Management Planning is implemented through Best Management Practice (BMP) application. These are some BMPs:

  • Regular soil tests. The Mississippi State University Extension Service recommends soil testing at least every 3 years. Always sample the same time of year for year-to-year comparisons.
  • Use realistic yield goals to determine nitrogen application rates if they are part of the recommendations.
  • Select the most suitable nitrogen fertilizer for the crop, application method, and climatic conditions.
  • Use the proper application technique for the situation.
  • Maintain and calibrate application equipment.
  • Avoid application to surface waters.
  • Time nutrient applications appropriately for most agronomic benefit and minimal environmental impact.
  • Control soil erosion because many nutrients move when soil particles move.
  • Properly control water flow. Slow water down when appropriate by conservation practices, or speed water movement when appropriate.
  • Use cover crops, and maintain crop residue on the soil surface.

 

Economics

The final decision about which fertilizer to use should be based on economics. Compare materials on the basis of a price per pound of actual plant food. To compare price per pound of plant food use this formula:

(Price per ton of fertilizer)/(2000 x plant food content as decimal value) = per pound of plant food

Here is an example with urea priced at $500 per ton:

Each ton of urea is 45% nitrogen, so each ton contains 900 pounds of nitrogen (2000/0.45). Dividing $500 by 900 shows the cost is $0.56 per pound of nitrogen in the fertilizer.

The maximum return per dollar invested in fertilizer is achieved from the first increment applied to a deficient soil or crop when the nutrient is needed. However, the maximum profit is achieved at a rate of fertilization that produces near maximum yield. At this economic optimum, the value of any yield increase produced by higher fertilizer rates just covers the cost of the additional fertilizer.

 

Dealer Service

Most fertilizer dealers handle high-quality products, often at similar prices. The quality of dealer service (reliability, timeliness, agronomic knowledge, other services offered) is a major consideration in choosing a fertilizer.

 


Publication 2500 (POD-05-17)

By Dr. Larry Oldham, Extension Professor, Plant & Soil Sciences.

 

Contact Your County Office

Authors

Extension Professor
Soil Health, Soil Fertility, Nutrient Management, Soil Conservation and Management, Certified Crop A

Your Extension Experts

Assoc Extension/Research Prof
RICE PRODUCTION SOIL FERTILITY AND NUTRIENT MANAGEMENT FOR RICE,SOYBEAN AND CORN
Extension Professor
Soil Health, Soil Fertility, Nutrient Management, Soil Conservation and Management, Certified Crop A

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