Inorganic Fertilizers For Crop Production

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: the right amount of the right fertilizer at the right place at the right 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, the right amount to apply is none.

If the laboratory results show response to added fertilizers is likely, there will be a rate recommendation. This is the right amount. The right time and right place depend 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, phosphate, and potash in the material, expressed in the order N-P-K. For example, 10-20-10 indicates the material is 10 percent nitrogen, 20 percent phosphate and 10 percent potash by weight.

Fertilizer ratio is the ratio of the weight percents of nitrogen-phosphate-potash 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, 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 nitrogen, 30 pounds of ph0sphate, and 10 pounds of potash. One hundred pounds of a 7-21-7 fertilizer has 7 pounds of nitrogen, 21 pounds of phosphate, and 7 pounds of potash. 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.

NPK Compound Fertilizer Granular High Tower Granulation Tumbling Granulation

Considerations in Using Fertilizers

If the soil test-based recommendations include supplemental fertilizer, several factors must be considered to select the right source. These include physical and chemical properties, environmental stewardship, and economics.

Fertilizer Formulations

Many different physical and chemical forms of commercial fertilizer are available (see Table 1). 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 fertilizer materials are solid, homogenous 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 fertilizers are 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 fertilizers are 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.