Chapter 21 Fertilizers 1. INTRODUCTION Besides the three basic elements of carbon, hydrogen, and oxygen that are common to all plants, there are 16 other elements known to be essential to good plant growth. Their percentages are given below. 95% basic elements 44% C, 6% H, 45% O 3.5% primary nutrients 2.0% N, 0.5% P, 1.0% K 1.3% secondary nutrients Ca, Mg, S 0.1% micronutrients B, Cl, Cu, Fe, Mn, Mo, Zn (Co, F, I in animals also) This chapter is concerned with the three primary nutrients making up most fertilizers: nitrogen, phosphorus, and potassium. The usual sources of nitrogen are ammonia, ammonium nitrate, urea, and ammonium sulfate. Phosphorus is obtained from phosphoric acid or phosphate rock. Potassium chloride is mined or obtained from brine and the sulfate is mined in small amounts. Potassium nitrate is made synthetically. These chemicals have already been described under inorganic chemicals of the top 50. Sources for the three primary nutrients are given in Fig. 21.1.
Nitrogen Phosphorus Potassium Raw materials Air, hydrocarbons, or hydrogen Phosphate rock Sulfur Potash* x. ; Primary nutrient chemicals Ammonia * Nitric acid Phosphoric acids SuI fun c acid Derived nutrient chemicals Ammonium sulfau* Urea Ammonium I nitrate* j Nitric phosphate Ammonium phosphate >, Triple ', -super- v phosphate * Normal superphosphate Mixers Bulk blenders of dry granular materials Liquid blenders nlmmoniaiorranulators Dry mixers of powder materials Retailers Dealers Consumers Farmers and commercial users * Major direct application materials sold through blender-retailers or dealers. Figure 21.1 Major fertilizer materials. (Source: Kline & Company, Inc.)
Phosphatic Pert. Nitrogenous Pert. Mixed Fertilizers Billions of Dollars Year Figure 21.2 U.S. shipments of fertilizers. (Source: Annual Survey of Manufactures) Billions of Pounds Nitrogen Phosphate Potash Year Figure 21.3 U.S. production of fertilizers. (Source: Chemical Economics Handbook and UN Food and Agriculture Organization)
2. HISTORY AND ECONOMICS OF FERTILIZERS Although the modern era of fertilizers began with the work of Justus von Liebig in 1840 and the first U.S. patent for a mixed fertilizer was granted in 1849, the use of large amounts of synthetic fertilizers was popularized only after World War II. Fertilizer consumption increased eight times between 1950 and 1980 worldwide. U.S. shipments of fertilizers is summarized in Fig. 21.2. Phosphatic Fertilizers (NAICS 325312) had a very fast increase from $1 billion in the early 1970s to $4.4 billion in 1980. Since then it has increased only slowly to $5.5 billion. Nitrogenous fertilizers (NAICS 325311) have also had a similar trend to 1980 and have dropped recently to that level now. Mixed Fertilizers (NAICS 325314) have had a more constant increase in the last few years. Fig. 21.3 gives trends in nitrogen, phosphate, and potash fertilizer production. Nitrogen and phosphorus production in billions of pounds have increased slowly through the years except for a few drops in the 1980s. Potash production is always much less and has been steady or decreasing in the last 25 years. Table 21.1 shows the uses of fertilizers on various types of crops. Note that nearly half of all fertilizers is used on one crop: corn. Wheat, hay, soybeans, and cotton consume most of the rest of fertilizers used on crops. 3. FERTILIZERMATERIALS Fertilizers may contain all three primary nutrients, in which case they are called mixed fertilizers, or they may contain only one active ingredient, called direct application fertilizers. Recently the ratio of direct application to mixed fertilizers is 60:40 in the U.S. Nitrogen and potash are consumed mainly from direct application fertilizers. Phosphate is applied mostly from mixed fertilizers. Table 21.1 Uses of Fertilizers on Crops Corn 44% Wheat 17 Hay 9 Soybeans 6 Pasture and range 5 Cotton 3 Other crops 10 Other feed grains 6 Source: Chemical Economics Handbook
Table 21.2 Percentage of Direct Application Fertilizers Nitrogen solutions 24% Anhydrous and aqueous ammonia 22 Potassium chloride 20 Ammonium nitrate 10 Urea 8 Superphosphates 5 Ammonium sulfate 3 Ammonium phosphate 2 Miscellaneous 6 3.1 Direct Application Fertilizers Table 21.2 lists all important direct application materials and their percentage of direct application fertilizers. Direct application use is increasing mainly because of anhydrous ammonia gas becoming popular. It can be pumped in 3-6 in. beneath the soil during plowing and is absorbed by the soil rapidly. Nitrogen solutions can also be applied in this manner (mixture of free ammonia, ammonium nitrate, urea, and water). 3.2 Mixed Fertilizers The primary advantage of mixed fertilizers is that they contain all three primary nutrients nitrogen, phosphorus, and potassium and require a smaller number of applications. They can be liquids or solids. The overall percentage of the three nutrients must always be stated on the container. The grade designation is %N-%P2O 5 -%K2O. It is commonly called the NPK value. Note that it is an elemental percentage only in the case of nitrogen. Phosphorus and potassium are expressed as oxides. Thus an NPK value of 6-24-12 means that 6% by weight is elemental nitrogen, 24% is phosphorus pentoxide, and 12% is potash. One way of remembering the order is that they are alphabetical according to the English name (mtrogen, phosphorus, potassium). A changeover to a grade designation by the three elemental bases is being resisted by the industry.
3.2.1 Nitrogen Sources The nitrogenous chemicals ammonia, urea, ammonium nitrate, and ammonium sulfate are used as sources of nitrogen in mixed fertilizers. A mixture is also quite popular and is relatively cheap, since the mixed nitrogen solution from which pure urea is made can be used as fertilizer. Nitrogen solutions have their own code number. An example would be 414(19-66-6), meaning 41.4% total nitrogen, 19% free ammonia, 66% ammonium nitrate, and 6% urea with the rest being water. Over 100 nitrogen solutions are marketed. Although the solutions are cheap, the solids do not have a vapor pressure problem and are more easily transported. The present breakdown of nitrogen fertilizer production is ammonia, 40%; nitrogen solutions, 30%; urea, 14%; ammonium nitrate, 8%; and ammonium sulfate, 8%. 3.2.2 Phosphorus Sources All phosphorus fertilizers come from wet process phosphoric acid or directly from phosphate rock. Normal superphosphate, triple or concentrated superphosphate, and ammonium phosphate are the three common types used. Normal or ordinary superphosphate (NSP or OSP) is mostly monocalcium phosphate and calcium sulfate. It is made from phosphate rock and sulfuric acid and is equated to a 20% P 2 O 5 content. It led the market until 1964. The production of normal superphosphate is similar to that for the manufacture of wet process phosphoric acid (Chapter 2, Section 3) except that there is only partial neutralization. Normal superphosphate is no longer used to any great extent. The following reaction is one example of an equation that represents this process. CaF 2-3Ca 3 (PO 4 ) 2 + 17H 2 O + 7H 2 SO 4 ^ 3[CaH 4 (PO 4 ) 2 -H 2 O] + 2HF + 7(CaSO 4 2H 2 O) normal superphosphate (NSP) Triple superphosphate (TSP), made from phosphate rock and phosphoric acid, is mostly mono- and dicalcium phosphate. It is equivalent to a 48% P 2 O 5 content. It led the market from 1965-1967. CaF 2-3Ca 3 (P0 4 ) 2 + 14H 3 PO 4 ^ 10CaH 4 (PO 4 ) 2 + 2HF triple superphosphate (TSP)
The ammonium phosphates took over the lead in 1967. Diammonium phosphate (DAP) is made from wet process phosphoric acid of about 40% P 2 O 5 content and ammonia. The usual finishing NH 3 :H 3 PO 4 mole ratio is 1.85-1.94:1. Monoammonium phosphate (MAP) is made with a final NH 3 )H 3 PO 4 ratio of 1:1. Current production percentages for phosphate fertilizers are DAP, 67%; MAP, 26%; and TSP, 7%. 2NH 3 + H 3 PO 4 ^ (NH 4 ) 2 HP0 4 (DAP) NH 3 + H 3 PO 4 > NH 4 H 2 PO 4 (MAP) 3.2.3 Potassium Sources Most potassium in fertilizers is the simple chloride salt, having a 60-62% K 2 O equivalent. Certain crops such as potatoes and tobacco do not like high amounts of chloride. For these crops KNO 3, K 2 SO 4, or K 2 Mg(SO 4 ) 2 may be used. Florida citrus crops need magnesium nutrients so K 2 Mg(SO 4 ) 2 is favored there. 3.2.4 Ammoniation When an ammonia fertilizer is mixed with a superphosphate there is a chemical reaction that occurs, changing the active ingredient's structure. The following equations illustrate this chemistry. (1) H 3 PO 4 + NH 3 ** NH 4 H 2 PO 4 (2) Ca(H 2 PO 4 ) 2 -H 2 O + NH 3 ** CaHPO 4 + NH 4 H 2 PO 4 + H 2 O (3) NH 4 H 2 PO 4 + NH 3 ^ (NH 4 ) 2 HPO 4 (4) 2CaHPO 4 + CaSO 4 + 2NH 3 ^Ca(PO 4 ) 2 + (NH 4 ) 2 SO 4 (5) NH 4 H 2 PO 4 + CaSO 4 H- NH 3 ^ CaHPO 4 + (NH 4 ) 2 SO 4 Reactions (1) and (2) are common for both normal and triple superphosphate. Reaction (3) is important in triple superphosphate because of the lack of large amounts of calcium sulfate. Reaction (5) is important
Table 21.3 Advantages of Fertilizers Liquids Lower capital investment by the company Less labor, handling, and conditioning costs More uniform composition More uniform distribution on land Solids Less corrosion of equipment Better economics of costs of storing smaller volumes Solubility restrictions are not present No crystallization problems in cold weather with normal superphosphate because of the large surplus of calcium sulfate in this formulation. 4. LIQUIDS VS. SOLIDS There are many different types of liquid and solid fertilizers but we give only some generalizations about advantages of each. Liquid fertilizers are a clear solution, a suspension of a solid in a liquid (aided by a suspending agent), or a simple slurry of a solid in a liquid. Solid fertilizers contain no liquid. Table 21.3 summarizes the advantages of liquids and solids. Mixed solid fertilizers can be made by either direct granulation methods (40%) or bulk blending (40%). Bulk blending is made by mechanical mixing of the separate granular intermediate materials. It is usually done in small plants near the point of use. This technique is employed because the fertilizer can be "tailor-made" to fit the exact requirements of the user. Fluid or liquid fertilizers (clear, suspension, and slurry) account for 20% of all NPK mixed fertilizers. 5. CONTROLLED-RELEASE FERTILIZERS Much recent research has centered on developing long-lasting slowrelease fertilizers to make application requirements less often. Ureaformaldehyde resins in nitrogen fertilizers tie up the nitrogen for a longer time, since degradation of the polymer occurs slowly by sunlight. This type of fertilizer is especially popular for the high nitrogen content of home lawn fertilizers. Sulfiir-coated urea (SCU) is also becoming a popular slowrelease nitrogen formulation. syw-tetrahydrotriazone, made by reacting urea, formaldehyde, and ammonia, can be added to urea fertilizers.
syw-tetrahydrotriazone Triazones form ammonium ions much more slowly than urea. Slowrelease potassium is also being developed. A coating of sulfur seems to delay its release. For phosphorus Mg(NH 4 )PO 4 is becoming popular because it has a slower dissolution rate in the soil. Despite the simple chemicals used in most fertilizers, some interesting research and formulation work will keep chemists involved in the industry for some time to come. Suggested Readings Kent, Riegel's Handbook of Industrial Chemistry, pp. 367-407.