Many questions have been raised concerning the use of fertilizers, why plants require them, and about the benefits they bring. Fertilizers have a significant effect on humanity, as they form the basis of food production. The addition of fertilizers assists enrichment of soil fertility and corrects nutrient deficiencies so that plants can grow more healthily, provide crops and food for people, and increase food production. In this way, fertilizers are socially relevant.
Fertilizers are used to improve the productivity and growth of plants; they can either be natural or manufactured chemical substances. In order to grow, plants necessitate carbon dioxide from the air, and water and essential elements from the soil. The latter are obtained by absorption through the roots of plants; however, if the soil is depleted of a specific nutrient, fertilizer is added to overcome these deficiencies. When plants are lacking in a particular nutrient, (i.e. when it is unable to attain enough of that specific nutrient from the soil for proper development), abnormal plant growth can occur.
The Chemical Properties
The three predominant elements in applied fertilizers are Nitrogen, Phosphorus and Potassium. Fertilizers are identified according to the percentage of composition of these three elements respectively. Actually, the amount of phosphorus and potassium in a fertilizer (i.e. indicated on the fertilizer bag) is usually representing the amount of phosphorus pentoxide (P2O5) and potassium oxide (K2O) that is present. For example, 5-8-7 indicates a fertilizer containing 5% nitrogen, 8% phosphorus pentoxide and 7% potassium oxide. Therefore it would be more accurate to say that fertilizers consist mainly of nitrogen, phosphorus pentoxide and potassium oxide. To simplify matters, they are thought of as just phosphorus and potassium. Some fertilizers do not even contain P2O5 and K2O, but they do contain phosphorus and potassium, and so its label would be representing the amount of P and K that is in the fertilizer if it were in the form of P2O5 and K2O (conversion factors are used to work out exactly how much of the element is present). Sulfur, magnesium and calcium (in the form of lime) are trace elements in fertilizers. Other elements that are required in smaller quantities, which are known as micronutrients, include boron, copper, iron, manganese, zinc, chlorine and molybdenum. Hydrogen, oxygen and carbon are also necessary and these are supplied from the air and water.
Nitrogen, the key element in amino acids, stimulates the leafy portion of the plant. Air contains about 78% N; plants cannot use it in this form, unless it is converted into ammonium or nitrate ions, which can be absorbed through the roots. This process is called nitrogen fixation. A bacterium, which is found in all legumes and converts N from the air into the forms that can be used by plants, is Rhizobium; it is found in the nodules of plants. Most nitrogen fertilizers are obtained from synthetic ammonia; this is produced from natural gas or changed into salts such as ammonium sulphate, ammonium nitrate and ammonium phosphate. The Haber-Bosch process converts N in the air into ammonia.
N2 (g) + 3H2 (g) 2NH3 (g) H = -92 kJ
The Haber-Bosch process is also used to produce nitric acid (HNO3). The richest source of N in fertilizers is anhydrous (containing no water) ammonia. Ammonium nitrate is a water-soluble, inorganic salt, containing 35% wt N. Its formation is caused by an acid-base reaction of HNO3 and ammonia in aqueous solution.
NH3 (aq) + HNO3 (aq) NH4NO3 (aq)
Ammonium sulphate, (NH4)2SO4, is also water-soluble and contains 21% N. It can be prepared by reacting calcium sulphate with Urea, (NH2)2CO, which is formed by reacting CO2 with NH2, and it is made up of 45% N. Its usage is gradually increasing in fertilizers.
The Nitrogen Cycle
Phosphorus or Phosphate (P2O5)
Phosphorus is important in the process of life. It helps transfer energy from sunlight to plants for its chemical reactions involved in the growth and reproduction of plants, as it increases the development of the reproductive parts. The soils of Australia are nearly all deficient in phosphorus. Superphosphate is one of the major phosphate fertilizers and is produced from the mineral apatite, CaF2.3Ca3(PO4)2. Tricalciumphosphate, Ca3(PO4)2, is an insoluble mineral, contained in phosphate rock, and it is treated with concentrated H2SO4 to convert it into soluble forms for use in fertilizers. The reaction for this can be shown as
Ca3(PO4)2 + 2H2SO4 + 4H2O Ca(H2PO4)2 + CaSO4.4H2O
Other phosphate fertilizers include other insoluble calcium phosphates, such as Ca5(PO4)3F, which also comes from phosphate rock or bones; superphosphate and triple superphosphate, which are produced from calcium phosphate with sulfuric acid and phosphoric acid.
Potassium or Potash (K2O)
Potassium is important in the balance of salts and is found largely in the vegetative part of the plant. It protects plants from diseases by developing thick, outer cell walls in plants. A lack of potassium in plants will result in sugar and nitrate buildup in plant tissues, which is food for attacking organisms. Potassium is found mainly in the minerals of mica type and on the surface of clay minerals. Potash is usually the name referring to the compounds of potassium, e.g. potassium chloride, oxide, carbonate and hydroxide, but in fertilizer trade, it usually specifies potassium chloride. The presence of potassium carbonate in the soil comes from orthoclase.
2KAlSi3O8 + 2H2O + CO2 H4Al2Si2O9 + 4SiO2 + K2CO3
Orthoclase Clay Quartz Potash
The most commonly used fertilizers are potassium sulphate containing 50% K2O, mined from potash deposits and other sources of potassium include woodash, seaweed and urine.
How Do Fertilizers Affect Society?
Fertilizers are essential to agriculture; however, excessive use of it can have harmful effects on plants and crops, on soil quality, the environment (as it contributes to soil and water pollution), and also on humans. Though fertilizers can have a negative impact on society, many benefits are also consequent of their use.
The increasing application of fertilizers and other agricultural chemicals to soils has led to concern over soil pollution. A buildup of toxic compounds, chemicals, salts, and radioactive materials, which have a negative effect on plant growth and animal health, can cause such pollution. Fertilizers that only contain the essential elements have not contributed to soil pollution, but the employment of trace elements has (e.g. the usage of heavy metals has led to cases of heavy metal poisoning).
In addition, both natural and manufactured fertilizers are hazardous towards water supplies. Soluble nitrogen forms can leach into groundwater and fertilizers are carried along in surface runoff. Leaching of nutrients into the water can lead to eutrophication; this occurs when nitrogen and phosphorus compounds, which are main elements in fertilizers, are washed into stagnant water. As the compounds provide nutrients for growth, specifically acting as fertilizer for algae, it can lead to a sudden growth of algae, or what is known as algae blooms . As the algae multiply, they consume a large portion of the available oxygen in the water, thus killing off all other plant and animal life in the water. They also release certain toxins into the water, which would affect human and animal health. Eutrophication can lead to ecological disasters, as was shown in the Great Lakes of America during the 1960 s and 1970 s.
Subsequently, the negative effects of fertilizer can also significantly impact agriculture. The effects would be quite severe in the quality of crops and plant growth; for example, an excess of the essential elements causes too much water to be produced in potatoes and in forage grasses. The grasses are vital for cattle, which in turn will have nutritional problems. The production of food decreases when plant growth is affected, and therefore will have an impact on the agricultural industry and the country s economy. Toxic effects can result from a surplus amount of micronutrients in soil. Some fertilizer elements may build up and accumulate, forming mineral salts and hence lowering the quality of soil. This can lead to abnormal growth of crops. Consequently, hunger or malnutrition may result from the reduction in the quality and production of food, which would also diminish the available food supplies.
Nutrient deficiencies, especially in those essential for the development of plants, can lead to deformed growth. An example of the deformities includes the distortion of colours in leaves of plants. Below is a table outlining briefly the symptoms if plants are lacking in a specific nutrient.
COMMON SYMPTOMS OF NUTRIENT DEFICIENCY IN PLANTS
ElementDeficiency Leaves to firstshow deficiency Symptoms
Nitrogen Old Leaves turn yellowish, may also turn reddish; stunting; premature maturity
Phosphorus Old Yellowing; erect habit; lacking in lustre; blue-green, purple colours; premature leaf fall-off
Calcium New Tip-hooking; blackening and death; yellowish leaf edges
Magnesium Old Patchy yellowing, may also turn reddish; brilliant colours, especially around edge
Potassium Old Scorched margins; spots surrounded by pale zones; withering of leaf edges and tips
Sulfur New Yellowing; smallness; rolled down; some pigmentation
Iron New Yellowing between veins; veins sharply green; youngest leaves almost white if severe; first seen in fast growing plants
Manganese New or Old Dead yellowish tissue between leaf nerves; veins pale green, diffuse; water-soaked spots; worst in dull weather
Copper New Dark blue-green; curling; twisting; death of tips
Zinc Old Yellowish areas between nerves, starting at leaf tip and edges; smallness; bunching; yellow-white mottling
Boron New Yellowing margins; crumpling; blackening; distortion
Molybdenum Old Mottling over whole leaf, but little pigmentation; cupping of leaves and distortion of stems
Although the use of fertilizers has many negative impacts, there are also positive effects due to their application. The use of fertilizers as a food source for plants enables the plants to grow and therefore provide food for people. With more effective use of fertilizers, they will have an important role in improving the standard of living, such as that in developing countries, where improvement of food production will reduce hunger and malnutrition.
Alternatives and Solutions
Certain alternatives and solutions for fertilizer use include that of sewage sludge and manure. However, their widespread use is delayed by many complications. Presently research is still underway to ensure they can be carried out safely, without increasing risks to the environment.
Liquid sludge could be used as a fertilizer. It contains soluble nitrogen, which is required for effective plant growth, and phosphorus compounds, as well as many of the secondary and trace elements that are useful for plant nutrition. However, there are certain elements that are present in sewage sludge that are toxic, like cadmium, mercury and lead and they are often too high to be used regularly on farmland. New methods for removing the toxic elements and infectious matter are being presently tested.
Manure contains many important nutrients and is rich in humus (decaying human and plant matter) but it lacks the three essential elements, nitrogen, phosphorus and potassium. A commercial fertilizer contains 20 times as much nitrogen, phosphorus and potassium as an equal amount of manure. This is why other fertilizers are usually used in conjunction with manure. It also helps to loosen soil and retain water. Techniques for removing disease micro-organisms by sterilization are being developed and also for converting manure into forms which can be transported around more easily. Since the methods are too expensive, manure fertilizers are not yet in competition with artificial fertilizers.
In summary, as the use of fertilizers offer a more efficient method of farming and providing food, plants and humans benefit enormously from them. When a deficiency is present in the soil, fertilizers are applied to compensate for the unavailable nutrients needed by the plants. With more effective use of fertilizers, poorer countries could improve their standard of living. Of course, problems can arise from the overuse of fertilizers. Excessive use can contribute to water and soil pollution and lakes and rivers can suffer from eutrophication, from the enrichment of the water with phosphate and nitrates compounds. However, despite the harmful effects that can develop, fertilizer is an essential nutrient provider for plants, and without it, the production of food globally would not be adequate for the entire human race.