38 Synthetic Fertilizers

Meenakshi Nandal

Objectives

To study Synthetic fertilizers- Definition and types

To study major synthetic fertilizers affecting plant growth To study the effects of fertilizers on soil and human beings

Introduction

39.1 Synthetic Fertilizers

In the human history agriculture has been depending on the use of natural fertilizers for increasing the nutrient levels in soil. As we approached the 20th century, synthetic fertilizers paved an entrance resulting in agricultural revolution of increasing crops yield. The three major compounds constituting modern synthetic fertilizers are nitrogen, phosphorous and potassium. There is a significant improvement in quality and quantity of the available food but at what cost is still questionable.

Fertilizer is added to soil in substantial form to improve the crop growth and yield. It was firstly used by some ancient farmers and this technology developed with time, as the needs for more yield increased due to increase in population. Fertilizer is defined as an organic or inorganic material, either of natural or synthetic origin added to soil to supply one or more essential nutrients for plants growth. A synthetic or chemical fertilizer may also be defined as an inorganic material of wholly or partially synthetic origin added to the soil to enhance plant growth. These provide supplementary nutrients to crops for enhancing plant growth and its yield. Naturally occurring nutrients are not present in usable forms for plants and have to be additionally supplied through these synthetic fertilizers. (Figure 39.1). Also, when h a r v e s t i n g o f crop plants is done nutrients are not returned to the soil and the quantity of soil nutrients can reduce over time. Hence, compensation of these essential nutrients needs to be done either through the natural process of decomposition in which the nutrients extracted are returned to the soil or by adding fertilizers. The growth goal of plants is achieved in two broad ways, the traditional one being as additives that provide nutrients and the second one by which some fertilizers enhance the effectiveness of the soil by modifying its water retention and aeration capacity. The modern synthetic fertilizers mainly consist of nitrogen, phosphorous and potassium blended compounds with addition of some secondary nutrients. Recently, the use of synthetic fertilizers has increased significantly resulting in improved quality and quantity of the crops available today. As we know, the living organisms as well as plants are composed of cells. For growth and reproduction of these cells, they are driven by numerous metabolic pathways that consist of vigorous chemical reactions. The nutritional requirement of plants is provided by soil that provides the basic constituents required for these metabolic chemical reactions. But with a limited supply, these components amount dwindles in soil as the plants are harvested causing a reduction in the quality and quantity of crop yield. These chemical components are replaced by fertilizers in the soil. These are specially designed to enhance the growth potential of soil, with a better surviving environment compared to natural soil.As we know that plants are composed of four main elements: carbon, hydrogen, oxygen, and nitrogen. Out if these, the three elements; C, O and H are easily available in form of water and carbon dioxide. The fourth element Nitrogen makes up a major part of but is not present in available forms in atmosphere. Being the most important nutrient as it

Figure 39.1: Organic vs. Synthetic Fertilizer Application Process for Plants Source: www.milorganite.com

forms proteins, amino acids, DNA and other components like chlorophyll, it must be available in fixed form. The only available form of N is through some bacteria present in nodules of leguminous plants. They can fix atmospheric nitrogen (N2) and convert it to ammonia. Fertilizers are typically provided, in varying proportions: (Dittmar et al.,     2009) Three main macronutrients: nitrogen (N), phosphorus (P), potassium (K)

Three secondary macronutrients: calcium (Ca), magnesium (Mg), and sulfur (S)

Micronutrients: copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn) and nickel (Ni), and sometimes boron (B), silicon (Si), cobalt (Co), vanadium (V). They are present in plant tissue in lesser amount (parts-per-million) and are needed by enzymes that control plant metabolism and enable catalysts to work on the active sites.

Today fertilizer has become essential to modern agriculture to feed the growing population. The chemical fertilizers enhance crop production but their overuse could damage the soil by hardening, decreasing fertility, strengthened pesticides, released greenhouse gases, polluted air and water, finally bringing hazards to human health and environment. It has already been proved how chemical fertilizers pose serious challenges to the balanced and sustainable growth. Therefore, overuse of fertilizers should be avoided by having early knowledge about the nutrient needs and proper application of fertilizers.

Fertilizers have been classified traditionally in two classes:

1. Straight fertilizers: This is a traditional term used for fertilizers, that are used for one major nutrient being either of the three nutrients N, P or K as opposed to multi nutrient fertilizers. When secondary nutrients are considered these include products consisting of elemental S, calcium oxide and magnesium sulphate etc. In micronutrients, borax, sulphate salts, Zn and Fe chelates of micronutrients are considered straight fertilizers. This is not a very accurate term to be used for them because many straight fertilizers also contain other essential plant nutrients, such as S in ammonium sulphate. These can also be termed single-nutrient and focus mainly on the most important nutrient. To be specific this term is used for products such as elemental S, urea, ammonium nitrate (AN).
2. Complex/compound fertilizers: These fertilizers generally contain at least two out of the three major nutrients; two-nutrient (NP) or three nutrient (NPK) fertilizers. They are generally solid granules and are formed by a chemical reaction between the raw materials with the desired nutrients. These are also called multi nutrient fertilizers, but they do not incorporate fertilizer mixture/bulk blends as no chemical reaction is involved in them.

39.2 Synthetic Fertilizers – Nitrogen (N)

The current environmental and economic issues have increased the need to know about the role of nitrogen (N) and its fate in crop production systems. Nitrogen is the most deficient nutrient in crops  resulting in substantial economic return for farmers. However, when the amount of N inputs in soil system exceeds the crop needs, there is a possibility that excessive amounts of nitrate (NO3-N) may enter either ground or surface water. Managing N inputs to achieve a balance between profitable crop production and environmentally tolerable levels of NO3-N in water supplies should be every cultivator’s goal. The behavior of N in the soil system is complex, so an understanding of these basic processes is essential for a more efficient N management program. The amount of nitrogen present in the soil depends on prevailing climatic conditions, physical and chemical properties of the particular soil. Nitrogen is fixed naturally by leguminous bacteria or by lightning and exists abundantly in our atmosphere as compared to its naturally available forms in soil. Majorly all nitrogen based synthetic fertilizers are developed by Haber-Bosch process (shown in figure 39.3) that includes combining nitrogen from the air and hydrogen at high temperature and pressure to generate anhydrous ammonia (NH3). This fertilizer is widely used but now its widespread adoption has resulted in unintended prominence to environment affecting the sustainability and quality of our food system. The table 39.1 below shows the different nitrogen based fertilizers.

Production of Ammonium Nitrate

Ammonia is oxidized in presence of air to produce nitric acid (HNO3). This nitric acid is then neutralized with addition of ammonia to produce a resultant product consisting of 83% ammonium nitrate and 17% water. This mixture can be further processed used to generate nitrogen fertilizer or solid ammonium nitrate (Fig 39.3).

Figure 39.3: Production of Ammonium Nitrate by Haber’s Process Source: https://thehaberprocesssjii.weebly.com

39.3 Phosphorus Fertilizer

Phosphatic fertilizers majorly consist of phosphorous in the form of calcium, potassium phosphate or ammonium. The phosphate part of fertilizers being available to plants may be fully or partly water soluble / citrate soluble. It dissolves slowly with comparatively more efficiency in the acidic soils. The P availability is mainly present as P2O5%, depending on concentration or the total phosphate present in fertilizer. Although the requirement of phosphorus to plants is low than other major nutrients, it influences the energy transfer and their early developmental growth stages. Phosphorus plays an essential role in stimulating young root development, early fruiting and controls several biochemical processes like photosynthesis, respiration, cell division, growth and development processes. The P Uptake as depending on pH occurs primarily in form of HPO42- and H2PO4-(orthophosphate) with latter being common in acidic soils. Table 39.2 describes various types of Phosphorous based fertilizers concentrations.

The amount of phosphorus that is made available to the plants through fertilizers depends upon the extent to which HPO4-or H2PO4– ions is supplied. On basis of solubility, the phosphatic fertilizers are divided in following categories:

1. Water soluble: These fertilizers containing phosphorus in available form and are readily absorbed by young plants in neutral soils. While in case of acidic and alkaline calcareous soils; the free iron forms in aluminium hydroxy phosphates is transformed to insoluble calcium phosphate. The common examples of these fertilizers are:

Rock phosphate is the basic raw material commercially used for manufacturing of most phosphate fertilizer. It is used as a major source of raw material in the food and chemical industry. Figure 39.4 shows the various steps used in manufacturing of various phosphate fertilizers in generalized form.

The initial product is phosphoric acid produced either by dry or wet process. The dry treatment occurs in an electric furnace with rock phosphate as substrate producing more expensive phosphoric acid (often called white/ furnace acid).

Potassium fertilizer

Potash or potassium (K) is an important nutrient for crops as it affects the yield and quality along with general health and vigor of a crop. The potassium Potash fertilizers are mainly water-soluble salts and the K concentration is still expressed generally as % K2O. The nutrient K is not present as K2O in soils, plants or in fertilizers but as potassium ion K+ in soils or plants and chemically as (KCl, K2SO4) compound blended in fertilizers. Potassium is essential considering water and crop growth as it

Figure 39.4 schematic process used for manufacture of phosphate fertilizers Source: www.extension.umn.edu

regulates water level within the crop. When plants have sufficient water, they remain turgid and upright and the osmotic concentration is maintained due to sufficient solutes presence within the cell sap. Majorly K+ is used as solute by plants and so this explains why it is important to maintain plant cells and tissues turgidity (rigidity). If the potassium is insufficient the leaf expansion and stem elongation process slow down during the initial stages of leaf canopy growth. It is also required for the sugar transport from the leaf to the growing regions of the plant and to storage organs such as grain in cereals and the tubers of potatoes where they are converted to starch, or to the storage roots of sugar beet where they are stored as sugar. It also affects indirectly the N usage efficiency of plants and so the plants can less effectively handle stress caused by frost, wind, water-logging and heat. Hence, it is essential to bring soils to the target Index for plant-available potassium (exchangeable K+) and then maintain this level by replacing the amount of potash removed each year in the harvested crops.

Potassium is present in three major forms in the soil:

1. Exchangeable/available potassium (K+), adsorbed by the soil CEC
2. Fixed by some minerals and releasing slowly to available form
3. Unavailable mineral form

Most of the potassium in soils is present in unavailable mineral form. Plants take up potassium as the K+ ion. The muriate of potash is the most common type of potassium bound fertilizer. As potassium chloride is highly soluble in water so, excess of the muriate of potash can cause salt damage to the plants. The mobility of potassium is between N and P because it is not as swiftly lost as N, but moves more quickly than P into the soil penetrating the roots. When potassium fertilizer is applied its fate depends on the CEC and clay minerals present. If sandy soils with low CEC are considered, movement of potassium is by mass flow and there can be significant loss from the surface of soil, especially when there has been heavy rainfall. The possible potassium movement can be leaching from the root zone before its interacts with soil solids (minimal), exchanges with other cations in the exchange complex and finally become available to plant roots for future use. In case of vermiculites or strongly charged montmorillonite clays, K may be fixed in interlayer region of these 2:1 clays and further released as the minerals slowly subjected to wetting, drying cycles or acid weathering.

Figure 39.5: Schematic representation of Production of Potassium

Production of Potassium Fertilizer

The majority of KCl is mined and used for generating various grade of potash fertilizers mainly designed considering particle size (granular, soluble, standard, fine). The granulated KCl is applied with other N and P based fertilizers in mixture form, generally to provide the nutrients required by the crops in a single application. Another potassium fertilizer in form of potassium sulfate is frequently used. It is mainly used for crops that require additional chloride that are not provided by common KCl fertilizer. Potassium sulfate can be either extracted from the mineral langbeinite or synthetized by reaction between potassium chloride and sulfuric acid at high temperature. If magnesium salts are added to potassium sulfate it produces potassium-magnesium granular compound fertilizer (Fig. 39.5).

Table 39.3 Different types of Potassium based fertilizers

Effects of Synthetic Fertilizers

Positive

The Synthetic fertilizers provide good amounts of required nutrients to the soil by acting on soil immediately. But the organic fertilizers need to break down before they could be absorbed by soil. This property of fertilizers is highly efficient in providing nutrients to malnourished plants. Also, they are easy to handle, store and use with immediate effects.

Negative

The synthetic fertilizers possess prolonged negative effects. The beneficial microorganisms that transform dead matter remains into organic residue are killed in the soil. These damage the natural characteristics of soil causing overfertilization of soil with minerals like iron, zinc, carotene, vitamin C, copper and protein. N and P based synthetic fertilizers cause groundwater contamination by leaching and toxicity increases leading to water pollution. They enter the lakes, streams, rivers, and other water bodies and disrupt the aquatic ecosystems. These increase the nitrate levels of soil that causes production of toxic nitrites when consumed by plants. These when consumed by living organisms react with the blood hemoglobin and cause methaeglobinaemia, causing suffocation and damaging the body systems that may lead to death in extreme conditions.

Minimization of Negative Effects

The synthetic fertilizer should be diluted and mixed well into the soil as improper dilution can burn and damage plants.

This Proper mixing will prevent runoff during heavy rain.

The produced yield of vegetables and fruit etc. from synthetically treated soil should be washed properly before eating.

The leftover or unused fertilizer should be safely stored away from water, pets and children.

At last, we should consider usage of organic fertilizers that are safer in comparison to synthetic fertilizer and cause no pollution

Summary:

We studied about the various synthetic fertilizers majorly including that of N, P and K. We studied about the synthesis procedure for various fertilizers.

We studied about the importance and types of various synthetic fertilizers

We studied about the positive and negative effects of fertilizer and minimization of these effects in soil

References

• Dittmar, H., Drach, M., Vosskamp, R., Trenkel, M.E.., Gutser, R., Steffens, G. Fertilizers, 2. Types, Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2009, DOI: 10.1002/14356007.n10_n01.
• Loneragan, J. F. and Asher, C. J. Response of plants to phosphate concentration in solution culture. II.
• Rate of phosphate absorption and its relation to growth. Soil Sci.103, 311-8, 1967.
• Mengel, K. and Kirkby, E.A. Principles of Plant Nutrition. International Potash Institute, Worblaufen-Bern, Switzerland, 1987.