MIR Plus An ECOWAS - UEMOA Joint Project implemented by IFDC
ISSUE N°7
FEBRUARY 2012
Information Bulletin Fertilizer toxicity: Myth or Reality?
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here is a widespread belief that mineral fertilizers are toxic to the environment, plants and public health while organic fertilizers are innocuous. This opinion is frequently referred to in various debates on the promotion of greater fertilizer use in West Africa. This paper is a contribution in this debate. What is fertilizer? Fertilizer is a natural or processed material that contains essential nutrients for the growth of plants. It is applied to the soil or to the plant foliage. Fertilizers can supply one or more nutrient elements. When it supplies only one nutrient, it is called a simple fertilizer. If it supplies more than one nutrient, it is called a composite fertilizer. One example of simple fertilizer is urea, which supplies only nitrogen to the plant. Examples of composite fertilizers are the various formulations of nitrogen/ phosphorus/potassium (NPK), which provide the three primary nutrients in various proportions. Fertilizer nutrients are drawn from nature - The main ingredients of current fertilizer products (nitrogen, phosphorus and potassium) are natural elements: Nitrogen comes from the air. In fact, 78 percent of the air we breathe is nitrogen. However, the nitrogen that we breathe is in a chemically inert form that most plants cannot use. Large amounts of energy (heat and pressure) are required to convert this nitrogen into a form that plants can absorb. Phosphorus is the fossil remains of ancient sea life. These fossilized
remains are mined in the form of phosphate rock from natural deposits throughout the world.
Potassium is found in the salts left behind after the evaporation of ancient inland seas. These salts are mined in the form of potash from natural deposits and are so benign (lacking adverse affects to human health) that many people use potash as a substitute for table salt, which contains more detrimental sodium.
A fertilizer plant
Fertilizer can be mineral (inorganic) or organic - Mineral fertilizers are manufactured through chemical synthesis using materials of a mineral nature. These types of fertilizers provide nutrients directly in inorganic form, and are typically absorbed through the plant’s root system. The mineral materials can be manufactured to become the source of all primary nutrients (N, P, K), secondary nutrients (calcium [Ca], magnesium [Mg], sulfur [S]) and micronutrients (boron [B], iron [Fe], sodium [Na], zinc [Zn], etc.) that plants require. Mineral fertilizers have the advantage of being highly concentrated, and nutrients can be blended (mechanically) or mixed (chemically) to meet required
specifications. Thus, mineral fertilizers can be better “targeted” to meet the nutritional needs of specific plants and soils. They are generally highly cost-effective, but do require an up-front investment that may be difficult for many smallholder farmers in West Africa to afford without subsidization. In contrast, the nutrients accumulated in plant tissues or excreted by animals (such as N, P, K and S) are not directly available for the plant after an organic fertilizer is applied to the soil. The organic material must go through a process of mineralization whereby nutrients transform to their mineral states, at which point nutrients can be taken up by plant roots. In addition to the concern of content and availability of plant nutrients, the application of organic fertilizers also has effects on other chemical and physical soil properties. The use of organic fertilizers can increase the soil's ability to hold and release plant nutrients (known as Cation Exchange Capacity), pH buffer capacity, and can affect physical properties such as the structure of the soil and its capacity to retain water. The content and uniform distribution of nutrients in quality mineral fertilizers is always known, while the content and distribution uniformity of the nutrients in organic fertilizers is largely unknown. This makes it difficult to guarantee the optimal balance or quantity of vital crop nutrients using only organic sources. For example, providing enough nitrogen for a crop by applying manure would mean adding four to five times more phosphorus than is needed.
Acknowledgment: The MIR Plus Project appreciates the contribution of our colleague Joaquin Sanabria from IFDC HQ, USA, Alabama and the Natural Resource Management Program team of IFDC, North and West Africa Division
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Plants have no preference between nutrients from mineral or organic sources - All plant nutrients – whether derived from organic or mineral sources – are the same. This fact was most effectively articulated by Nobel Prize winner, the late Dr. Norman Borlaug, when he stated that, “it makes no biological difference to the plant whether the nitrate ion it “eats” comes from a bag of fertilizer or decomposing organic matter.” Ideally, however, mineral fertilizers should be used together with organic fertilizers. Manufactured mineral fertilizers help overcome the limitations of organic fertilizer, while organic fertilizers improve the efficiency of mineral fertilizer use. Combined use also may reduce the total cost of improving soil fertility, making it more affordable for many smallholder farmers in West Africa to address their soil fertility challenge. The sources of fertilizer toxicity are many - The presence of toxic or harmful substances to plants, animals or humans in mineral fertilizers can be controlled. The most common causes of such eventualities are fertilizer adulteration, contamination and manufacture with primary materials that contain harmful compounds. Toxic, carcinogenic or otherwise dangerous substances can contaminate fertilizer products – the result of attempting to dilute the nutrient content of a mineral fertilizer with foreign materials that contain harmful substances, or poor manufacturing processes that allow contamination with these harmful substances. The natural deposits of minerals used in fertilizer manufacture can contain chemical substances that can be potentially harmful for organisms; such substances can be heavy metals, radioactive materials and carcinogenic or toxic elements. The use of mineral deposits with these characteristics and dangerous agents as the source of primary material for fertilizer manufacturing is forbidden if there is not 100 percent assurance that the materials have been reasonably cleared of the dangerous substances. The risk of contaminating soil and
plant products with different dangerous chemicals or pathogens can be higher with the use of organic fertilizers. Sludge from city sewage systems or materials from city solid waste deposits can be loaded with heavy metals, pharmaceutical residues and pathogens. Organic materials from these sources should be used as fertilizers only after the process of decontamination and sterilization. Manures also hold contamination potential due to the presence of residual pathogens and drugs, and often the composting process is not enough to rid them from these harmful agents. In other words, one can equally pollute the environment using only manure. Because the nutrient content of manures cannot be effectively separated to control the nutrient balance, meeting the elemental requirement of one nutrient can cause the overapplication of one or more other nutrients. To believe that mineral fertilizers are intrinsically harmful due to their synthetic character, and that organic fertilizers are innocuous due to their natural origin, is therefore incorrect. Any fertilizer product, inorganic or organic, has the potential of causing environmental damage when misused. Clearly, poor management of plant nutrients – whether from organic amendments or mineral fertilizers – can cause the loss of some nutrients to the environment where they can upset the balance of natural ecosystems or lead to their accumulation in larger proportions in food. Nitrogen may also volatilize, or transform into a greenhouse gas that affects the atmosphere. But if a farmer uses appropriate agricultural practices, the crop will absorb most of the applied fertilizer.
ICS Fertilizer ready for shipping
How do we avoid fertilizer toxicity? - The best way to avoid harmful compounds in mineral and organic fertilizer products is through the development and implementation of regulations for the production and trade of fertilizers, both at national and regional levels. The laws and the mechanisms to apply them, as well as educational campaigns for agro-dealers and farmers, will ensure that toxic products are not added to fertilizers due to ignorance, negligence or fraud. The management of fertilizer in terms of choosing the right product, time, rates and method of application should correspond with crop nutritional needs, soil nutrient availability, texture, depth, slope and its hydrodynamic properties. The amount of rain, its distribution and intensity of storms should also be considered in making decisions on fertilizer management to avoid the movement of excessive amounts of N and P to surface and ground water. The nitrates that result from fertilizer application in the soil are highly soluble and move out of the fields to water sources through runoff and leaching. Most of the applied phosphorus is bound to soil particles and eventually migrates to water bodies through soil erosion. Large amounts of N and P disrupt the ecology of aquatic systems, causing the overgrowth of some organisms (algae, bacteria), depleting the oxygen content of the water and adversely affecting the normal development of other organisms. Excessive levels of N and P also make water unsuitable for human consumption. Agricultural research and extension organizations together with environmental protection agencies should work to develop technologies that are appropriate to specific crops, soils and agro-ecological zones of different countries or regions, and advocate for legislation to regulate fertilizer production and trade to avoid the environmental damage and the human health issues that may result from the inappropriate management of fertilizers. ■
MIR Plus Contact : Georges Dimithe (PhD), Project Coordinator P.O. Box 10948 Garki, Abuja, Nigeria - Telephone: +234 703 897 4713 - Email: gdimithe@ifdc.org