MIR Plus An ECOWAS - UEMOA Joint Project implemented by IFDC
ISSUE N°7
FEBRUARY 2012
Information Bulletin Fertilizer toxicity: Myth or Reality?
T
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