From organic to mineral fertilizers

Plants use nutrients in mineral form for their growth, regardless of whether the nutrients originate from organic or mineral sources. Of all plant nutrients, nitrogen is the one usually required in the highest quantities. A wheat crop, for example, needs about 20 kg of nitrogen (N), 4 kg of phosphorus (P) and 5 kg of potassium (K) to produce one tonne of grain per hectare.

Nutrients in organic fertilizers
Plant material contains nutrients that have been taken up from the soil. If plant material is returned to the soil, it will decompose and release the nutrients again – that is why it is called organic fertilizer. Organic fertilizers are plant residues that remain on the farmer’s field after the harvest and which may be removed, composted and re-applied to the field. Crops are used as fodder for animals. The animals in turn produce milk, meat and organic waste, which is recycled to the field as organic fertilizer (manure or slurry). Common to all organic fertilizers is the fact that the nutrients are bound into organic molecules: nitrogen, for example, in protein. Before crops can use these nutrients, the organic molecules have to be decomposed by micro-organisms in the soil. The nutrients in organic fertilizers are therefore not immediately available to the plants, as their availability depends on the activity of these micro-organisms. How long it takes, before the nutrients from organic fertilizers are available to plants, depends on several factors. A good water supply and high temperatures will increase the speed of organic matter decomposition.

Low nutrient concentrations
Characteristic for all organic fertilizers is their low concentration of nutrients, compared to mineral fertilizers (Fig. 1). Hence the total amount of fertilizer needed to supply a given amount of nutrients is much higher for organic than for mineral fertilizers. For instance, to supply 100 kg of mineral N – which is about the average N rate applied to agricultural crops in Europe per hectare – 20 tonnes of cattle manure are required, compared with only 286 kg of mineral fertilizer (ammonium nitrate) (Fig. 1). It is obvious that the transport and application of 20 tonnes of manure require a greater logistical effort than does the spreading of a small amount of mineral fertilizer.

Figure 1.
The nutrient content in mineral fertilizers is much higher than in organic fertilizers.
Example: Nitrogen content in ammonium nitrate and cattle manure:

Essential role for mineral fertilizers
The main reason, however, for the essential role of mineral fertilizers for sustainable agricultural production is the fact that the nutrient cycle in agricultural production is not closed (Fig. 2). There are unavoidable losses of nutrients at different stages of the cycle. Crop production is always associated with unavoidable losses through the leaching of nutrients, erosion and gaseous emissions. Animal production, in particular, leads to nutrient losses, e.g. loss of nitrogen due to ammonia volatilisation. The export of food for human nutrition also results in nutrient losses, since human excreta and other organic waste is only recycled to agricultural soils in small quantities. These nutrient losses would decrease the amount of nutrients running through the agricultural cycle if fertilizers did not replace them, a phenomenon often described as the “spiral of poverty”. Without fertilizer input the inevitable agricultural losses would lead to fewer nutrients producing lower yields, which in turn leaves fewer nutrients for recycling. The input of external nutrients, in the form of mineral fertilizer,  is necessary to compensate for the losses.

Figure 2.
Nutrient cycle in agriculture.

Constraints of organic fertilizers
Even if there were not any nutrient losses in the cycle, manure is not available in sufficiently large quantities to provide enough nutrients for the amount of crops grown today throughout the world. For example, Vaclav Smil (Enriching the Earth, MIT Press 2001) estimated that a worldwide organic farming mandate would require the manure from an additional 7–8 billion cattle to replace the mineral nitrogen fertilizers farmers currently apply year by year. The forage requirement for the additional cattle would leave almost no land available on which to grow crops for human nutrition.

Calculations show that mineral fertilizers will become even more important in the future. Figure 3 reveals that the world’s population will increase to about 8 billion in 2020. At the same time, agricultural land worldwide can hardly be extended. The available agricultural area per person will therefore decrease sharply during the next 20 years. As a consequence, the intensity of agricultural production per unit of land will have to be increased to provide more food for the growing population. Only mineral fertilizers can provide the additional nutrients. Mineral fertilizers provide therefore the only long-term source of nutrients to sustain future yield increases.

Figure 3.
Mineral fertilizers are the only source of nutrients to sustain future yield increase.