Nutrient resources

Because nutrients are so vital for agriculture and for the population that depends on it, it is natural that questions are posed about the long-term viability of nutrient resources. Nutrients are not consumed in the sense that fuels are. Nutrients continue to exist in natural cycles, but the cost and technical feasibility of recovering those nutrients varies. Whereas nutrients are eternal, high-concentration sources of nutrients may be finite.

The four nutrients used in the greatest quantity for agriculture are nitrogen (N), phosphorus (P), potassium (K) and sulphur (S).

Nitrogen is now sourced from the atmosphere, which is composed of 78% N. Before the advent (in the early 20th century) of the Haber-Bosch process for synthesizing ammonia from atmospheric nitrogen, the availability of nitrogen in plant-available forms was a significant limit on global food supplies. Although the nitrogen supplied from the air is virtually limitless, the process also requires hydrocarbon (primarily natural gas) or coal feedstocks. These perform a dual role as sources of both hydrogen and energy. Resources are considered adequate for the foreseeable future. (USGS 2009a)

Phosphorus is a common element, ranking eleventh in order of abundance in the Earth’s crust. It is currently mined from rock deposits, which are finite and should be managed efficiently. The most important of these are sedimentary, although there are also some igneous deposits.

Deposits are classified as reserves if they can be economically extracted or produced at the time of determination. Since technology and economic conditions evolve constantly, the amount of reserves are in constant flux. Furthermore, since phosphate is a strategic resource, not all reserves may be publicly divulged. For example, China recently revealed that its reserves are much higher than previously acknowledged. Combining the estimates of the United States Geological Survey (USGS 2009b) of current phosphate reserves with IFA statistics on phosphate consumption, it is possible to calculate that current reserves should be sufficient for at least a hundred years.¹

The reserve base – which includes those resources that are currently economic, those that are marginally economic, and some of those that are currently subeconomic – is much larger. Furthermore, as the most economical resources are used, the incentive increases for technological breakthroughs that would make it viable to recycle much more of the P found in urban, industrial and agricultural waste streams, as well as tapping the vast amounts of phosphorus found in the oceans.

Potassium is mined from buried deposits of marine origin, but a significant portion of production also comes from the evaporation of surface brine deposits. Potash deposits occur on every inhabited continent, but production is currently dominated by 12 countries. On a worldwide basis, there appear to be as many as 100 large buried deposits that could produce products of commercial value and about the same number of significant brine deposits. (UNIDO/IFDC 1998) In its January 2009 Mineral Commodities Summaries, the United States Geological Survey indicates that world potash resources would most certainly suffice for at least a century and possibly thousands of years at current consumption levels. (USGS 2009c)

Sulphur is the sixteenth most abundant element in the Earth’s crust. It can be mined in its elemental form, but virtually all elemental sulphur is currently obtained as a co-product recovered from oil and gas production. This source is considered adequate to meet world demand for the foreseeable future. (USGS 2009d)

¹ Syers et al. 2008 estimate that current resources could last between 105 and 470 years, although they note other estimates of up to 1000 years.