Fertiliser availability in a resource-limited world: Production and recycling of nitrogen and phosphorus

Abstract

Without the input of fertiliser nitrogen it is estimated that only about half of the current global population can be supplied with sufficient food energy and protein. The anticipated increase in the population to 2050 will increase the dependency on fertiliser inputs. The paper examines the different potential sources of energy and hydrogen required for this essential fixation of atmospheric nitrogen into plant-available nitrogenous fertiliser and concludes that methane from natural gas is clearly the most suitable source. In the absence of a cost-effective alternative source of hydrogen it is recommended that an on-going requirement for methane is acknowledged and that consideration be given to strategic reserves for the production of food. Phosphorus is also an essential and unsubstitutable nutrient for plants and animals, but while the global reserves of atmospheric nitrogen are effectively unlimited, the reserves of phosphate rock are finite. Recent estimates of the reserve suggest that at the current rate of use this resource will become exhausted within some hundreds of years. The annual increment of phosphorus contained in the human population is estimated to be in the order of 1 Mt/yr, which is a small proportion of the quantity mined. There is a clear requirement to ensure that phosphorus is recycled to a large extent, so that the rate of exhaustion of the reserves of phosphate rock is significantly reduced. Legislation relating to the management of phosphorus appears entirely associated with its potential to upset natural ecosystems, with apparently no regulations yet requiring the efficient use and reuse of a scarce resource.

Introduction

Fertile soils are the key to sustainable commercial-scale production of crops for food, feed and fibre. Very few agricultural soils are fertile without addition of plant-available nutrients either recovered and recycled in origin such as farmyard manure, or manufactured such as fertiliser. Most soils are not sufficiently fertile, requiring periodic but regular treatments with macronutrients such as nitrogen (N), phosphorus (P), potassium (K) or other nutrient elements required by all higher plants. Of these, some are in abundant and non-limiting supply, such as carbon, hydrogen and oxygen from the air and soil solution. Calcium, sulphur (S) and magnesium are generally available macronutrients, but may need application to some soils. Others, such as boron, chlorine, copper, iron, manganese, molybdenum and zinc (the so-called micronutrients), are required in small quantities that many ‘fertile’ soils are usually capable of supplying without amendment. In quantitative terms however, it is the role of N, P and K in commercial agriculture that predominates and hence is a critical dependency in any strategy designed to result in food security. Global population growth, which has accelerated markedly in the past 150 years, correlates closely to the discovery and development of industrial-scale fertiliser production. Fertilisers do not make plants grow, but rather it is the lack of the nutrients they contain, or an imbalance in supply, that can prevent the plant from expressing its full productive potential.

Crop nutrients are available in a naturally recycled form, such as in crop residues and manures. The management of these nutrient sources underpinned agricultural production for many centuries, but from an empirical rather than scientific understanding of processes and fluxes involved. As pressure on productive soils grew during the 19th century in the wake of industrialisation and population growth, agricultural soils grew steadily less productive, with fertility falling as nutrients were removed in the harvested crops. The first limitation was an increasing deficiency of P. If the commercial manufacturing process for phosphatic fertiliser had not been discovered in the 1840s, life in the UK and in other industrialised countries would have become unsustainable. The global population which was threatened by a lack of soil fertility in 1850 was less than 1.5 billion; by 2050 this may have grown to 9.2 billion, all of whom will depend on a sustainable solution to the supply of nutrients to soils to replace those removed at harvest.

The benefit of N was known through the use of ammonium sulphate, but the true potential of N only became apparent following the alleviation of the severe P deficiency. In the 19th and early 20th century reactive¹ N was in very limited supply, and pressure was intense to find a cost-effective technology for ‘fixing’ N from the air. The goal, which eventually won a Nobel prize, was finally achieved in 1909 as the so-called Haber Bosch process for the manufacture of ammonia. Although there were other uses for ammonia, including as a precursor for the production of explosives, this breakthrough led to the establishment of the N fertiliser industry.

Of N, P, K and S, the four major nutrient inputs required for productive agriculture, K and S are not anticipated to be in limiting supply, nor is there a significant energy requirement in their processing. This paper therefore considers the critical and mutually dependent roles of N and P. The finite and depleting nature both of the energy sources currently used in the production of ammonia for N fertilisers and of known global phosphate rock reserves requires our attention. While the timescale of depletion is measurable over centuries rather than decades, timescales potentially at odds with present-day commercial interests, new awareness of the need for resource conservation and sustainability invites us to consider ways of extending that timescale out by thousands of years, if not indefinitely. This objective will require a major change of thinking on the part of all stakeholders in the food production and consumption continuum, including the fertiliser industry. Since 1945 developed countries have focused their food policies on quantitative goals, driven largely by the experience of food shortages and rationing during and after World War 2. This policy has probably been too successful: among its unintended consequences are a commoditisation of both food and fertilisers and a consequential breakdown in society’s understanding as to the true origins, and costs, of the food it eats. A major change of direction is required, starting with a rethink of food policy and the business model on which the ‘farm to fork’ supply chain rests. This need is well articulated in the ‘reconnection’ agenda set out in the report of the UK Policy Commission (Curry, 2002). Modifying stakeholder behaviours, for example in respect of diet and food waste prevention, will also be critical to future sustainability, while strategic plans for land use, input management and food security require both stakeholder accountability and international collaboration. At the same time, any change of policy direction must factor in the need to maintain the viability of the fertiliser industry on which food security will continue to depend.