Nitrogen Fixation

The threat to food security

Oil is not as abundant as it used to be and reserves are becoming more difficult and expensive to extract and the days of cheap oil are coming to an end. This tipping point is known as peak oil. Arguably peak oil has already been reached and, with increasing demand from emerging markets, the price of oil will continue to rise dramatically.

With the reduction in oil will come a peak in gas supplies. Cheap flights, cheap imports and the global distribution of food will be things of the past. Intensive agriculture and the global food trade depend on large quantities of fossil fuels for transport and for the industrial production of nitrogen fertiliser. Nitrogen is important as it is the nutrient that’s availability most often limits the growth of plants.

Industrialisation

With the growth of towns and cities and the increases in population that came with industrialisation, the importing of food from rural areas to urban areas meant a loss of nutrients from soils without replenishment. These imported nutrients found their way into sewers; little, if any, were available for recycling in the field of origin. During the 19th century supplies of nitrates for use as fertilizer were obtained from South America. Peru had massive reserves of guano. Guano is the name given to any excrement from birds, seals or bats with value as fertilizer. In this case bird droppings that had built up to many metres thick on the rocky coast. Chile had a good supply of sodium nitrate, a source of highly soluble nitrogen that was mined from the largest source in the world in the Atacama Desert in Northern Chile. These days the vast majority of the food consumed in this country, and up to 40 % worldwide, is grown from fertilizer produced using the Haber-Bosch process.

The Haber-Bosch process

At the beginning of the 20th century, a time of tension in Western Europe, Germany felt vulnerable because it was reliant on supplies of nitrates from South America not only for use as fertiliser but for the production of explosives. Germany felt it must become self sufficient in nitrates as supplies from overseas could be cut off by a blockade at any time.

In 1909, chemist, Fritz Haber devised the chemical solution whilst chemical engineer Carl Bosch developed Haber’s technique into a working plant which began producing nitrates in 1913. World War 1 broke out in 1914 and supplies of nitrates from South America were stopped from reaching Germany. The Haber-Bosch process ensured German self sufficiency in nitrates and historians argue, helped Germany sustain a war that would have otherwise ended much sooner.

The Haber-Bosch process began to be used worldwide during the 1940s and the basic chemistry is still used today for the industrial production of nitrogen fertiliser. Hydrogen gas from fossil fuel, often from natural gas (CH4) is combined with nitrogen gas from the air (N²) at high temperatures and pressures, using nickel and iron as catalysts – a catalyst being a substance that speeds up the chemical reaction without being consumed itself.

Nickel/750degrees C/30 atmospheres
CH4 (Natural Gas) +2H2O (Water)———–CO2 (Carbon Dioxide) + 4H2
(Hydrogen)
Iron/400 degrees C/200 atmospheres
N2 (Nitrogen) +3H2 (Hydrogen)——————2NH3 (Ammonia)

The ammonia produced is then combined with other compounds for use as fertiliser, such as with sulphate to make ammonium sulphate, (NH4)2SO4.

As can be seen within the chemical equation it not only takes fossil fuels to provide a source of hydrogen to combine with nitrogen – in order to create a form of nitrogen that will eventually be able to be used by plants – it also takes masses of fossil fuel energy to create the vast temperatures and pressures needed for the process. This is truly unsustainable especially with the worries of peak oil, not to mention the fossil fuel use that goes into the production of pesticides and herbicides.

As is stated in Craig Sam’s Little Earth Book: A farmer using pesticides and (synthetic) fertilisers uses 12 calories of energy from fossil fuels to produce 1 calorie of food energy. A subsistence farmer with a hoe uses 1 calorie of energy to produce 20 calories of food. A sustainable ‘revolution’ lies somewhere between these extremes of high and low tech farming.

Nitrogen fixation in nature and the organic way

Almost 80% of the earth’s atmosphere is composed of nitrogen gas (N2), a molecule of which consists of 2 atoms of nitrogen held together by a strong triple bond. Most plants do not have the means to break this triple bond and make use of nitrogen for growth by this method, instead relying on other plants that have broken down re-releasing nitrogen in a more readily taken up form.

However some plants and organisms are able to break the bond and are essential to the cycling of nitrogen within the terrestrial environment. The alder tree and bog myrtle are two species able to fix nitrogen and are found in waterlogged soils in this country. Lichen are also able to fix nitrogen for growth. Some free living bacteria can fix nitrogen: in rivers and seas, blue green bacteria or cyanobacteria are important free living fixers of nitrogen in the sea.

The biggest plant group able to fix nitrogen and the most important to the organic farmer are the legumes, otherwise known as the pea and bean family, which includes clovers and fenugreek. Legumes form a symbiotic relationship with Rhizobium bacteria. (A symbiotic relationship is a close association between two different species where both organisms benefit from the relationship.) Rhizobium bacteria possess an enzyme that can break the strong triple bond present in nitrogen gas in the air. Leguminous plants provide the bacterium with a supply of sugars for energy and an oxygen free home (important because the enzyme is poisoned by oxygen) in knobbly nodules on the side of the roots.

Non-organic farmers apply industrially fixed nitrogen from a bag in the hope that the nitrogen will be taken up by plants, but it is possible nitrogen may change its form and be lost from the soil as gas or washed from the soil leading to a risk of pollution. Organic farmers, on the other hand, grow legumes in order to make use of their capacity to fix nitrogen naturally from the air, reducing external inputs to the farm, as opposed to using nitrogen from a bag fixed industrially in the Haber-Bosch process.

Through growing legumes, organic farmers incorporate nitrogen biologically into plant material, boosting levels of organic matter which is important for soil structure, regulating plant nutrition and keeping the soil alive with bacteria, fungi, worms and other soil organisms – thus sustaining the growth of healthy plants and a healthy balanced field scale ecosystem.