The N word

Why the fixation with nitrogen fertilisers?

There are presently 7.8 billion people on Earth and 50% of these people rely upon nitrogen fertilisers to provide food. Around 83 million tonnes of nitrogen (Advances in Agronomy, vol 87, 2005 pp 85 – 156) are used annually which is a hundred-fold increase over a century. Growing rice, wheat and maize accounts for 60 % of global nitrogen use.

The British Survey of Fertiliser Practice ( Fertiliser use on farm crops for crop year 2019) states that “most agricultural soils in Great Britain contain too little naturally occurring plant-available nitrogen to meet the needs of a crop so supplementary nitrogen applications must be made each year.”

Since 1966 the UK has used 62,656,000 tonnes of nitrogen on its arable and grassland. Nitrogen use efficiency (NUE) is generally between 30 – 50% with less than 7% of the unused nitrogen being available for following crops so somewhere between 17,481,025 and 29,135,040 tonnes of nitrogen have been lost from the soil-crop system to cause disruption in ecosystem functions. There are, of course, the natural gas used and emissions created by the manufacture of the lost nitrogen to be considered too.

This is not sustainable!
From just those few statistics a number of questions are raised:

  • Where is all the lost nitrogen going?
  • Why have there been no improvements in NUE?
  • Why do we keep doing it?

The answer to the first question is: into the air and into water. The British Geological Survey have warned of the UK’s “Nitrate time bomb” because the historic high levels of artificial nitrogen fertilisers have not all filtered down into the groundwater aquifers due to underlying geology slowing down its progress. The true peak of nitrogen pollution in groundwaters might not become apparent for another 60 to 100 years. In 2015, already, 37% of groundwater bodies were failing there regulatory objectives because of nitrate and hundreds of millions of pounds were being invested by water companies in equipment for dealing with high nitrate levels in groundwater.

Although livestock farmers are said to be responsible for the bulk of nitrogen emissions (ammonia and nitrous oxides) a portion will be down to volatilisation etc of nitrogen fertilisers. To the extent, in fact, that the UK government are in the process of considering a ban, or restriction in use, of solid urea because of the potential for ammonia emissions.

NUE has not greatly improved in many years because there are so many factors affecting the uptake of nitrogen from the soil by plant roots e.g. soil type, plant species and environmental factors as well as the timing and formulations of nitrogen fertilisers applied. Plant breeding can improve the potential for nitrogen uptake but this can be limited by the chemical and physical properties of the soil in which they are grown (pH, texture, aeration, water etc).

A small number of high-quantity applications of nitrogen fertiliser can overwhelm the soil-crop system. The plant can only work at a predetermined rate so any excess is washed away. The flush of growth that you see after a dose of nitrogen might be a welcome sight after a long, cold winter but it is actually the crop responding to toxic levels of nitrate being taken up.

If Nitrate is overly abundant in the soil the plant has to take on board a lot of water to dilute it to prevent it’s toxic effects – the solution to pollution is dilution! Plant cell walls are put under incredible strain which makes them susceptible pest and disease attack. The plant also needs up to 15% of it’s energy to convert the nitrate to useable nitrogen so if energy (sunlight) is limiting there will be products of incomplete metabolism present and many of these have been shown to attract insect pests.

So why do we keep doing it? Why are we addicted to soil-applied nitrogen fertilisers?

Because we have always been told that it works, because we have been able to see yield improvements from nitrogen fertilisers and because, so far, the soil has been resilient enough to take it. We have an awful lot of knowledge but not much understanding. Research works by breaking down systems into smaller, easier to understand units from which we determine actions to improve performance – there can be many knock-on effects which then have to be researched to re-balance the system. Take pill A to cure disease X but then you need pill B, C and D to counter act the side effects of pill A……. which can be worse than the symptoms of disease X. Sound familiar? Increasing fungicide use with increasing nitrogen inputs?

Plants need nitrogen and we are far from going cold turkey on solid nitrogen fertilisers – there are literally billions of mouths to feed – but there is growing pressure at all levels, from governments down to individual farmers, for a re- examination of how things are done. We have the knowledge, now we need the understanding.

“There was a kind of superb arrogance in the idea that we only had to put the ashes of a few plants in a test tube, analyse them, and scatter back into the soil equivalent quantities of dead minerals……. but it was expecting altogether too much of the vegetable kingdom that it should work only in this crude, brutal way.”

The Soil and Health, Albert Howard 1945