Controlling nitrate


Agricultural nitrate has polluted water for decades. Today various techniques make it possible to more effectively control the use of this necessary fertiliser. It’s a much more complex problem than is generally believed.

No nitrogen, no crops! That’s the inescapable starting point of responsibly managing this chemical element: agriculture can’t do without nitrogen (N) or the nitrate which derives from it (see box). But it must be used in a certain form and way.

First World War consequences

For centuries, farmers relied on the mineralisation of organic matter in soil and gave nature a boost by spreading manure. Crops and livestock were closely linked. Thanks to bacteria, these organic materials created nitrate, which can be absorbed by plants, whereas, with some exceptions, airborne nitrogen cannot. But in 1909, the famous German chemist Fritz Haber discovered the chemical process of synthesising ammonia (NH3) from nitrogen in the atmosphere and, four years later, the German company BASF, through the cutting-edge Haber-Bosch industrial process, was able to manufacture derivative products such as nitrate, and especially nitric acid, the precursor to many explosives. The technique would be used extensively during the First World War. When the war ended and the demand for explosives fell, factories were reconverted to produce nitrate for agricultural use. With the ability to create infinite amounts of this fertiliser, the industry overcame what had always limited agricultural production: dependence on natural soil mineralisation or organic fertiliser from distant deposits such as the well-known Peruvian guano. Yields increased, peaking with the post-Second World War agricultural revolution and the introduction of intensive agriculture.


The problem,’ says Prof. Richard Lambert, director of the UCLouvain Michamps Agri-environmental Centre, near Bastogne, and member of the Louvain4water research cluster, ‘is that the plant responds to nitrate up to an amount beyond which yield doesn’t increase. Farmers wanted high yields, but they didn’t have the tools to calculate optimal doses. So they often tended to spread more than was necessary.’ Exacerbating this was the development on some farms of enclosed livestock production whose manure and slurry were sometimes spread on nearby fields. In short, for decades, plants overdosed on nitrate. The consequences? Nitrate is soluble in water; if not absorbed by roots, it will percolate into groundwater or surface water. As early as the 1960s, its concentration increased both in groundwater (thus posing a drinking water hazard) and in surface water (resulting in eutrophication).

Nitrate directive

In 1991, Europe adopted a directive to reduce agricultural nitrate water pollution; the Walloon Region transposed the directive into its legislation while creating a structure (Nitrawal, today PROTECT'eau) to help farmers improve their nitrogen management practices. Its scientific members are the UCLouvain Earth and Life Institute, Gembloux Agro-Bio Tech (University of Liège), and the Walloon Agricultural Research Center (CRAw). ‘One of our missions,’ explains Marc De Toffoli, head of the UCLouvain scientific unit at PROTECT'eau, ‘is setting up and monitoring a network of baseline farms in which we carry out measurements of potentially leachable nitrogen, which define annual standards for each crop.’ This is where it gets complicated, because no two plants, or seasons, or soils, are alike.

Potentially leachable nitrogen

Every spring, scientists project soil nitrate content using samples from baseline farm plots. They measure the sample’s nitrate, then estimate the amount that will be produced by both soil mineralisation and the farmer’s organic fertiliser (manure, slurry). From these three values, and according to crop and region, scientists deduce how much nitrate will be required to achieve a realistic yield. At the end of the season, after harvesting, they will measure the amount of residual nitrate, the so-called potentially leachable nitrogen. Every year, for each crop, they set standards that should not be exceeded. End-of-season inspections of farmers selected by lot determine whether standards have been violated.

Nitrate traps

Determining exactly how much nitrogen to apply to satisfy crop needs is obviously an important first step’, says Prof. Lambert. ‘But it’s not enough. We’re also working on how to use less chemical fertiliser, through better use of farmyard fertiliser, fractionating inputs, and understanding more fully the soil mechanisms that mineralise organic matter.

Chemical fertilisers are indeed expensive for farmers and their synthesis consumes a lot of fossil energy. One farmer confessed to Prof. Lambert, ‘If I’d always known what I now know today and had applied your methods from the beginning, I could have bought a villa on the Côte d'Azur!’ Today, there are many ways to reduce chemical fertiliser use and the losses they entail. The first is to use ‘nitrate trap’ crops. ‘Take the example of a cereal that was harvested early in the season’, Mr De Toffoli explains. ‘As mineralisation of the soil’s organic matter continues and there are no more plants to use the nitrate it produces, it’ll accumulate in the soil and be washed away by water in winter.’ The idea, after harvesting the grain, is to sow mustard, for example, which continues to absorb nitrate and prevents it from being leached. The mustard is not harvested but, after winter, buried in the soil and broken down by soil micro-organisms, which will provide input for the main crop. This is now a practice so well-established that a farmer told Mr De Toffoli, ‘I produce one crop for the market and the other for the soil!

The UCLouvain team aims to assess as precisely as possible the influence of such green fertilisers, as well as of farm manure. Regarding the latter, farmers have long been unable to do what they want. Legislation sets a timetable and maximum doses for spreading manure and slurry. ‘We’re now working on nitrogen production standards for livestock’, Prof. Lambert says. ‘In the Walloon Region, a dairy cow, for example, produces an average of 90 kg of nitrogen per year in her excrement. This standard is used to calculate what is known as the farm soil binding rate.’ Suppose a farmer livestock produces more organic nitrogen than he can spread on his land; he must then find farmers to buy his manure according to contracts sent to the Walloon administration. Each farmer receives his soil binding rate each year. If it’s less than one, he can use his manure on his farm; if it’s greater, he must find farmers to buy his excess manure. If he finds none, he’s liable to penalties.

Results that percolate…slowly

The few examples shown above show that sustainable nitrogen management exists. Does this mean that we can do without expensive, energy-intensive chemical fertilisers? No. And certainly not if we want to maintain current yields. Because nature remains the master of the game. The potato is a good example. It has high nitrogen requirements but a very short nitrogen-absorbing period, and its shallow root system doesn’t exploit soil well. Since the process of releasing nitrate from organic fertilisers is slow, very large quantities of organic fertiliser would be required to produce enough nitrate over a short period of time. But it also means that the release of nitrate will extend beyond the plant’s period of need. ‘So it's best to work with a base of organic nitrogen and supplement with mineral fertilisers that have a much faster effect. It’s also possible to apply mineral fertiliser precisely in the root zone or fractionate the inputs and thus intervene during cultivation, when the plant’s needs are acute.

That said, today’s good practices are beginning to bear fruit. They remain difficult to assess at groundwater level, however, because nitrate percolates at about one meter per year on average. It takes about 20 years for nitrate to reach a water table 20 meters below ground level, and as much time to see effects on the table of sustainable agricultural practices. All the more because tables have large volumes; it will take years for cleaner water to reach them and lower the overall volume’s nitrate concentration.

An uncertain future

But researchers are especially worried that progress will be undermined, first by changes in farming practices. There is indeed less livestock, hence less organic fertiliser, but also less grassland, which is a store for carbon and nitrogen. On average, the amount of nitrogen in grassland is 5,000 to 15,000 kg/ha, whereas in arable land it’s only 2,000 to 4,000 kg/ha. The destruction of a meadow causes rapid and significant mineralisation of nitrogen and releases nitrate in quantities often much higher than what the crops that follow need. There are also fewer beet and other crops that absorb nitrogen over a longer period and more deeply in the soil, unlike potatoes which, as we have seen, leave behind a lot of nitrate. Finally, our climate shouldn’t be experiencing droughts as frequently as it has been in recent years. ‘They result in lower yields and plants mature more quickly, so there’s less nitrogen removal’, Prof. Lambert explains. And in winter, there’s less water to recharge aquifers and therefore less nitrate dilution.’ Unfortunately, the impact can’t be predicted at the moment nitrogen fertilisation forecasts are necessary.

Nitrogen, nitrate, plants and water

Nitrogen (N) is the majority component (78%) of the earth's atmosphere (in its molecular form N2). It’s a chemical element essential to life (for building amino acids and proteins), and thus to plant growth. Like most living things, plants are unable to extract nitrogen from the air; they must find it (or have it provided) in a form other than N2. Legumes (clover, alfalfa, beans) are an exception: their roots have nodules home to bacteria (Rhizobium spp.) that can absorb airborne nitrogen to make amino acids. Other plants absorb nitrogen mainly in the form of nitrate provided by synthetic fertilisers or produced from organic matter. In the soil, bacteria transform organic fertilisers and organic matter containing nitrate (NO3-), which is absorbed by plants. This nitrate ion is soluble in, and thus carried by, water (it’s leachable).

An excessively high concentration of nitrate renders water unfit for consumption (above 50 mg/l according to the World Health Organization) and causes eutrophication (resulting in excessive growth of algae).

In Wallonia, in 2015, the concentration of nitrate was not good (>25mg/l) in 20% of surface water and about 30% of groundwater, but with very contrasting situations. It’s in the areas of Comines-Warneton, Hesbaye and Pays de Herve that aquifers are impacted most, while the basins of Namur, southern Luxembourg and the Ardennes are much less affected.

A glance at Richard Lambert

Richard Lambert was reluctant to take over the family farm before earning a bachelor’s degree in bioengineering at UCLouvain in 1989. His PhD thesis was devoted to grass and grassland growth as a function of climate and nitrogen (2001). At the university, he works for the scientific unit within the Walloon framework formerly called Nitrawal, today PROTECT'eau. In 2007, he took over the management of the Michamps Agri-environmental Centre, near Bastogne, a centre for agronomic, agri-food and environmental analysis and advice funded by UCLouvain and the Province of Luxembourg..

A glance at Marc De Toffoli

After studying industrial agricultural and environmental engineering in Huy, Belgium, Marc De Toffoli worked for 10 years on the development of environmental and climatic agricultural methods for the Walloon Region, and provides expert advice to farmers. In 2008, he replaced Prof. Lambert as an engineer in the UCLouvain scientific unit at PROTECT'eau.

Published on March 28, 2019