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What can be done about badly depleted nitrogen levels in Africa’s soil

Nitrogen is one of the most important nutrients for soils and Africa doesn’t have enough. Reuters/Siphiwe Sibeko

African soils have been mined for their nutrients for far too long. Nutrients are removed in harvesting across the continent, but they aren’t being returned to the soil. This usually happens in the form of manures or fertilisers. As with a bank so too with soil: if you don’t deposit as much as you withdraw you’ll be left impoverished.

Nitrogen is a vital nutrient for healthy soils and healthy people. To feed its growing population in 2050, Sub-Saharan Africa will need to at least double its nitrogen inputs. Where will this nitrogen come from?

Nitrogen: the good and the bad

Nitrogen can be added to the soil via synthetic fertiliser, manure, or leguminous plants like soybean, peanuts and certain tree crops. Unfortunately the manure available to most African farmers is low in nutrients. This is because the livestock are not well nourished. Leguminous tree crops can provide nitrogen rich green mulches. But they require space that edible crops can use and they take several years to grow.

According to the integrated soil fertility management approach, organic inputs of good quality manure, green mulches and crop residues will be needed to build healthy soils. But many soils are currently so badly depleted in nutrients that inputs of synthetic fertilisers are needed to get decent plant growth that can start the process of building soils and boosting crop yield.

But there are dangers. If nitrogen inputs are not done carefully, there will be unintended consequences for the environment and human health. For example, if fertilisers or manures are applied excessively a form of nitrogen called nitrate can leach into ground water and streams. This threatens drinking water supplies and causing harmful algal bloom in lakes and estuaries.

Other forms of nitrogen can escape from the soil. These cause local air pollution and contribute to global climate change. The amount of added nitrogen that goes to the crop instead of being lost to the environment is called nitrogen use efficiency. The United Nations Sustainable Development Goals to end hunger and promote sustainable agriculture should use nitrogen use efficiency as one of the indicators of progress.

Historical trend of two measures of the efficiency in crop production, namely yield nitrogen and nitrogen use efficiency. Xin Zhang

A tale of two efficiencies

Adding the right amount of nitrogen to cropland is a worldwide challenge. The use of synthetic nitrogen fertiliser grew rapidly in North America and Europe after World War II. It led to increasing crop yield, a measure of land use efficiency in crop production - but also to low nitrogen use efficiency and growing air and water pollution.

Recent technological advances and environmental policies have reversed this trend. More nitrogen is going into the crops and less as pollution. Consequently, both nitrogen efficiency and crop yield increase.

Unfortunately, much of Asia is now following the same historic path of increasing fertiliser use and its accompanying pollution. This is because there is not enough attention on managing nitrogen for high efficiency.

Brazil, on the other hand, is following an intermediate path by doing this with growing nitrogen fertiliser use, growing crop yield, medium efficiency levels, and more modest increases in pollution than in other transition countries.

Countries in Sub-Saharan Africa clearly have been using too little nitrogen. They are depleting their soils and producing poor crop yield and this means unhealthy soils and people. The challenge lies in how to get farmers to increase their nitrogen input while maintaining high nitrogen efficiency and avoiding pollution.

In most African soil more nitrogen could be added before leaching or gaseous loss becomes a serious risk. There are exceptions, like leaching through very sandy soils or poorly timed additions. Fertilisers must have the appropriate balance of all essential plant nutrients, so that the crops can efficiently use the nitrogen.

High-efficiency pathway for Africa

How Sub-Saharan Africa moves forward in the next few decades is critical. A nitrogen intensive, low efficiency pathway like China today, the US and western Europe post-WWII is the costly path for both economy and environment. Inputs would need to quadruple, and about half of that would be lost to the environment as air and water pollution. That scenario is neither economically nor environmentally sustainable.

If the region can forge a new high-efficiency pathway to development, leap-frogging the low-efficiency phase and moving directly to higher nutrient inputs with high efficiency, then fewer expensive nitrogen fertilisers will be needed and human nutrition can improve with little environmental harm.

But following the second pathway demands much more effort, both domestically and internationally. It will involve stable and transparent governance, appropriate policies, technological transfer and capacity building.

It should be possible to triple current average cereal crop yields, from 1 ton/ha to 3 ton/ha while also managing for high efficiency and low pollution. This will require some subsidies of fertilisers and seed sources to make them affordable. They will also need market access for farmers to purchase inputs and sell goods and improved extension services to ensure their proper use.

The ratification of the Sustainable Development Goals by countries has shown the political will for reducing hunger and developing sustainable agriculture. Technologies and policies to help Africa move towards the high-efficiency pathway are available. The challenges are whether the political will in international and national arenas will be translated into support for farmers, tailored to local climate, soil type, and socio-economic conditions. This is essential to obtain and wisely use nutrient inputs to make soils and families healthier.

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