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Nitrogen in the soil
How it gets lost and how to keep it by Rivka Fidel, University of Arizona If you garden, do lawn maintenance, or farm, you’ve probably added nitrogen fertilizer
the Southwest. In wetter areas, about 30% of nitrogen fertilizer is lost due to leaching. This leached nitrogen contributes to pollution in groundwater, lakes, rivers, and even the ocean.
Nitrogen is the most common nutrient to limit plant growth – because plants need quite a lot of it (10-60 g per kg of plant mass, to be exact). It also doesn’t stick around very long in the soil. Instead, it ends up in places where we don’t want it: in groundwater, water bodies, and even the atmosphere.
How can I reduce leaching?
to your soil.
But, why? And how can we get nitrogen to stay in the soil, where plants need it? To answer that question, we need to take a closer look at the nitrogen cycle. The nitrogen cycle, in which nitrogen moves through soil, water, air, and organisms, is one of the most complex element cycles. Luckily, keeping nitrogen in the soil simply involves reducing the losses of nitrogen from the soil. Losses are ways that nitrogen exits the soil.
UAC MAGAZINE | SUMMER 2019
To keep nitrogen in soil, we need to reduce four key losses:
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1 Leaching 2 Ammonia volatilization 3 Denitrification 4 Harvesting
What are leaching losses? Leaching happens when water travelling through the soil dissolves nutrients from the soil, and carries them downwards into the groundwater table. Leaching losses are largest in wet climates, especially regions with enough rainfall to support plant growth year-round. In the United States, for example, wetter regions east of the Mississippi River have much higher leaching losses than drier regions in
Leaching losses are greatest following rainfall or irrigation. So, reducing them is mostly a matter of timing. If rain is in the forecast, wait until after it rains to apply nitrogen fertilizer (a little drizzle won’t matter, but watch out for steady rain or thunderstorms). If rain is a long way off, say a week or more, it is ok to apply fertilizer ahead of time. The type of fertilizer also matters. Nitrate doesn’t “stick” well to soil, and so it is lost very easily. To reduce leaching losses, avoid nitrate (NO3–) fertilizers, and instead choose ammonium (NH4+) or organic fertilizers (including urea, composts and manures).
What are ammonia volatization losses? Ammonia volatilization happens when ammonium (NH4+) loses a hydrogen (H+) and becomes ammonia (NH3). Ammonia is a gas, and so it can rapidly leave the soil to pollute the atmosphere. Ammonia volatilization is most likely to happen in alkaline soils (pH > 7) and when the soil is warm.
How can I reduce ammonia volatilization? Avoid applying ammonium fertilizers, composts, and manures on warm to hot days – especially if your soil is alkaline.
What are denitrification losses? Denitrification is a multi-step process where microbes convert nitrogen (in nitrate form, NO3–) into various nitrogen gases. One of these gases is nitrous oxide, N2O, a potent greenhouse gas. The other is nitrogen gas, N2, which is harmless and comprises 79% of the atmosphere. Denitrification happens when the
