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Synthetic Fertilizer – Where it Comes From and What it Does and Doesn’t Do

This week John Marble writes about the economics of adding fertilizer to pastures, so we thought it was a good time to take a closer look at nitrogen and fertilizer. This piece was inspired by some info John provided about nitrogen and it’s potential impacts.

A Little Background

As an example of the importance of guano, the Chincha Islands War began in 1864 when Spain seized the guano-rich Chincha Islands off the coast of Peru and Chile. Photo courtesy of Wikimedia commons.

In his 1898 inaugural address as president of the British Association for the Advancement of Science, renowned chemist Sir William Crookes made a dire prediction: The wheat eating peoples of the world were going to start running out of food in the 1930s. The reason – a shortage of nitrogen fertilizer.

Nitrogen is a critical plant nutrient and farmers of the day were using nitrogen in the form of ammonia from guano shipped from South America to improve crop yields. But guano was a limited resource, so Crookes urged scientists to find another solution.

Since nitrogen makes up about 78% of the earth’s atmosphere, German scientist Fritz Haber focused on a method for drawing nitrogen from the air. He and his assistant, Robert le Rossignol, developed a way to catalyze ammonia from hydrogen and atmospheric nitrogen. Carl Bosch, a BASF scientist, scaled up Haber’s tabletop machine to industrial-level production, and the Haber-Bosch process was born.  By 1913 one German plant was producing 20 tonnes of fertilizer a day using this process.

The Haber-Bosch process did exactly what William Crookes hoped and more. In fact, it’s estimated that about half the global population is supported by synthetic fertilizers. That’s 3 to 3.5 billion people fed thanks to the Haber-Bosch process.

Graphic from

The Good

Today, we use nitrogen from the atmosphere and hydrogen from the methane in natural gas to produce chemical fertilizer. That means that the price of fertilizer rises and falls with the price of natural gas. We also often combine nitrogen with other nutrients like Phosphorous, Potassium or Sulfur, nutrients that can be lacking in soils.

With proper fertilization, crop yields generally increase by 30 to 50 percent over what farmers would have gotten otherwise. Pasture yields can increase as well. Research in Iowa has shown that grass yield, measured in terms of dry forage, cow-days of grazing, or live weight gains of yearling steers, can be increased two to three times or more with adequate N fertilization.

Whether or not you choose to fertilize will depend on the need for additional forage, your legume content, your management, and the cost to profit ratio. For more on thinking about cost vs profit, check out John’s piece this week. For more on the ins and outs of how, when and why to apply fertilizer, check out this two-part series:

When Does Fertilizing Pasture Pay? – Part 1

When Does Fertilizing Pasture Pay? – Part 2

The Bad and the Ugly

When we spread fertilizer of any kind on a pasture or hay field, a portion of the N may volatilize (evaporate). Under good conditions, most of the N is absorbed by the soil, where soil micro-organisms convert it into a form that plants can use. But nitrogen can also be lost to run-off and erosion, increasing the threat of N molecules finding their way into surface waters, where they cause problems for biological systems. Nitrogen can also leach into groundwater supplies creating an acute public health threat in some places.

Adding Nitrogen to your soils can also discourage legumes in your pastures from naturally fixing nitrogen. After all, why do all that work if there’s plenty of free nitrogen already available in the soil? Fertilizing with Nitrogen also increases the possibility of excessive Nitrates in your forage. Finally, depending on your soil type, adding chemical Nitrogen fertilizer can ultimately drive the pH of your soil down, making it more acidic as time goes on. In fact, in areas where soils are not acidic enough, one of the prescriptions can be to fertilize.

Does Chemical Fertilizer Kill Soil Microbes?

The myth that synthetic fertilizer kills microbes has gotten a lot of play lately. The reality is quite the opposite, so let’s take a look at what we know about how things work.

First, there is no chemical difference between a nitrate molecule from an organic source of nitrogen and a nitrate molecule from a bag of synthetic fertilizer. Labs can’t tell the difference, and neither can plants. What makes organic fertilizer different is its slow rate of release compared to synthetic fertilizer which becomes available as soon as the fertilizer dissolves in water.

Could this rapid release of nutrients be dangerous? When researchers ran trials they found that adding synthetic fertilizer resulted in no change in bacteria or fungi numbers while organic fertilizer showed a slight increase in both. In addition, a ten year study looking at the difference showed that, when applied properly, nitrogen had minimal effects on soil microbes, soil biochemical properties or soil structure.

Dr. Ray Wiel is the author of The Nature and Properties of Soils the hallmark text on the soils. He tells us, “Most fertilizers actually stimulate microbial growth, either because they provide nutrients that the microbes need or more often because they stimulate plant growth, and the plant stimulates the microbes.”

He adds, “The main situation in which fertilizers actually kill the microbes is anhydrous ammonia injected in the soil in bands.” While the soil is fairly sterilized in a two or three inch diameter area around the injection site, the microbes quickly recolonize once the ammonia gas dissipates or dissolves in water, then becomes ammonium and it taken up by plants.

But what about the salts in synthetic fertilizers?

Here’s where the language of chemists and the language of the rest of us leads to confusion. To a chemist a salt is a compound made up of two or more ions. Table salt, or sodium chloride, is made up of sodium and chlorine ions. Ammonium nitrate fertilizer is made up of ammonium and nitrate ions, so they call it a “salt” as well.

But this kind of salt’s ions behave differently than sodium chloride. When it rains after we apply fertilizer, the water dissolves the fertilizer into ions and washes them into the soil. The ions don’t harm microbes or plants, rather they’re the food that they absorb. The process is the same for organic fertilizers like compost and manure. The process is just longer because the larger proteins and carbohydrates have to decompose and convert into ions – the exact same ions that fertilizer produces.

You may not have thought you needed to know all this about nitrogen and fertilizer, but now that you do, you can use this information to make better decisions about fertilizer applications, or impress your friends and colleagues with some interesting trivia. 🙂

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