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Want Good Soil? Feed the Microbes

Farmers in the U.S. also use burning to deal with wheat stubble as in this Kansas field.

In June of 2014, Grist reporter Nathanael Johnson reported on a battle between two men in New South Wales Australia. Clive Kirkby and John Kirkegaard were having it out over the proper handling of crop residues after harvest. Kirkby was trying to get farmers to stop torching wheat stubble. Rather than letting fire release all that carbon into the atmosphere, he told them that they could increase soil organic matter and build healthier, carbon-rich soils by leaving the stubble in the field. John Kirkegaard, an agronomist, told Kirkby he was wrong. The practice of burning and cultivating was what was growing the best crops.

As most folks will tell you nowadays, cultivating, or plowing, disrupts soil microbes and releases even more carbon into the air. That’s why no-till is becoming increasingly popular. But the practice that Kirkby was promoting didn’t seem to be making a difference either. After six years of leaving stubble in the field, Kirkegaard’s data showed that soil organic matter and the carbon it holds wasn’t increasing, and in some cases, it was even decreasing.

Farmers have been encouraged to leave stubble in the field for the same reason that management-intensive grazing proponents leave plenty of forage behind in pasture: It’s food for the soil. Put more precisely, it’s fuel for a complex, not entirely understood food web of fungi, insects, and microbes eating the residue and each other and transforming plant remains into stable, carbon-rich soil.

But why wasn’t it working as Kirkby expected? Though he was near retirement, the mystery captured Kirkby and he headed back to school. He earned a PhD and, working with Kirkegaard, gathered the evidence that would solve this problem for farmers and ranchers.

It’s the Nutrients!

Electron micrograph of soil microbes. Photo credit: Alice Dohnalkova of PNNL

We talk a lot about microbes “eating” which paints a picture of chewing up bites. But consider the cow and her rumen for a minute. After she’s chewed and swallowed, her rumen microbes take over, and the chemical reactions they produce turns grass into sustenance for the cow’s body. The microbes are more successful when they’ve got the right mix of nutrients. For example, a little extra protein from a supplement makes it possible for a cow to survive on poor quality grass. The nitrogen in the protein supplement helps the rumen microbes turn what would otherwise have been unusable into good quality nutrition.

It’s the same for the soil microbes. Like all creatures on earth they need carbon, nitrogen, phosphorus, sulfur, oxygen and hydrogen to live. By leaving stubble on the ground, farmers gave them access to plenty of carbon. They can get oxygen and hydrogen from the air. But without nitrogen, sulfur and phosphorous, they can’t perform the chemical process to turn the stubble into soil carbon.

When he provided these nutrients, Kirkby and Kierkegaard found that everything changed. The soil microbes could finally consume the plant residue. You can see the difference in the two pans of sandy soil below. On the left is the untreated soil. The light specks are bits of wheat straw. On the right is what happens when nutrients are added. There are microbes in both pans of soil, but only the ones on the right had the right stuff to get their jobs done.

Photo courtesy of CSIRO Plant Industry/CSIRO Agriculture (CA Kirkby, JA Kirkegaard, AE Richardson

As Kirkby told Johnson in his interview, “With the right balance of nutrients, you get a population explosion. And that’s what you want. The carbon is in the soil’s organic matter, and that’s essentially dead bug bits. And live bugs. Humus!”

What This Means for You

The healthier your microbes, the better your soil, the more forage you produce, the more animals you can feed, the better your bottom line. That’s what’s in it for you. While we’ve known for decades that there’s a golden ratio of carbon, phosphorous, sulfur and nitrogen that create humus, Kirkby and Kirkegaard have taken it the next step to show us that the microbiome depends on these nutrients to function and create the humus.

Kirkby and Kirkegaard note that farmers typically fertilize just enough to get optimum plant growth. Their discovery that feeding the microbiome creates healthier soils means that farmers should consider fertilizing in ways that meet the needs of the microbes as well.

What Can You Do With This?

This photo from TomKat Ranch shows what 1/4″ of compost looks like in pasture. Research has shown that this treatment can increase carbon sequestering for up to 30 years.

Organic matter is microbes, and if you want more you need to give them what they need. You can fertilize with these ingredients. But that can be expensive. Here are two alternatives: One is to manage your livestock so that they spread their nitrogen rich manure and urine around the pastures, giving those microbes that added boost they need.

Another option is compost. Farms that apply compost have more soil organic matter and store more carbon in the soil. In fact, research in California has found that applying a half inch of compost doubled forage production, increased soil water holding capacity and increased net ecosystem carbon storage by 25-70%. Check out this two-part series for more about how you can use compost to improve soil health and carbon sequestration, while growing more forage for your livestock. It’s a win-win!


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