Andrew McGuire

Does synthetic nitrogen fertilizer burn up soil organic matter? Whether you are focused on soil health, soil sequestration, or soil carbon credits, this is an important question. The persistent claim is that synthetic N fertilizer can “burn” soil carbon by supercharging the soil microbes. This claim mainly arises from a 2007 research article from researchers at the University of Illinois (Khan et al., 2007; open access here) and has recently resurfaced in another article (Jesmin et al., 2021) and the resulting (flawed) media coverage. However, a single study is far from conclusive – so what does the broader scientific literature say? And what have we learned in the last few decades on the relationship between synthetic N and soil organic matter?

You may know that it is difficult to increase soil organic matter, but how difficult is it, with numbers? First, your crop harvest removes up to 50% of the biomass grown. Then, about 90% of the remaining crop biomass is decomposed by soil organisms leaving only 10% contributing to soil organic matter.  You also have to account for the annual 1-5% losses of existing soil organic matter. Using these and other estimates, let’s do some rough calculations so you know what to expect. The task is difficult, but the math is easy, I promise.

In a 2012 book, Donald Maier asked, “What’s so good about biodiversity?” He describes how difficult it is to critique principles of biodiversity because all the value of the natural world has been bestowed on…

Although I work in irrigated agriculture, the views on my morning commute are all sagebrush, or the shrub-steppe as this native plant community is called. And cheatgrass, a lot of cheatgrass. Where there have been recent fires, stands of cheatgrass thrive. Sagebrush, the iconic plant of the shrub-steppe ecosystem, is having a hard time. The combined effects of fire frequency, climate change, and cheatgrass invasion have made sagebrush recovery an uphill battle. Will the shrub-steppe recover to its former subtle beauty, or should we get used to the cheatgrass prairie?

In a realistic scenario, where not everyone gives up eating meat, where some in the developing world eat more like us, and where food waste is not zero, feeding 9+ billion people will require a lot more food. Ideally, this additional production would be from existing cropland, with better input efficiency, and fewer off-farm effects. How are we going to do this, both in currently high-yield agriculture and where significant yield-gaps exist? This is the topic of an important book chapter from Hunt, Kirkegaard, Celestina, and Porker (2019): Transformational agronomy: Restoring the role of agronomy in modern agricultural research.

I have been called a reductionist quite a few times. I never know how to respond. Am I a reductionist? If so, is that a bad thing? Why is there resistance to the approach? So I did some investigation.

I have seen it work. As a graduate student, I researched cover crops in a California dryland wheat system, comparing a wheat-fallow system to one with a cover crop replacing fallow (McGuire et al., 1998). A wet winter allowed for successful wheat yields in both systems. However, research results suggest that this is often the exception in dryland agriculture. More often, water use by the cover crop reduces the yield of the following cash crop.

Long-term cropping systems research is expensive, difficult to manage, and therefore rare, especially for vegetable crops. So when results are published for potato cropping systems, it is worthwhile taking a look. A team from Maine recently reported the results of experiment comparing four cropping systems:

As I wrote in my last post, the focus and excitement around soil biology leads many to believe that they have a soil biology problem. To fix this perceived problem, some recommend trying various types of soil amendments or inoculants to enhance the soil’s microbial community. This includes compost, manure, and their extracts and teas. There is a problem with this strategy though; the microbes at home in manure, compost, or in their extracts, tend to die off in the soil.

I am not sure of the causes. Perhaps it’s the demand for soil health information which far surpasses the supply of science-based content? Or maybe the speculation and exaggeration that this high demand produces? The intense focus on soil biology for soil health (Coyne et al. 2022) is surely a factor. And the excitement that comes with learning about the new discoveries in soil microbiology. Whatever it is, the results are clear.