Andrew McGuire

The Pyramid Scheme of High Organic Amendment Rates

There are many soil health benefits of applying high rates of organic amendments. For example, a recent meta-analysis found adding organic amendments increased soil organic matter across multiple studies by an average of 29% in croplands and 34% in grasslands (Figure 1 below, Beillouin et al., 2023). This is the power of winning a pyramid scheme. What the studies don’t consider, however, is what happens to the losers of a pyramid scheme. What do I mean by all this? With organic amendments, all of it had to grow somewhere.

A pyramid of squares leading to a top winner.

Nitrogen Fertilizer and Soil Organic Matter: What Does the Evidence Say?

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?

Graphic showing proportions of soil organic matter

Putting Numbers to the Difficult Task of Increasing 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.

What it takes to increase soil organic matter from 1% to 1.1% in the top 6”.

Sagebrush Shrub-steppe or Cheatgrass Prairie?

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?

Sagebrush with mountains in the background

Meeting Food Demand through Agronomic Engineering and Incremental Transformation

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.

Inforgraphic of agronomy considerations

Water Use Limitations of Cover Crops in Dryland Cropping

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.

Hand drawn figure of different fates of water

A Problem with Soil Inoculants

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.

Drawing of the environment selecting microbes for the conditions