Contrary Science; Cover Crop Mixtures, Monocultures, and Mechanisms

This post from Andrew McGuire of WSU is part of an ongoing scientific discussion about the relative effects of cover cropping and monoculture. This excellent summary of historic and ongoing research helps to bring us up to date on this complex, fascinating, and timely subject.

Excerpt from the article:

Soil diversity not linked to plant diversity

One last contrary finding. Another common belief is that a diverse mixture of plant species will drive greater diversity in the soil than a monoculture. This is often given as a reason to grow cover crop mixtures. Here again, we have a new analysis of the accumulated research investigating this mechanism. Zhou et al. (2020) analyzed 1235 experiments done around the world on the effects of what they call global change factors on soil microbial diversity. These change factors included land use change, such as occurs when a natural ecosystem is converted to agriculture, and nutrient fertilization. From this analysis they make several surprising conclusions.

First, most changes in the number of species (alpha diversity) can be explained by a change in pH. pH! It truly is the master variable in the soil. This is good news, as we can and often do manage soil pH.

Second, as they state it, “Conversion from highly diverse natural ecosystems to homogeneous agricultural monocultures has a positive effect on microbial alpha diversity.”

Read that again, a positive effect. The same goes for conversion to pasture.

What is more surprisingly contrary is that this implies, and the authors state this, that changes in soil microbial diversity (number of species) are NOT linked to changes in plant diversity. I know, heresy, but thus sayeth the science.

This does not mean that the conversion to agriculture is all positive. The microbial biomass is reduced, probably due to the change from perennial to annual plants and the associated decrease in carbon flow to the soil3. Agriculture also changes the structure of the microbial community. What these structural changes mean in the crazily complex soil is a difficult question to answer, but they may all not be beneficial with respect to the soil or the environment. This does, however, remove another purported reason of growing cover crop mixtures.

Read Andrew’s full article here:

Fixing Climate Change – Boosting Nature’s Cooling System – Fixing The System

This exciting article from Fixing the System discusses soil as the engine of our planet’s cooling and carbon capture system.

Here’s an excerpt from the article:

“Rich soils exponentially increase the capture of water and carbon
What is powerful about healthy soil, and it really is the central element of the whole sponge discussion, is that now we have 66% of the volume of the matrix which is available for infiltrating and retaining water. That retained water is what can sustain plant growth. Because of these voids, and the increased surface area exposed by them, this healthy soil can vastly increase the availability of nutrients. Now we have the phosphorus, the calcium, and the zinc all exposed for microbial activity.

So the bio-productivity of that soil increases exponentially, simply by creating those voids. The rootability of these soils vastly increases, that is the roots can grow, and penetrate and proliferate. Instead of 6 inches, they can grow down to 6 feet, or 20 feet, so the volume of soil resource that is now available for plant growth, and the drawdown of carbon that we mentioned earlier, is exponentially increased.

Soil formation is the engine of nature’s cooling and carbon capture system
So the whole bio-productivity of these healthy soils, the resilience of those soils, the capacity to infiltrate, to buffer, to extend life vastly increases. This process is what nature did to create the biosystem, to create the hydrology, and in very simple terms, that is all we have to do.

The process is taking sunlight, carbon dioxide and water to produce plants, using photosynthesis to create sugars, and fungi and microorganisms that convert those sugars into stable soil carbon, which is just the carbon based organic detritus or ‘bed springs’. This process is how the Earth ran 95% of its heat dynamics and its natural hydrological cooling.

So, if we have to draw down 20 billion tonnes of carbon, if we have to rebuild this soil-carbon sponge, we simply need to copy nature, and speed up the soil formation process.”

Soil Health Quality Cards

The race is on during the National Decade of Soil. The states are racing to find better ways to measure soil health. As the new soil health tests are published by federal, state, and local resource managers, we will share them here.

REGIONAL CARDS

Oregon Soil Quality Card

Palouse and Nezperce Prairies Soil Quality Card

NATIONAL CARDS

Georgia Soil Quality Card

Maryland Soil Quality Card

North Dakota Soil Quality Card

New England Soil Quality Card

Ohio Soil Quality Card

Pennsylvania Soil Quality Card