For several years, I have occasionally encountered the name, Dr. Christine Jones, an Australian soil microbiologist. Perhaps because I’ve been spending more time recently reading about soil biology, I’ve encountered more references to her. Out of interest, I decided to investigate further. That was not too difficult given that she has provided links to her writings on a web site called, Amazing Carbon . The following is based on several articles posted there.

Dr. Jones is obviously deeply committed to soil improvement and the importance of soil microbial activity. What she promotes is a combination of both the orthodox and the unorthodox. There is no hint that any of this is done for personal gain or product promotion, other than for her own services as a consultant.

I like her emphasis on plant roots and root exudates because they are significantly more important for building soil organic matter than aboveground crop residues. She also emphasizes the importance of arbuscular mycorrhizal fungi (AMF) to soil health and plant growth.

Some of her other statements, however, make me quite uneasy.

She states in a March 2015 interview with Acres, the publication of an alternative-agriculture group in the United States, “If a plant photosynthesizes faster it’s going to have higher sugar content and a higher Brix level. Once Brix gets over 12, the plant is largely resistant to insects and pathogens.” I doubt that many plant pathologist and entomologists would agree that what is, effectively, lusher plant growth ensures pest resistance.

In the same interview, she expresses major concern about cancer caused by pesticides and other crop inputs: She states, “Not that long ago the cancer rate was around one in 100. Now we’re pretty close to one in two people being diagnosed with cancer. At the current rate of increase, it won’t be long before nearly every person will contract cancer during their lifetimes.” No mention is made of the age effect on cancer incidence (modern people live longer). When the age effect is removed statistically cancer rates are mostly stable over time or going down.

Dr. Jones expresses concerns elsewhere in her writings about claimed links between nitrogen fertilizers and health, and the general unhealthiness of modern farm crops and foods. These are claims that I don’t believe are much supported by most credible research.

On nitrogen, I also found this statement puzzling: “the application of high rates of inorganic nitrogen has many unintended negative consequences on-farm. These include deterioration of the water-stable aggregates important for soil structural stability.” Most soil scientists would argue, I believe, that nitrogen fertilizer at recommended application rates stimulates plant and root growth, thereby increasing the amount of soil organic material and helping to build water-stable soil aggregates. Of course, much depends on the definition of “high.” Maybe extremely high rates do affect soil as she states.

This excerpt, which appears in identical format in at least two or her articles, implies that crop plants are less affected by drought stress if surrounded by other plants (including weeds, I assume).

I was once involved in research on drought in corn and this disagrees with what we believed true then.

I am as keen as other farmers on cost-effective ways to increase the organic matter of soils. Dr. Jones devotes major attention to this subject. She expresses disappointment in scientists who say it’s difficult to do this, and presents her own numbers implying the converse.

I was intrigued by these paragraphs from a 2010 presentation at an agriculture and greenhouse emissions conference:

“Recent research by United States Department of Agriculture (Liebig et al. 2008) investigated soil carbon sequestration under a perennial native grass, switchgrass (Panicum virgatum) grown for the production of cellulosic ethanol.

“Despite the annual removal of aboveground biomass, low to medium rainfall and relatively short growing season, the USDA-ARS research, averaged across 10 sites recorded average soil carbon sequestration rates of 4tCO2/ha/yr in the 0-30 cm soil profile and 10.6tCO2/ha/yr in the 0- 120 cm profile (Liebig et al 2008).

“The best performing site was at Bristol [South Dakota], where soil carbon levels increased by 21.67 tonnes in the 0-30 cm soil profile over a 5 year period. A soil carbon increase of 21.67tC/ha equates to the sequestration of 80tCO2/ha.”

I checked the Liebig et al paper (link here). You might want to do that too. It involves 10 farm sites located in Nebraska, South and North Dakota.

As it turns out, the 21.67tC/ha value is not statistically significant different from zero at even P<0.10; hence, it’s the same for the 80tCO2/ha.

At some locations, for some soil horizons, there was a statistically significant (P<0.05) increase in calculated amount of soil organic matter (SOM) per ha. (In one case, there was a significant decrease.) However, this was often the result of an increase in soil bulk density, over time, rather than a change in percent SOM per se. There were also examples of decreases in soil bulk density and that can lead to spurious errors in calculations of percent SOM. (See explanation at the end of this column.)

I’ve no doubt that long-term production of switchgrass will increase SOM, though probably not nearly so much as implied by Dr. Jones in her reference to this paper – or in other writings involving other cropping practices too. It would be nice to believe that there is some combination of crops, soil microbiology and management that will lead to large, rapid increases in SOM content, but the science I’ve seen it’s highly unlikely.

The part about Dr. Jones writing which intrigues me most involves her claims that a ‘healthy soil’ can virtually eliminate the need for synthetic fertilizer applications. I can understand this to some extent with nitrogen where a high SOM content coupled with high microbial activity will lead to rapid SOM oxidation and N release. But the P (phosphorus) part is more complicated. We do know that soils contain lots of elemental P that is generally in forms unavailable to plants. Can the right combination of roots, mycorrhiza and other microbes release enough to make an agronomically significant difference? Dr. Jones says yes. It would be great if she was right.

Unfortunately, there is a substantial probability that, just as in other areas as shown above, she’s off the mark on this one too.

I emphasize again that I am not questioning Dr. Jones’ commitment to better soil health, and have no thought that she’s presenting anything other than what she believes to be 100% true. As for me, I’ll be looking for support from credible, peer-reviewed scientific literature.

—-

How bulk density affects measurements of soil organic matter

In an untilled soil, or even one lightly cultivated or disked but not mouldboard plowed, the organic matter (OM) percentage is highest near the soil surface and declines with depth.

Consider a naturally compacted soil with an OM percentage of, say, 5% in the upper 5 cm, 4% in the next 5 cm and 3% for 10-15 cm  of depth That’s an average OM of 4% for a 15 cm soil core.

Now consider what happens if the top soil is loosened substantially, say with a tillage tool or biological activity. In this case, a 15 cm core may now sample soil that was originally in just the upper 12 cm of depth. The average OM percentage is now calculated as (5% x 5 cm + 4% x 5cm + 3% x 2cm)/12 cm = 4.25%.

The measured OM percentage has been increased from 4 to 4.25% simply by soil loosening.

The reverse occurs with compaction.

Some researchers try to adjust for this simply by adjusting for the change in core weight. For example, if the density drops by 20% with loosening, to use the example above, the researcher would multiply the OM percent after loosening by 15cm/12cm, or 1.25, in computing total OM content in the soil. Unfortunately, this does not eliminate the sampling error.

A more proper technique is to sample the soil to varying depths depending on soil density so that the same weight of soil is sampled for all comparisons. Other approaches involve measuring the density and OM percentage for 5 cm soil depth increments and making calculations based on what would occur if the bulk density was the same for all layers.

Bottom line: Be very cautious in interpreting published data involving differences in percent soil organic matter when there are also differences/changes in soil bulk density (which is usually the case).

While I am at it, I believe there is another common error/bias in soil OM data involving comparisons of tillage with no tillage. With tillage, OM is incorporated into the soil and all soil samples taken thereafter include the incorporated OM. However, with no tillage, above-ground residue remains on the surface and is usually avoided in taking soil samples. Hence, the OM benefit with no tillage may be underestimated in these comparisons.