Healthy soils produce healthy crops, right? Nope. Although it would seem the very definition of soil health, this popular thinking does not match what plant pathologists find (Janvier et al., 2007). It’s not true.
You can have soil that is healthy in all other ways and still have problems with soilborne disease. But then, one may ask, is it really healthy? We can define away the dilemma – if you have soilborne disease problems, you don’t have healthy soil—but then we may never have healthy soil. Instead of the soft thinking of catchphrases, let’s look at the hard reality.
Soil health reduces disease potential
First the easy part. So far as it improves soil structure, drainage, and aeration and reduces compaction, soil health also reduces the potential for soilborne disease incidence and severity. This is the environmental corner of the plant disease triangle.
Soil health and soilborne disease suppression
What “healthy soils produce healthy crops” is really getting at is soilborne disease suppression. Rather than just reducing the potential for disease, suppression acts against the actual pathogens in the soil. It is the soil version of immunity; pathogens can be present in the soil, but plants don’t get the disease. It is the Mega-Millions jackpot-win of soilborne disease control, something often pursued but rarely achieved.
General suppression is related to soil health
Plant pathologists identify two types of soilborne disease suppression, general and specific (Schlatter et al., 2017). General suppression is related to higher populations and activity of the general microbe population, so it is related to soil health. The good news is that general suppression is broad spectrum. The bad news is that it does not completely suppress every pathogen in all conditions.
Although general suppression can be achieved through improved soil health, research results are inconsistent because of the many factors involved: pathogen, the soil type, crop, and climate. Soil health may improve general disease suppression but does not produce complete immunity to diseases implied in “healthy soils produce healthy crops.”
Specific suppression is not related to soil health
The Jackpot-Win of disease suppression is called specific suppression. Unlike general suppression, it is linked to the activity of a smaller number of specific microbes. And it is complete suppression, but only of one pathogen. Unfortunately, the properties that make soils suppressive are specific to each soilborne disease; a soil that suppresses one disease in one crop will not suppress another soilborne disease in another crop. It’s like immunity to measles but not to the common cold. The best-known examples of specific disease suppressive soils are take-all in wheat, Rhizoctonia bare patch, also in wheat, and common scabs in potatoes (Schlatter et al. 2017).
How to get specific disease suppression
Since it is complete suppression, it would be useful to achieve specific disease suppression when and where we want it. Here is how you get it for potatoes: grow nothing but potatoes for 20+ years on the same soil (Lorang et al., 1989; Menzies, 1959). The same can be done for wheat (Cook, 2003). The best examples we have of specific disease suppression are the result of long-term monocropping.
Researchers have studied disease suppressive soils, hoping to identify how they suppress disease so as to come up with non-monocropping methods of attaining them, but with little success. The mechanisms are complicated. One research team identified 17 distinct microbes involved in one instance of specific suppression of Rhizoctonia (Mendes et al., 2011).
A few other observations of specific suppression:
- A break in the mono-cropping that creates suppression will often break the suppression (with take-all suppression but not with Rhizoc, Schlatter, et al., 2017).
- Composts with specific qualities can sometimes promote disease suppression (Hoitink et al., 1997)
- Green manures can also sometimes promote disease suppression (Abawi and Widmer 2000; McGuire 2003; Wiggins and Kinkel, 2005)
- Soil biodiversity does not guarantee disease suppressiveness (Hossain et al., 2021)
Practices vs. soil health for disease suppression
So, going broke monocropping is not a practical option. Nor is achieving soil health a complete solution: “No soil health management practice consistently suppresses disease.” (Soilborne Plant Pathogen Fact Sheet, 2021). The big lever here is crop rotation, not soil health. It is the traditional practice used to avoid soilborne disease problems in annual cropping. As with nutrients and water, soil health is fine-tuning for soilborne disease.
Given all this, we should consider suppressive soils and healthy soils as two different things (Figure 3), especially when we consider multiple crops and diseases in a rotation. And abandon the “healthy soils produce healthy plants” thinking. It does not describe reality, which is much more complex.
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Washington State University