In my last post, we found that soil health was not a major factor for crop yield but is useful in fine-tuning crop nutrient supply. Here, we will look at soil health fine-tuning of soil water use.
Soil Health and Water
Soil health, through increased soil organic matter, can add nutrients to the cropping system. It cannot do this with water. However, it can help to conserve the precipitation that falls. This is the fine-tuning. Here, we need to distinguish between practices and actual soil health.
Some practices that end up improving soil health have direct effects on soil water not related to soil health. No-till, by eliminating tillage passes and associated losses of soil water, conserves soil water. And the residue cover resulting from using no-till, by reducing evaporation and thus reducing losses of water from the soil, does the same. This does not rely on improved soil, but only on the practices themselves.
While those practices reduce losses of water, soil health, mainly through increased soil organic matter, can help store more water. Despite the wild exaggerations of the increases in soil water holding capacity resulting from higher soil organic matter levels, studies show that the benefits are more modest. A review of US soils found that, on average, a soil organic matter increase of 1% (i.e. from 2% to 3%) results in a 0.1” increase in water storage in the top 6” (Libohova et al., 2018). The amount of silt in a soil is a much bigger driver of available water storage capacity than soil health (Libohova et al., 2018).
Finally, there is the possibility that increased soil health results in increased infiltration rates, allowing more water to soak into the soil and be of use to your crop. Both improved soil structure and porosity, and residue or living cover crop cover can contribute to increased infiltration rates.
Climate Determines the Size of Water Effects
The result of these water-related benefits of both practices and improved soil health for crop yield depends highly on your water supply (climate). If this combination of reduced tillage, residue cover, increased infiltration, and higher water storage capacity results in enough extra water to allow you can go from a wheat-fallow system to a yearly cropping system (think the eastern edge of the High Plains), then the benefits are great for yield. Most everywhere else, they are smaller than the other major lever factors except in droughts when the combination mentioned above may have higher benefits. And not in extreme or extended droughts – soil health does not produce water – but in shorter periods of drought during the growing season, soil health may reduce crop stress.
This means that with sufficient water, soil health is not a major factor for crop yield, a conclusion suggested by research results. Here are a few of the many examples:
Even in soils where mycorrhizae have beneficial effects on wheat without irrigation, the benefits disappear with irrigation (Duan et al. 2021)
In potatoes, soil health benefits to yield related to water-holding capacity were positive under rainfed conditions but much reduced or eliminated under full, well-managed irrigation (Larkin et al., 2021).
Irrigation Diminishes Soil Health Benefits
Finally, look at Figure 1 below from Egli and Hatfield (2014). The NCCPI on the x-axis is a combined measure of productivity due to climate and soil properties. Many of the soil properties are not manageable, such as soil texture and depth. Others like pH are. It includes some soil health-related factors like soil organic matter levels and bulk density. Soybean yields (similar graphs can be made for wheat and corn) increase with increasing NCCPI as they should. But look at the circled triangles from Nebraska. They don’t follow the trend. Why? Because they are irrigated, which is why NCCPI is only estimated for dryland yields; irrigation eliminates the effects of those soil differences, including soil health, on yield.
With sufficient water, soil limitations can be overcome or are not important for crop yield. I wrote about this here for the irrigated Columbia Basin of Washington state, where I work.
Soil Health is Fine Tuning
Contrary to current headlines, soil health is unnecessary for high yields. This is why it was not a factor in producing the remarkable results of the green revolution, and why we got away with ignoring it for so long. In soils with sufficient nutrients and water and without other serious problems such as compaction, drainage, or severe erosion, soil health has minor effects on crop yields. Rather than a major factor, soil health is the fine tuning of the system.
The graph below shows this well. It depicts soil organic carbon (organic matter) for two sites, one in Florida and one in Texas. The difference between the two sites’ soil organic matter levels (~soil health), L, is due to differences in climate and perhaps soil texture. Compared this difference L to the differences resulting from soil health management at each site, SH. Soil health is fine tuning within the given environment. Soil health is local. If we removed all limitations due to water, nutrients and sunlight/temperature, we could expect similar yields from both locations regardless of all but the most extreme soil health problems.
Soil Health ROI
Back to your soil health return-on-investment. There are potential small gains, as we have shown (with exceptions). And it is possible to have decreased yields while increasing profits. To some extent, using soil health to fine-tune nutrient management can be useful here. However, as with all fine-tuning, there is a danger of losing sight of the primary factors. Too much focus on soil health can degrade to unimportant fiddling: managing your soil’s microbes, worrying about its biodiversity, or searching for the perfect cover crop mixture. As you read and learn about soil health, just remember that in your management toolbox, soil health is a relatively small tool among the many larger tools that are used to manipulate crop productivity.
Duan, H.-X., C.-L. Luo, J.-Y. Li, B.-Z. Wang, M. Naseer, et al. 2021. Improvement of wheat productivity and soil quality by arbuscular mycorrhizal fungi is density- and moisture-dependent. Agron. Sustain. Dev. 41(1): 3. doi: 10.1007/s13593-020-00659-8.
Egli, D.B., and J.L. Hatfield. 2014. Yield Gaps and Yield Relationships in Central U.S. Soybean Production Systems. Agronomy Journal 106(2): 560–566. doi: 10.2134/agronj2013.0364.
Larkin, R.P., C.W. Honeycutt, T.S. Griffin, O.M. Olanya, and Z. He. 2021. Potato Growth and Yield Characteristics under Different Cropping System Management Strategies in Northeastern U.S. Agronomy 11(1): 165. doi: 10.3390/agronomy11010165.
Libohova, Z., C. Seybold, D. Wysocki, S. Wills, P. Schoeneberger, et al. 2018. Reevaluating the effects of soil organic matter and other properties on available water-holding capacity using the National Cooperative Soil Survey Characterization Database. Journal of Soil and Water Conservation 73(4): 411–421. doi: 10.2489/jswc.73.4.411.