How much is enough soil organic matter? “The more, the better” is often the assumed answer, or at least as much as the native soil had before crops were grown. There are a few papers on the topic (Loveland and Webb, 2003), some recent (Schjønning et al., 2018 and Oldfield et al. 2020) But is this the right question?
Janzen (2006) wrote, “organic matter is most beneficial, biologically, as it dissipates by microbial activity…”
So, the flow of organic matter derived energy through the soil is at least as important as how much is stored as organic matter, I thought.
Organic matter is like stored inventory, in a warehouse.
I’ve read that maintaining warehouse inventory is expensive.
Is it expensive in the soil? Yes, it is difficult to increase and difficult to maintain high levels in annual cropping systems.
What did I learn back in high school about just-in-time management…?
Just-In-Time Soil Health
I am not generally interested in business concepts, but for some reason the idea of just-in-time production intrigued me from when I first heard of it back in the 1980s. “The process involves ordering and receiving inventory for production and customer sales only as it is needed to produce goods…” The supply of parts or materials is delivered Just-In-Time to the manufacturing process on an as-needed basis. Toyota is famous for starting the concept. It allows businesses that make something, like Toyota, to keep their inventory low and warehouses smaller and so to cut costs. But it requires accurate forecasts of demand, reliable suppliers, synchronizing supply with demand, quick delivery of supplies, and maintaining a steady production rate.
Could it apply to soil management? Janzen is a researcher in Canada whose thinking about soil carbon cycling focuses on carbon’s flow through the soil; “organic carbon may be best viewed, not as a reservoir entrapped in soil, but as a stream of atoms flowing through.” (Janzen, 2015)
At risk of being accused of being a reductionista, I am going to compare the soil to a factory. And although I do not usually go for conjecture, that is what much of this is. Nevertheless, think about it; the parts and materials of the soil are photosynthesis-produced carbon, really the energy, in the soil. Soil organic matter is the soil’s warehoused inventory of this energy with particulate organic matter in a nearby warehouse, and therefore more available for use, while the mineral-associated organic matter warehouse is further away, less available. And just as with a factory, it is difficult/expensive to maintain a high level of this inventory in the soil.
The soil’s product is microbial life and all the functions it provides. This microbial life is dependent on the flow of photosynthetic energy from plants. “Plant input fuels the whole system and drives the microbial pump,” (Kastner and Miltner 2018). If it could be done, we would want the benefits of the energy flow through the soil but without the need to build and maintain a massive warehouse full of organic matter; we want Just-In-Time soil health.
Here is how it could work: 1) If we provide enough just-in-time energy we will 2) attain desired soil function, and 3) reduce the need to maintain high inventories of soil organic matter. The key to this, just as in a factory, is an accurate forecast of the demand: when is it needed and how much? Demand here is the flow of photosynthesis-derived C-energy through the soil needed to provide the desired function. Then we need to be able to provide enough energy at the time it is needed, just-in-time.
Demand can be either productive or unproductive. Productive energy demand goes to soil biological function, maintaining soil structure, suppressing soilborne pests making nutrients available, etc. Unproductive demand is from tillage looting our soil warehouses for some short-term gain but at great expense in lost inventory. Unlike a factory, we cannot be very exact about demand and energy flow rates in the soil, but we can make educated guesses about when extra just-in-time flow would be useful and how we might provide such a flow.
When is demand high and how might this be managed in a soil? The period of crop stand establishment is crucial for the rest of the growing season. Seeds germinate, roots grow, shoots emerge. However, cooler soil temperatures and lack of living plants can limit the energy flow from decomposition of our warehoused soil organic matter. This is when we should attempt to insure a good flow of C-energy through the soil.
How can we provide this just-in-time flow? Planting green, including a carbon source with fertilizer, C-containing seed treatments, some relay crops, some cover crops, green manures, manure, and compost applications. Not every use of these practices is a JIT example, but they all can be.
(Planting green photo by Michael Strang, Pelleted compost photo by Thad Schutt, Compell, both used with permission)
Consider the use of mustard green manures here in the Columbia Basin of Washington state. A late fall incorporated green manure crop can produce JIT flow to an early planted potato crop the next spring. It starts with a large amount of green, easily decomposed biomass incorporated into cooling soils, which slows the resulting burst of microbial growth, perhaps just enough to make it effective for potato seedlings when the soil begins to warm the next spring. The resulting flow improves water infiltration and resistance to wind erosion and provides some soilborne pest suppression in low organic matter soils. Although the effects are not long term, they are often enough for the following spring’s potato crop to become established and close canopy before dissipating.
Location is important for the JIT soil health. Seed treatments and furrow applications will be most likely to affect seeding growth. Pelleted compost applied in the seed furrow is being tested. This would ensure needed flow at the right locations for growing seedlings. The 4R’s of nutrient management apply here too; right material, rate, time, and location.
These and other similar Just-In-Time soil health management strategies are being recommended at some grower meetings. Some, like cover crops and green manures, have been the focus of research but many others have not. I found a few published papers (if you know of more, please mention them in the comments). An early foray into this strategy was undertaken by Ritz et al. (1992). They observed the effects of N fertilization of potatoes with and without C as straw or sugar (sucrose). Combining N+sucrose provided increased microbial biomass for up to 25 days after incorporation, enough time for the seedlings to get established.
In a lab experiment with field soils (Stenstrom et al. 2006), addition of glucose induced a quick transition from dormant to active microbial states that lasted at least 27 days.
There are other studies, but not many. In their review, Managing Soil Microorganisms to Improve Productivity, Welbaum et al. (2004) have a section on JIT strategies which they call Feeding Soil Microbes, mainly using sugars. It provides an informative discussion of the few published research results but highlights the need for more research in this area.
Once established, photosynthesis in seedlings can begin to produce their own flow of energy to the soil through root exudates. Soil energy flow for the rest of the season is provided through these exudates and from organic matter.
1Technically this is rhizodeposition which includes exudates and sloughed off cells. Total flow quantities crops ranges from 0.3x to 1x that from the decay of soil organic matter which varies by decay rates (1-5% of total SOM annually). Flow from annual crop roots increases rapidly for a few months and then declines (Pausch & Kuzyakov, 2018). 2 Just-in-time flow is much smaller than other flows but comes at a critical time for the crop and when other flows are lower because of low soil temperature. It is also often concentrated at or near the germinating seed.
Just like with JIT manufacturing, there are risks for this soil management strategy. If the energy flow to the soil is early or late or broken, the benefits are missed. If the demand is higher than anticipated or more than our chosen practice can supply, the benefits are missed. This is where the Just-In-Case management comes in.
Just-in-Case Soil Health
The storage organic matter should not be neglected as it provides a base flow of energy in the soil. In the business comparison, this is the Just-In-Case inventory.
There is nearly always a flow of energy coming from the decay of your soil’s organic matter, organic amendments, and dead plant roots and shoots. Its rate varies by the amount of these materials present and the temperature and water status of the soil. We can increase this flow by building up inventory levels Just-in-Case something goes wrong. Higher soil organic matter levels can keep the base flow rate at a higher level, reducing risk of low supply at the wrong time, but there is always a cost to doing this. Just as in a factory, there is a trade-off between the risks of just-in-time management and costs of just-in-case management.
Source or Practice
Source or Practice
Dispersed in soil
Soil organic matter decay
Broadcast manure and compost
Molasses and other sugar-based products
Cover crop biomass
Crop root exudates
Cover crop exudates
How much is enough?
As with soil organic matter, we would like to know how much energy flow in the soil at the critical times is enough. One problem here is measurement of that flow; we don’t have an accurate way of doing it. It can be imperfectly measured with active carbon tests (POXC) and soil respiration-related tests like the Solvita. However, these provide only snapshots of the flow and so should be measured regularly, or at the critical times for your crop growth.
Finally, a reminder that I have indulged in speculation here. Although the mechanism behind just-in-time interventions seems reasonable, whether they can consistently make a difference in yield, or crop health, or in input reduction remains to be seen. The determining factors will be soil organic matter levels, soil temperature and water, and crop stage. For now, I think both just-in-time and just-in-case strategies—carbon-energy flow and soil organic matter—should be part of soil health management.
What about the piece I had planned to write: how much soil organic matter is enough? That is a complicated question and will have to wait for a future blog post.
Janzen, H.H. 2006. The soil carbon dilemma: Shall we hoard it or use it? Soil Biology and Biochemistry 38(3): 419–424. doi: 10.1016/j.soilbio.2005.10.008.Janzen, H.H. 2015. Beyond carbon sequestration: soil as conduit of solar energy. European Journal of Soil Science 66(1): 19–32. doi: 10.1111/ejss.12194.
Kästner, M., and A. Miltner. 2018. SOM and Microbes—What Is Left from Microbial Life. In: Garcia, C., Nannipieri, P., and Hernandez, T., editors, The Future of Soil Carbon. Academic Press. p. 125–163
Loveland, P., and J. Webb. 2003. Is there a critical level of organic matter in the agricultural soils of temperate regions: a review. Soil & tillage research 70(1): 1–18.
Oldfield, E.E., S.A. Wood, and M.A. Bradford. 2020. Direct evidence using a controlled greenhouse study for threshold effects of soil organic matter on crop growth. Ecological Applications n/a(n/a). doi: 10.1002/eap.2073.
Pausch, J., & Kuzyakov, Y. (2018). Carbon input by roots into the soil: Quantification of rhizodeposition from root to ecosystem scale. Global Change Biology, 24(1), 1–12.
Ritz, K., B.S. Griffiths, and R.E. Wheatley. 1992. Soil microbial biomass and activity under a potato crop fertilised with N with and without C. Biol Fertil Soils 12(4): 265–271. doi: 10.1007/BF00336042.
Schjønning, P., J.L. Jensen, S. Bruun, L.S. Jensen, B.T. Christensen, et al. 2018. The role of soil organic matter for maintaining crop yields: Evidence for a renewed conceptual basis. Advances in Agronomy. Elsevier. p. 35–79
Stenström, J., K. Svensson, and M. Johansson. 2001. Reversible transition between active and dormant microbial states in soil. FEMS Microbiol Ecol 36(2–3): 93–104. doi: 10.1111/j.1574-6941.2001.tb00829.x.
Welbaum, G.E., A.V. Sturz, Z. Dong, and J. Nowak. 2004. Managing Soil Microorganisms to Improve Productivity of Agro-Ecosystems. Critical Reviews in Plant Sciences 23(2): 175–193. doi: 10.1080/07352680490433295.