“When you start farming regeneratively, you rely a lot less on external inputs, such as fertilizers…” – Tom Tolputt

One of regenerative agriculture’s extraordinary claims is that it can drastically reduce or even eliminate nutrient inputs, fertilizers. How is this possible? The go-to explanation is often “soil biology” – revved up soil biology makes nutrients available that plants can’t normally access. As it often the case, there is a bit of truth here. Regenerative ag can reduce inputs, and soil biology is involved along with other natural processes, but the whole truth may be much more ordinary.

Reduce exports to reduce inputs

The key principle of most definitions of regenerative agriculture is the integration of livestock into cropping systems. The grazed livestock component of regenerative agriculture allows for several advantages that annual cropping cannot give including improved soil health. It also reduces the need for nutrient inputs though a series of system changes that start with reduced exports.

Consider the export of nutrients from a grazed pasture vs. a harvested crop. Whitehead (2000) estimates the yearly nitrogen removal in the beef produced on a moderately-intensive grazed pasture1 at 22 lb./ac. Similarly small amounts of P, 2 lb./ac, and of the rest of the important soil nutrients are removed. Compare this to the 134 lb. of N and 27 lb. of P removed per acre in an average 180 bu/ac corn crop (Ohio State Extension); the switch from a harvested crop to beef production reduces the export of N by 84% and P by 92%. This reduction in export means that these nutrients do not have to be replaced in the system. Here is the mechanism for the largest reduction of nutrient inputs in regenerative agriculture; it drastically reduces exports.

Reduce cropping intensity; The time factor

Interacting with this reduction in exported nutrients is the longer time that grazing allows the soil to be undisturbed compared to annual crops, even those under no-till. This time allows for greater buildup of nutrients through the natural processes, especially of biologically fixed nitrogen in association with legumes in annual cover crops or perennial pastures.

A short 60-90-day cover crop of mixed cereal-legume species grown in the non-cropped beginning or end growing-season-window fixes a limited amount of nitrogen. However, including livestock grazing provides a financial return that enables a cover crop to be left in the field for much longer than a normal cover crop, even for growing the cover as a substitute cash crop during the main growing season. These are not so much cover crops as they are annual forage crops, and managed as such, the included legumes can fix significantly more N than an ungrazed cover crop.

Grazing perennial pasture is even better for nutrient cycling. A perennial grass-legume pasture can fix a significant amount of N over a year or two, easily enough to replace the N being removed in the system. In fact, using the same study as above (Whitehead, 2000), the system is gaining 50 lb./ac of nitrogen every year without any N-fertilizer inputs. Most of the N in the grazed herbage is recycled to the soil through manure and urine which is much more efficient at conserving nutrients than annual cropping systems.

The additional time also gives soil microbiology opportunity to work on the legacy P (Sharpley et al., 2013). It’s called legacy P because it was originally applied as fertilizer but has become mostly unavailable to plants. It is “slowly exchangeable” (van der Bom et al., 2019), and therefore more available than P in or on soil minerals. Microbes, root exudates, and biofertilizers can all play a part in solubilizing this P to make it available for plants (Menezes-Blackburn et al., 2018).

This Strategic Phosphorus Reserve exists to some extent in all our farmed soils that have a history of fertilizer P applications. The more time this process has to work without P removal, the more quantity of P will be available. One recent paper (Menezes-Blackburn et al., 2018) estimates that legacy P could supply P for the next 100 years, not necessarily at a rate sufficient for high yield annual cropping, but certainly more than enough for a beef grazing system. However, while we should draw down levels of legacy P, this is not a natural source of the nutrient and will not last forever.

Another way that nutrient inputs are reduced with the integration of livestock grazing is by having active plants growing year-round (a soil health principle), especially deep-rooted perennial species as is common with grazing systems. This reduces losses of nutrients from leaching and tightens nutrient cycling.

How much does actual soil weathering, the release of nutrients from the mineral portion of the soil, contribute? Release rates may be higher than previously thought, (Bormann et al., 1998), and active soil biology does play a role here, increasing the rates of nutrient release. However, estimated rates remain much lower than the other mechanisms mentioned above, and far too low to support moderate yields of annual crops.

Essentially what regenerative ag is doing with livestock grazing is reducing cropping intensity and therefore yield intensity, substituting production of meat and milk as a trade-off for crop export. If exports are reduced to the point where the soil’s internal ability to provide N, P, and K and other needed nutrients is able to replace all the nutrients removed, then yes, we can have a system where no external nutrient inputs are needed.

Why grow crops at all?

Given all this, why not just skip the crop production altogether? Indeed, many regenerative agriculture practitioners do this, and nutrient inputs are drastically reduced or eliminated. They produce meat, eggs, milk, but no crops. This is great for their soil, and sometimes for grower profits. We can do with less feed and biofuel crops; if these are the crops being replaced by grazing, there is no downside other than the reduced meat production per acre (compared to exported feed crop systems), which is perhaps offset by more sustainably produced meat. Some growers, however, can’t or don’t want to raise livestock.

Can inputs be reduced without livestock integration?

Livestock and cropping have long been separated in most of our modern agriculture. It is not always easy or feasible to integrate them. What is the potential to reduce inputs but only grow crops? There are many opportunities to improve the nutrient use efficiency of annual cropping; more diverse crop rotations, rotations that include legumes, perennials and cover crops, fertilizer formulation and management, combining organic soil amendments with synthetic fertilizers, etc.; and all of these can reduce the need for nutrient inputs (Drinkwater et al., 2017). These systems can also take advantage of the legacy P. Low-input systems using a combination of these methods can drastically reduce nutrient inputs by improving the cycling of nutrients within the system. There is often a modest yield decrease that accompanies these changes, with optimal rather than maximum yields the goal. An all-crop system can be sustained without external inputs of nitrogen (Drinkwater 1998), although other inputs of other nutrients may be needed. When done within a no-till system, it fits within regenerative ag IF regenerative agriculture can be done without livestock, a topic for debate.

Non-grain food crops are even less regenerative than annual cereal, biofuel, or feed crops. Consider that a 32 ton/ac russet potato crop here in the Columbia Basin of WA will remove 282 lb. N, 38 lb. P, 311 lb. K per acre plus micronutrients from the field. That would take a whole lot of grazing time to replace without inputs.

What I have not addressed are those regenerative annual cropping systems that do not integrate livestock yet claim to have drastically reduced or eliminated nutrient inputs. Other than the processes already mentioned, I have no explanation for those claims. However, I did find one published example of a cropping-only system that could potentially eliminate all external nutrient inputs. Crews (2005) using a perennial crop system speculates that relying on what he calls “endogenous nutrient supply” (no external nutrient inputs) might just be possible with if the following factors are all aligned:

Perennial grain crops are successfully developed and used exclusively.

Timing of N release from soil (legumes, bacteria, etc.) must match uptake timing of crop.

Crops grown on young to middle-aged soil.

A climate favorable for weathering reactions; moderate temperature and precipitation.

Exported yields lower than today’s high yields, perhaps much lower.

Such a system could theoretically continue for many decades without nutrient inputs (Glover et al., 2010). This would be the ultimate in regenerative agriculture, but we are not there yet. Some think we may never get there without major concessions in yield (Denison, 2012).

A Beneficial Tradeoff?

This brings us to the inevitable tradeoffs we face in agriculture. There is a basic tradeoff between crop yield and inputs. Because of the economics, this tradeoff will most likely take place in lower value annual crops (feed, biofuel, etc.) and not in staple, vegetable, or fruit crops. If this is the case, it can be a good strategy for farmers and the soil. For the individual farmer, increased profits may tip the scales in favor of reduced yield and reduced inputs, or increased soil health but less yield. Farmers should do what they need to do to stay in business. However, regenerative agriculture in this form may not be a good strategy for reducing inputs in staple food-crop production, at least not until our population stops growing.

Yes, regenerative agriculture does reduce inputs, but the primary mechanism by which it does this is the reduction of nutrient exports from the field. All the other factors; soil biology, mycorrhizal fungi, diversity effects (other than including legumes), and mineral weathering, are minor factors.

1While regenerative agriculture practitioners generally use intensive grazing, they do not fall into the intensive input use, so these estimates for moderate intensity grazing are probably more accurate than those for intensive systems which add a lot of inputs. Also, there are many factors that affect nutrient flows in grazing systems, so these numbers are only used for example and should not be considered to represent any generalized system.

References

Bormann, B.T., D. Wang, M.C. Snyder, F.H. Bormann, G. Benoit, et al. 1998. Rapid, plant-induced weathering in an aggrading experimental ecosystem. Biogeochemistry 43(2): 129–155. doi: 10.1023/A:1006065620344.

Crews, T.E. 2005. Perennial crops and endogenous nutrient supplies. Renewable Agriculture and Food Systems 20(1): 25–37. doi: 10.1079/RAF200497.

Denison, R.F. 2012. Darwinian Agriculture: How Understanding Evolution Can Improve Agriculture. Princeton University Press.

Drinkwater, L.E., M. Schipanski, S. Snapp, and L.E. Jackson. 2017. Chapter 7 – Ecologically Based Nutrient Management. In: Snapp, S. and Pound, B., editors, Agricultural Systems (Second Edition). Academic Press, San Diego. p. 203–257

Drinkwater, L.E., P. Wagoner, and M. Sarrantonio. 1998. Legume-based cropping systems have reduced carbon and nitrogen losses. Nature 396(6708): 262–265. doi: 10.1038/24376.

Glover, J.D., S.W. Culman, S.T. DuPont, W. Broussard, L. Young, et al. 2010. Harvested perennial grasslands provide ecological benchmarks for agricultural sustainability. Agriculture, Ecosystems & Environment 137(1): 3–12. doi: 10.1016/j.agee.2009.11.001.

Menezes-Blackburn, D., C. Giles, T. Darch, T.S. George, M. Blackwell, et al. 2018. Opportunities for mobilizing recalcitrant phosphorus from agricultural soils: a review. Plant Soil 427(1): 5–16. doi: 10.1007/s11104-017-3362-2.

Sharpley, A., H.P. Jarvie, A. Buda, L. May, B. Spears, et al. 2013. Phosphorus Legacy: Overcoming the Effects of Past Management Practices to Mitigate Future Water Quality Impairment. Journal of Environmental Quality 42(5): 1308–1326. doi: 10.2134/jeq2013.03.0098.

van der Bom, F.J.T., T.I. McLaren, A.L. Doolette, J. Magid, E. Frossard, et al. 2019. Influence of long-term phosphorus fertilisation history on the availability and chemical nature of soil phosphorus. Geoderma 355: 113909. doi: 10.1016/j.geoderma.2019.113909.

Whitehead, D.C. 2000. Nutrient Elements in Grassland: Soil-plant-animal Relationships. CABI.