Regenerate Soils? A Good Goal, But Where Will We Get the Nitrogen?

People are rallying around a set of principles called “regenerative agriculture” to improve soil health. Initiatives such as “4 per 1000” and The Carbon Underground want to sequester carbon in the soil to address climate change. To achieve their goals, both want to increase soil organic matter levels. Soil organic matter is the part of the soil that consists of plant and microbial-produced biomass in various stages of decomposition. Organic matter is a small but crucial part of soil. If we can maintain a soil’s organic matter levels, we have gone a long way in maintaining soil health and function. Increasing soil organic matter levels would produce even greater benefits, including sequestering carbon from the atmosphere. One of the issues that these efforts overlook (I wrote about another here) is that increasing levels of soil organic matter requires nitrogen. Where will that nitrogen come from?

The soil’s supply of organic matter can be viewed as a bank. In agriculture, we make deposits through high-residue grain crops, perennial crop roots, cover crops, and organic amendments. We make withdrawals through tillage, low residue vegetable crops, and accounting for the annual nutrient release from organic matter and uptake by crops. To maintain soil organic matter, we balance deposits and withdrawals. To increase organic matter we make large deposits, but small withdrawals, for many years. Attached to all these transfers is nitrogen.

Organic matter averages 5% nitrogen. That might not sound like much, but even a modest increase in organic matter, for example from 2% to 3%, requires 1000 lbs. of nitrogen per acre. This nitrogen is not lost, but is, for the most part, removed from the system. Only a small part of it, 1-3%, becomes available to crops each year. The rest is locked in the soil organic matter, not available to current crops. This is the nitrogen cost of increasing soil organic matter levels.

All the different versions of regenerative agriculture agree on one goal: regenerate soils by increasing their organic matter levels. But they don’t just want to increase levels; they often aim at restoring them to native levels. In effect, they want to make deposits that will cover the large withdrawal that our predecessors made when the land was cleared and plowed. In opening up the land for crops, they caused organic matter levels to drop, rapidly releasing large amounts of nutrients, which allowed them to grow crops for years, albeit with the lower yields of the past, with low levels of nutrient inputs. The nitrogen that was released came from natural fixation, mainly by legumes, slowly built up over time. Restoring this will require a lot of nitrogen.

For example, increasing organic matter levels from 2% to 11% over 20 years (see Gabe Brown TED talk) would require 450 lb. per acre each year, or a total of 9000 lb. of nitrogen per acre. Considering that most regenerative ag proponents also claim that their practices allow them to reduce and eventually end their use of nitrogen fertilizer, where will all that nitrogen come from?

Addressing climate change by sequestering carbon in the soil through increased organic matter levels faces a similar nitrogen challenge. For example, the “4 per 1000” initiative proposes a 0.4% annual increase in percent soil organic matter. Authors of a paper critiquing this initiative (van Groenigen et al. 2017) estimate that globally, this effort would require over 110 million tons of nitrogen each year, which is about 75% of current global N-fertilizer production, or a doubling of current N fixation rates (legumes) in agricultural systems. Where would all this nitrogen come from? It will come from either biological fixation or synthetic fertilizer.

Blaustein-Rejto and Blomqvist point out the limitations of relying on biological nitrogen fixation for crop production. The nitrogen needed for increasing soil organic matter, as proposed for both regenerative ag and as a tool to address climate change, would be in addition to what is needed for crop production. Where will this nitrogen come from if not from synthetic N fertilizer?