Today’s annual intense burning observed in Russia is especially ironic because the earliest research documenting the deleterious impacts of open burning actually occurred in the former Soviet Union, as early as the 1930’s. This research found that, In contrast to traditional belief, burning does not “enrich” the soil, but instead deprives it of both nutrients and structure.
The practice primarily decreases soil’s productive capacity by destroying the humus (organic matter) and soil consistency vital to good yields. With each successive burn, soils lose more nutrients – not only nitrogen (N) and phosphorus (P), but also carbon, increasing the need for fertilizer. Lack of humus and the high heat of burning also compacts the soil, making it more brittle and prone to erosion by both wind and water. Such erosion is most evident on hillsides, but also occurs on flatlands, where soil levels drop each year as burning and plowing cause topsoil levels to diminish. In one comparison between a no-burn farm in Chile and its neighbors that practiced burning, land levels remained 1-2 meters higher compared to neighboring burned fields over 30 years of measurement. Similar impacts can be seen on agricultural lands where cement wells and water cisterns, originally dug at soil level or below now sit some meters above the current ground level. Where irrigation is necessary, larger amounts of water must be used to compensate for more brittle soils, further depleting supplies already under stress in a warmer climate.
After burning and related erosion, whatever remaining fertility naturally remains in the soil comes from the deeper layers under the burned portion; reaching in other words successively deeper and deeper into the earth as soils erode until at some point, no topsoil will remain. This reality means that farmers must increase fertilizer use, not only because of lower nutrient levels, but to compensate for greater fertilizer run-off from brittle soils less able to maintain the fertilizer in-place. This results in higher supply costs to the farmer, as well as greater rates of pollution and eutrophication in nearby rivers, lakes and waterways, negatively impacting local populations and eco-systems, including eco-system based services and industries such as drinking water, fishing and tourism.
This loss of fertility and increased erosion is highly significant: humus content may shrink to well under 1%. To rebuild humus may take many years, especially with conventional agriculture and plowing. Most studies, including the earliest ones of the 1930’s focused primarily on this issue of organic matter. The “black earth” regions of Ukraine and Russia, for example in an unburned state might have close to 12% humus content, making them among the most naturally fertile lands in the world. Soil measurements there today, on fields burned over many years sometimes show humus levels hovering at or even below 2%. Although not naturally as high originally, one conventional farm in southern Sweden with annual monitoring showed an increase in humus content from a low of 2.1% when burning ceased in the mid-1980’s, to around 4.5% today – in other words, higher than the famous “black earth” lands in their current condition, but still an agonizingly slow build-up. There is some evidence however that “conservation agriculture” or no-till methods (covered in more detail below) can increase fertility far more quickly. One farm in Chile with early adoption of no-till practices, including cover crops and livestock manure injection for example, reports an increase in humus levels from 0.6% when they first ceased burning, to over 5% in the first inch of soil alone.
Fewer studies have focused on damage to crop yields directly as opposed to studies of humus; in part because other factors such as weather and pests can so easily influence the result, and in part because fertilizer use (which almost always is higher than actually needed by crop vegetation) compensates for nutrient loss. However, in one study focused on California’s rice sector, where fertilizer was not used to compensate, the burned fields showed an average 30-40% decrease in yields each year, with the impact increasing over time. Of interest, the study actually was focused on providing farmers with guidance on how much they could decrease fertilizer loads as California moved to a ban on field burning. The conclusion was that farmers should drop their fertilizer application by a minimum of 20%, and could perhaps decrease the amount by over one-third (125 lbs/acre to 100 or 75 lbs/acre).
These negative impacts hold for all uses of fires in the agro-forestry sector; though of course are greatest where use of fire occurs most often (annually or even more often), or where fire occurs on marginal lands such as those in the Amazonas region, or mountainous regions with only a thin layer of topsoil. Use of fire even once in such ecosystems can limit agricultural use to just a few seasons or crops; after which time the user moves on, leaving a depleted and eroded landscape difficult or impossible to restore to its prior condition. This rule also holds for forestry burning use to clear before tree harvest on steep lands with little topsoil. Once the trees are harvested, the remaining land, even if planted with new seedlings, remains much more prone to erosion and landslides, especially in the first seasons after harvest. Pasturelands similarly do not benefit in “fertility” after a burn: while some non-grass species may be removed, research has shown that the quality and livestock nutrition of such pastures is lessened after each burn unless fertilizer is used, an investment rarely employed especially in developing countries.
To summarize, while burning may meet certain short-term needs, the overall damage to soil structure, fertility and even loss due to erosion make its use in the agricultural and forestry sectors rarely if ever motivated from any longer-term economic standpoint, let alone those of sustainable agriculture.
A note on wildlands: Agro-forestry sector burning for the above purposes should never be confused with practices used in the maintenance of “wildlands” (natural, especially virgin forests and grasslands), including immediate wildfire protection. Wildland ecologists and managers have fought for years for recognition of the role natural fires play in preserving some of these ecosystems, where periodic fire plays an integral role in their natural cycle; and without which they soon degrade. Under natural conditions however, such fires occur in wildland systems only on decadal or more time scales, quite different to annual or even more frequent burning on lands already used by humans for agriculture or forestry, and which are no longer in anything resembling a natural or “wildlands” state.