The Importance of Crop Residue
Mar 30, 2016
Soil consists of four components--minerals, primarily from rock broken down by wind and water over geologic time, organic matter from decaying plants and animals, moisture, and air. Soil that lacks organic matter or has excessive concentrations of certain minerals, such as too much salt or too acidic, cannot be used to grow plants unless the farmer adds something to the soil. That is why the Matt Damon character in the 2015 movie The Martian had to fertilize the barren Martian soil with his own excrement in order to grow the potatoes that kept him alive. Damon’s role as a farmer on Mars earned him an Academy Award nomination as Best Actor, the first farming role to receive such an honor since 1984, when Sally Fields was nominated for her performance in Places in the Heart.
The organic matter contained in soil is generally characterized by its carbon content, since about 60 percent of plant matter is carbon. Depending on climate and geography, undisturbed ‘virgin’ soil that has never been cultivated generally has carbon content between 10 and 14 percent in the first ten centimeters of the soil profile. Those soils lose between one half and one third of their carbon almost immediately after the ground is first cultivated. A significant share of that carbon is released into the atmosphere as carbon dioxide through a process known as soil respiration, contributing to climate change. Net emissions of carbon from land use changes accounted for about 12 percent of global greenhouse gas emissions between 1990 and 2010, according to a 2012 article in Biosciences.
Around the world, soil carbon content is lowest in places where consistently high or low temperatures and/or low rainfall have limited vegetative growth over the last several millennia, such as the deserts of the Middle East and Western Australia, or the ice caps covering Greenland and Antarctica. In regions where crop production is feasible, soil carbon content is relatively lower where cultivation has been underway for several centuries, such as in Western Europe, Eastern and Southern Africa, and southern China.
A 2011 study reported in the South African Journal of Science found that 58 percent of South African soils contained less than 0.5 percent organic carbon, while only 4 percent had more than 2 percent organic carbon. From the early 1900s to the 1990s there was an estimated loss of 40 percent of soil carbon from the central U.S. Corn Belt in the top 20 centimeters (cm) of soil. Despite that decline, soil organic carbon was estimated to average 12.2 kg/m2 across the Midwest in 2000, as compared to 8.7 kg/m2 in the South and 5.5 kg/m2 in the Northeast.
Using soil amendments such as nitrogen fertilizer can substitute for the nutrients that crops would otherwise draw from organic matter in the soil, but it does not rebuild soil carbon. In developed countries, where farmers generally have ample resources or access to operating loans to acquire purchased inputs, they can maintain their crop’s productivity, for a price. According to data collected in the 2010 Agricultural Resource Management Survey conducted by USDA’s Economic Research Service and National Agricultural Statistics Service, the average U.S. corn producer spent $112 per acre to apply fertilizer to his or her crop. That expense was easier to bear when the corn price was $4.50/bushel in that year, as compared to the $3.60/bushel currently projected for the 2015/16 crop year by USDA.
By contrast, most farmers in Africa have a much more difficult time acquiring fertilizer to bolster productivity. According to FAO data, only 7 kilograms of fertilizer was used per hectare on average in Africa between 2006 and 2008, less than 10 percent of the average application rate elsewhere in the developing world. Due to cost and availability constraints, millions of smallholder farmers in Africa use no fertilizer at all.
The adoption of conserving agricultural practices such as conservation tillage or cover cropping, leaves more crop residue in the fields to decay and replenish soil carbon over time than conventional cultivation practices. Such actions can stabilize or even reverse the decline of soil organic carbon, especially when used in combination on the same field. In a 12-year study conducted by scientists at the University of Illinois, stocks of soil organic carbon rose 30 percent for a combination of no-till and cover-cropping, compared to using moldboard plows and no cover crops on similar fields.
It appears that more American farmers have started to utilize these practices, although the data are not clear on this score. For example, the 2012 Census of Agriculture found that 34 percent of U.S. cropland was cultivated using no-till practices, but did not track what share of that land was continuously no-tilled. The Conservation Technology Information Center (CTIC) at Purdue University has been conducting annual surveys on farmers’ use of cover crops since 2012 and has found that farmers are consistently experiencing improved yields with this practice. However, the survey design does not shed much light on how many farmers are adopting it nationally.
In Africa, many smallholder farmers strip their fields bare of crop residue before planting the next crop because it makes it easier to achieve with manual labor. The removed crop residue can also serve as a source of fodder for livestock. A partial FAO survey of no-till use in 10 African countries in 2007 identified less than half a million hectares cultivated under such practices, more than 70 percent of the total no-till area found in South Africa.
Farmers around the world can help both themselves and the rest of the world by adopting conserving agricultural practices that leave more crop residue in their fields. However, smallholder farmers who lack the resources to purchase fertilizer have much more to gain by making such a change.