Crop Residue - A Valuable Resource

AgMRC Renewable Energy Newsletter
October 2009

Don HofstrandDon Hofstrand
Professor Emeritus Agricultural Economist
Iowa State University

dhof@iastate.edu


Crop residue, traditionally considered as “trash” or agricultural waste, is increasingly being viewed as a valuable resource.  Corn stalks, corn cobs, wheat straw and other leftovers from grain production are now being viewed as a resource with economic value.  If the current trend continues, crop residue will be a “co-product” of grain production where both the grain and the residue have significant value. 

The emergence of crop residue as a valuable resource has evolved to the point where there are competing uses for it.  In this article we will discuss these competing uses and recent research on the topic.

Cellulosic Ethanol Production

With the emergence of the ethanol industry, considerable attention is focused on utilizing crop residues as a feedstock for cellulosic ethanol production.  It is often referred to as Phase 2 of the biofuels industry with corn ethanol being Phase 1.  Cellulosic ethanol has been highly touted as superior to corn ethanol due to its improved energy balance (more Btus produced per Btu of fossil fuel used in the process), lower carbon emissions and less direct competition with food production. 

Cellulose to ethanol requires an additional step in the production process that is still being investigated and developed.  In addition, the high processing cost, difficulty, and cost of gathering, storing and transporting crop residues is problematic.  Regardless, the push to develop commercially viable cellulosic ethanol processes is moving forward.  This has been intensified by the cellulosic ethanol mandates outlined under the Energy Independence and Security Act (EISA) passed by congress in 2007.  The act mandates that, starting in 2010, 100 million gallons of cellulose-derived ethanol will be blended in the nation’s gasoline supply. The mandate increases annually with blending in gasoline to reach 16 billion gallons in 2022.  This is more than the corn-starch ethanol mandate of 15 billion gallons in 2022. 

Although a variety of biomass sources can be used for cellulosic ethanol production, a significant portion of the feedstock is expected to come from crop residues, especially in Iowa and surrounding states where the bulk of the corn-starch ethanol production exists.  More information on the current state of the cellulosic ethanol industry is available in AgMRC Renewable Energy Newsletter Article, Cellulosic Ethanol: Will the Mandates be Met?

Other uses

Crop residues have traditionally been used for animal feed.  In many parts of the country, beef cows are placed in corn fields after harvest to graze on the residue and any grain remaining in the field.  Also, crop residues are harvested, stored and fed to livestock during the winter.  Crop residues, especially straw from small grains, are used for livestock bedding.

A variety of commercial uses for crop residues are in various stages of development.  Crop residues can be a feedstock for composite products such as fiberboard, paper, liquid fuels and others.  Several straw-to-fiberboard business ventures have emerged in recent years with mixed success.  Likewise, crops residues have been investigated as a feedstock for pulp for making paper.  Conservative estimates indicate that there are enough crop residues to expand the supply of papermaking fiber by up to 40 percent.

Crop residues can be used as a feedstock in the gasification (thermo-chemical) process for making syngas (synthetic gas) which contains carbon monoxide (CO) and hydrogen (H2).  Syngas can be used for several purposes including producing electricity, producing certain chemicals and making ethanol, gasoline and diesel. 

Biomass can be used in the production of biogas, which is composed mainly of methane (CH4) and carbon dioxide (CO2).  Biogas can be used in many parts of the world for low-cost heating and cooking.  It can also be used to generate mechanical or electrical power.  Biogas can be compressed, much like natural gas, and used to power motor vehicles. Crop residues can also be burned directly to produce heat and steam.

The investigation of alternative uses for crop residues to make commercial products will continue to grow as traditional feedstocks become limited and the need for renewable sources of feedstocks expands. 

Carbon Sequestration

Forests are often discussed as vehicles for storing or sequestering large amounts of carbon.  However, we should not underestimate the potential of our soils to sequester carbon.  Global storage of soil organic carbon is more than two times the amount stored in either vegetation or the atmosphere (1).  In addition, increasing soil organic matter (stored carbon) has the additional benefit of improving soil productivity (discussed below). 

A variety of methods are being examined as ways of sequestering additional soil carbon.  A price placed on carbon through a cap-and-trade system or a carbon tax would create an additional income stream for landowners.  So, utilizing crop residues to enhance the sequestration of soil carbon is a competitor to enterprises designed to remove crop residues for commercial uses. 

The original breaking of the native grasslands started the process of releasing enormous amount of sequestered carbon as discussed below.  Programs to reduce soil tillage are designed to increase and sequester soil carbon.  For example, no-till farming has been touted as sequestering soil carbon when compared to moldboard plowing.  The programs are designed to provide an income stream to no-till farming based on the value of the carbon sequestered.  However, there is uncertainty as to the amount of carbon sequestered from no-till farming.  Recent research studies show that soil carbon may not increase due to no-till compared to moldboard plowing (2, 3, 4, 6).  These studies show that no-till farming increases soil organic carbon in the upper layers of many soils.  However, these same studies show that the soil organic carbon is higher in lower layers of many soils from moldboard plowing.  When both layers are taking into account, there is little difference in soil organic carbon from no-till compared to moldboard plowing.  Additional research is needed to increase our understanding of the impact of various tillage practices on soil organic carbon.

Other ways of sequestering carbon in our soils are being investigated.  An especially promising method is with the use of “biochar” which has the potential to sequester large amount of carbon for long periods of time while also improving soil productivity.  Biochar will be explored further in an upcoming article.

Soil health and productivity

Are crop residues really a renewable resource?  Or, are there trade-offs when crop residues are removed?  Our soils are an amazingly complex, diverse and valuable resource.  Examining the impact on soil health and productivity from the removal of crop residues is critical before another use for crop residue is implemented.

Soil Erosion – Soils are degraded due to the movement of soil caused by the erosive action of wind and water erosion.  For example, it is estimated that China currently loses 18 tons of farmable soil through erosion for every ton of food consumed (5).  Much of the eroded soil ends up in streams and waterways that reduce water quality and deposits soil sediments.  Eroding soil takes with it important crop nutrients that need to be replaced to maintain crop yields.  These nutrients further degrade water quality in lakes and streams and contribute to environmental problems such as the “dead zone” in the Gulf of Mexico. 

Leaving crop residues on the soil surface is critical for protecting soils from wind and water erosion.  Soil erosion decreases exponentially as soil cover increases (5).  So minimizing soil erosion to tolerable levels limits the amount of crop residue that can be removed.  The Revised Universal Soil Loss Equation (RUSLE2) (http://www.ars.usda.gov/research/docs.htm?docid=6010) can predict the amount of crop residue required to reduce soil erosion to a tolerable level under various soil types, topographical conditions, etc.

Soil Organic Matter – Productive soils are the critical resource behind U.S. agriculture’s enormous production capacity.  Many of these soils are black because they contain generous amounts of organic matter that accumulated over centuries when lush native grasses grew and decomposed.  Similarly, productive brown colored soils developed in the eastern Corn Belt from decaying forest vegetation.  Maintaining adequate levels of organic matter is directly related to the health and productivity of soils.  Soil organic matter retains and recycles nutrients, improves soil structure, enhances water exchange characteristics and aeration, and sustains microbial life within the soil. 

When native prairies were broken, organic carbon was released into the atmosphere and reduced the amount of soil organic matter.  It is believed that 30 to 50 percent of our soil organic matter has been lost since the time of the native prairies.  The rate of loss was greatest when the prairies were first plowed.  Over the years the rate of loss has decreased to the point where organic matter levels in the soil have become somewhat stabilized.  However, it is believed by many authorities that organic matter loss under current crop production practices is still occurring, although at a much slower rate. 

Removing crop residues will increase the rate of organic matter loss from soil.  Changing from just corn harvest to the harvest of both corn and residue results in a loss of organic carbon, especially in year right after the change (1).  Removing as little as 25 percent of the residue results in the loss of soil organic carbon (1).  The greater the amount of corn residue removed, the greater the loss of soil organic carbon.  It is believed that removing crop residues reduces soil organic matter which reduces crop yields in subsequent years and, will in turn, lead to further reductions of soil organic matter.

However, growing temperate-zone perennial grasses such as switchgrass, miscanthus and native grasses sequesters soil organic carbon (1).  Growing perennial grasses on carbon depleted soils provides immediate carbon accumulation (1).  This also provides the economic benefit of generating an income stream from the harvest of perennial grasses (e.g. cellulosic ethanol) on degraded agricultural soils.

The amount of crop residue required to maintain soil organic carbon is greater than the amount required to limit soil erosion to tolerable levels.  So, soil organic carbon is the factor that should be used in determining the amount of crop residue that can be removed.  RUSLE2 can predict the amount of crop residue required to reduce soil erosion to a tolerable level under various soil types, topographical conditions, etc.  A similar methodology for computing the amount of crop residue to maintain organic matter does not exist.  Further research is required to develop a methodology for maintaining soil organic matter.

Healthy soils are critical in meeting the food needs of the world’s expanding population. In 2008, the world population stood at 6.7 billion, up from 2.5 billion in 1950.  It is expected to be 8.9 billion in 2050 (33 percent increase) and 9.7 billion in 2150 (45 percent increase).  Population growth, along with expanding diets as millions of people move from poverty to the middle class, will put enormous pressure on food production systems. 

Increasing soil organic matter could increase global food grain production by about one billion bushels per year (5).  As a comparison, the U.S. produced about 2.5 billion bushels of wheat in 2008.  So, in 2.5 years the increase in annual global food production would be equivalent to the entire U.S. wheat production.

Below are the benefits of using crop residues to maintain healthy and productive soils: 

  1. Soil benefits:
  • Increases soil productivity (yields).
  • Sustains soil organic matter.
  • Improves soil structure.
  • Controls soil erosion by buffering the soil against forces of raindrop impact and wind shear.
  • Increases water infiltration rates.
  • Conserves soil moistures.
  • Recycles plant nutrients.
  • Provides habitat and an energy source for soil organisms including earthworms and microorganisms.
  1. Environmental benefits:
  • Mitigates flooding by holding water on the land rather than allowing it to run off into streams and rivers.
  • Reduces surface runoff and decreases sedimentation.
  • Improves water quality by denaturing and filtering of pollutants.
  • Reduces nonpoint source pollution.
  • Minimizes risks of anoxia and dead zones in coastal ecosystems (e.g. Gulf of Mexico).

Soil Fertility

Crop residue returns fertility back to the soil.  In Iowa, it is estimated that the removal of corn residue removes 20 pounds of nitrogen, 2.9 pounds of phosphate and 25 pounds of potash per ton of dry matter (7).  So, the nutrients need to be replaced, probably with commercial fertilizers.  Although commercial fertilizer prices are quite volatile, recent prices indicate approximately $21.02 of commercial fertilizer is needed to replace the nutrients removed with the corn stover.
   
    20 lbs. nitrogen x $.35 per lb. = $7.00
    5.9 lbs. phosphate x $.30 per lb. = $1.77
    25 lbs. potash x $.49 = $12.25
    Total = $21.02

Implications

Using crop residues as a feedstock for producing renewable energy and other valuable products has received considerable attending in recent years.  However, these uses must be balanced against the long-term benefits of maintaining and improving the productivity of our soils.  Our soils are a valuable resource critical for meeting the challenges of the next century.

References

  1. Kristina J. Anderson-Teixeira, et al. 2008.Changes in soil organic carbon under biofuel crops. GCB Bioenergy.
  2. Anita Gál, et al. 2007.Soil carbon and nitrogen accumulation with long-term no-till versus moldboard plowing overestimated with tilled-zone sampling depths, Soil and Tillage Research, ScienceDirect,
  3. John M. Baker, et al. 2006. Tillage and soil carbon sequestration – What do we really know?, Agriculture Ecosystems and Environment, ScienceDirect.
  4. Humberto Blanco-Canqui and R. Lal. 2008. No-Tillage and Soil-Profile Carbon Sequestration: An On-Farm Assessment. Carbon Management and Sequestration Center, Ohio State University. Soil Science Society of America Journal. 72: 693-701.
  5. Douglas L Karlen, et al. 2009. Crop Residues: The Rest of the Story, Environmental Science and Technology.
  6. W.W.Wilhelm, et al, 2007. Corn Stover to Sustain Soil Organic Carbon Further Constrains Biomass Supply. Agronomy Journal. 99: 1665-1667.
  7. William Edwards. 2007. Estimating a Value for Corn Stover. Ag Decision Maker.