Why Sustainable Agriculture Needs A Market For Corn Stover
Iowa State University
The primary reason sustainable agriculture needs a market for corn stover is to eliminate the need for soil tillage.
Heavy crop residues, unless tilled into the soil, delay warming and drying of soils in the spring which delays planting and early plant growth that are essential for maximum yields. Crop residues of corn, cereal grains, and rice also make weed control and uniform stand establishment more difficult. Tillage to prepare a seed bed is the reason that soil organic matter continues to decline and soil structure (health) deteriorates. Deep tillage returns the old and more resistant-to-decay organic matter to the surface where it is exposed to higher levels of oxygen and an environment where biological combustion is rapid.
The second adverse effect of tillage is that formation of soil aggregates (the crumb structure of soils) is a long-term process in which the end products of microbial activity such as lignin, gums and waxes stabilize soil aggregates. When returned to the surface, raindrops and other forces at the surface destroy the soil aggregates and this decreases air and water movements within the soil that are needed for root growth.
Baling standing corn stover after combining grain removes mainly the stalk part of the plant and leaves the other one-half of the plant that contains most of the nutrients in the field. Simple thrash wipers in front of the planter shoe creates about and 8-inch band of black soil that warms rapidly. With this practice most farmers have the skills needed for no-till planting.
The trouble is that farmers are born with the need to till the soil. And, even after all many others and I have said, the least risky way to farm is to use deep-fall tillage. For example, I want fall-tilled soils for my research plots. In the late 1970′s the Extension Service convinced farmers that tillage resulted in a loss of about 2 inches of soil moisture reserve.
In northwest Iowa farmers stopped tilling soybean fields in 2 years and there has been a progressive increase in no tilling in Iowa until 1993. With heavy rains that year, the media said the soil had been compacted (which would have been loosened by freezing and thawing in the surface) so farmers began deep fall tillage and tillage has increased until last fall it was difficult to find a field of soybean land that was not tilled. This spring we had enormous sheet erosion and gulley development. For sustainability of the productivity of our soils it is imperative that tillage be reduced. Developing a market that covers the cost of baling corn stover and other waste crop residues is essential. In Iowa the energy in baled stalks (5 ton/acre) is equal to that used by all of our utility industry and the energy is renewable in contrast to that from coal or oil.
Anaerobic Digestion Systems
Anaerobic digestion of organic matter to methane and carbon dioxide occurs when oxygen is excluded in the presence of two groups of bacteria. The first group of bacteria is commonly present in the environment. This group converts the sugars of fibers and many other substances to organic acids (mainly acetic acid). In normal aerobic metabolism by plants and animals the acids are oxidized to carbon dioxide plus chemical energy, but in the absence of oxygen the acids accumulate.
The second group of bacteria converts acids to methane gas and carbon dioxide. These bacteria are unique; for example, they require nickel, vanadium, and chromium for activity. An inoculum of these bacteria is usually obtained from another anaerobic digester. The first group is referred to as acid producing bacteria and their optimum pH for metabolism is about 5.5 or relatively acid. They also may adapted to temperatures of 120°F – 135°F at which temperature they have a rapid hydrolytic activity. The second group of bacteria is called methanogenes and require a neutral pH of 7 to a slightly alkaline pH of 7.4. The methanogens convert acetic acid to methane gas and carbon dioxide. Methane gas can fuel an internal combustion engine powering an electrical generator. With a phaser the generated electricity can be fed into utility power lines and runs the owners electrical meter backward.
There are many possible methods for conducting anaerobic digestion of agricultural waste biomass. The simplest is a single stage digester in which both types of bacteria are in the same tank or lagoon. The acid bacteria are many fold more active than are the methanogens. At the start up of a digester being fed corn stover, acid production is rapid and lowers the pH to the point the reactor fails because the methanogens are inactivated and die. To prevent this we maintain the pH above 7.0 with alkali and began with low rates of feeding corn stover.
The health of the digester is measured by the ratio of alkalinity to acidity and is simple to do with a pH meter. If the ratio is above 2.0 there is a balance between acid production and acid use for methane production. Rates of feeding corn stovers are increased until the ratio drops to 2.0 and then this becomes the feeding rate for corn stover. In the single stage digester neither group of bacteria are at their optimum pH and temperature so the rate of use of corn stover and methane production are slower than with two-stage digestion. For demonstration of single-stage digestion we will use a 150-gallon aerated soak tank.
Three days before starting we will add 15 pounds of ground corn stover and 72 gallons of water. On the day we start we will began daily additions of 5 pounds corn stover with 24 gallons of water to the tank. At 6-hour intervals a pump will transfer 6 gallons from the soak tank to the methanogenic tank containing about 1200 gallons of primary digester liquid from Ames Municiple Sewage Plant. We will monitor the alkalinity to acidity ratio and add lime if acidity is too high. To obtain a stable methanogensis we may need to reduce the rate of feeding corn stover for a few days but more probably we will be able to increase the rate above 5 pounds/day. When the methanogenic tank fills to 1500 gallons we will begin returning 24 gallons from it to wet the corn stover instead of using water. After 6-12 months we will need to empty about one-third the volume of the methanogenic tank to reduce solid waste content.
For demonstration of two-stage digestion the soak tank will pump into a 300-gallon acid tank instead of into the methanogenic tank. The acid tank will start with 100 gallon of primary digester liquid with some corn stover and over a period of 10 days the temperature will be progressively increased to 125°F to develop an acid and high temperature tolerant bacteria. The pH will become sufficiently acid to kill all methanogenic bacteria in the acid tank. After the adaptation period a pH controller in the large methanogenic tank will turn on a pump from the acid tank to the methanogenic tank whenever the pH is above 7.3 and turn off the pump when enough acid had been added to lower the pH to 7.2. We will adjust the amount of corn stover added to the soak tank to be adequate for the needs of the methanogenic tank.
We will add another digestion system with liquid swine manure used to wet corn stover. After these pilot studies we will test a much larger methanogenic tank before moving to a lagoon system. We also are studying the production of acetate and other chemicals from corn stover using the anaerobic digestion bacteria. These bacteria are more efficient, and much less expensive than enzymes or pure bacterial cultures in converting lignified fibers to sugar for production of chemicals. (8/00)