- Agronomic: canola, corn, oats, rapeseed, rye, soybeans, wheat, grass (misc. perennial), hay
- Animal Production: feed/forage
- Crop Production: continuous cropping, cover crops, no-till, nutrient cycling, conservation tillage
- Education and Training: on-farm/ranch research, participatory research
- Natural Resources/Environment: biodiversity
- Pest Management: mulches - killed
- Soil Management: green manures, organic matter, soil analysis, soil quality/health
The goal of this research was to use cover crop mixtures to improve soil quality and diversify the corn-soybean cropping system in the eastern Corn Belt. Specific objectives were to evaluate the effect of cover crops on soil aggregation, water infiltration, and microbial community structure. Soil aggregation and water infiltration were increased by the growth of the cereal (wheat or rye) cover crops in the mixtures. The effect of one season of cover crop growth on microbial communities was small but observable. On many soils, several years of cover crops may be needed before soil quality is improved.
Many farmers have a great interest in improving soil quality within their fields, but they often don't know practical ways to improve their particular soils within their overall farming system. Some farms may be able to reintroduce hay crops and animals into a longer rotation, but other strategies are needed for many farms in the eastern Corn Belt. Winter cover crops are one such tool available to improve soil structure, biological diversity, and overall quality. A mixture of morphologically diverse cover crops may be able to mimic the beneficial effects of longer (3-4 yr) rotations within a shorter time period. For example, some of the benefits of longer, sod-based rotations include breakup of pest cycles, different rooting patterns proliferating in different depths of soil, greater soil biological diversity due to differences in plant root and residue substrates, and greater protection of the soil surface by crops that grow earlier in spring and later in the fall. By using a mixture of two cover crops each winter in a 2-yr corn-soybean rotation, many of these same benefits may be achieved.
For cash-grain farmers, the use of cover crops can be a way to diversify the landscape and increase the resilience of the cropping system to temporary weather stresses. In addition to the benefits discussed above, improved soil quality can result in improved water infiltration and water-holding capacity, better root growth and aeration, and greater resistance to soil erosion. Cover crops also contribute to weed suppression and may result in less residual herbicide use.
Although generally it is known that winter cover crops can improve soil structure, the particular cover crops to choose for a given soil type, climate, crop sequence, and degree of soil improvement desired is generally not known. Cover crop performance has been unpredictable in much of central and northern Indiana, and many farmers are reluctant to try cover crops because of their sometimes negative impact on yield of the cash crop. Studies are needed on a variety of soils to determine their effect on soil structure, biological diversity, nutrient cycling, and crop performance. Few studies have considered combinations of cover crops with different root system morphologies for their potential synergistic effects on soil quality.
Crop rotation, the practice of growing a sequence of crops on the same land, has been a valuable tool for farmers almost since farming began. Rotations traditionally have been important in maintaining soil productivity through management of fertility or crop insect, disease, or weed pests. During the 1950s and early 1960s the availability of synthetic fertilizers and chemical pesticides led some to think that rotation could be eliminated without loss of yield (Bullock, 1992). That line of reasoning proved to be false and the current consensus is that crop rotation allows for sustained production and improved yield (Mitchell et al., 1991). However, modern rotations have become more short-term and usually are only 2 or 3 years in a cycle. The inclusion of long-term meadow or forage crops has nearly disappeared as a normal production rotation. With economic pressures pushing producers to move away from long-term rotations, the ability to use winter cover crops in short rotations to keep some of the advantages of long rotations could be a valuable tool to crop producers.
Cover crops may be defined as crops that are grown specifically to cover the ground with living or dead mulch that will help to improve soil structure, soil fertility, and nutrient and pest management. There are many different niches or purposes that cover crops may fill in a particular field. When selecting appropriate cover crops, a producer must identify the primary goals or niches for the cover crop as well as some secondary goals, and then try to match the plants with the goals. The primary goal for our work is to improve soil structure, nutrient conservation and availability, and microbial diversity.
Bullock (1992) concluded that the shift from extended rotations to short rotations has resulted in the degradation of soil structure as measured by soil aggregate stability, bulk density, water infiltration rates and soil erosion. Classic review articles by Allison (1968) and Harris et al. (1966) explained that rotations involving sod, pasture and hay improve soil aggregate stability better than short rotations because they contain varied species with different growth habits and morphology, and there are generally longer periods without tillage. In general, as soil aggregation improves soil structure and tilth also improve (Allison, 1968).
Cover crops have been shown to improve water-stable aggregation and increase water infiltration rates compared to soil without cover crops (McVay et al., 1989). The beneficial effects of plants on soil aggregate formation come from several factors including: protection of the soil surface from raindrop impact; the formation of extensive root systems that help to break the soil apart and open it to aeration and water infiltration; the addition of organic matter through the death and decay of root and stem tissues; the supplying of a food source for soil microbial populations that in turn may directly or indirectly play a role in soil aggregate formation. These effects are generally accepted to be present in long-term rotation systems. The introduction of winter cover crop mixtures that mimic the various root system morphologies of the species found in traditional long-term crop rotations may be one strategy to improve soil structure while meeting the economic demands for short rotations.
The issue of species selection is another significant unknown in the use of cover crops for on-site nutrient retention and enhanced nutrient-use efficiency of the cash crops in a cropping system. In the eastern Corn Belt, the majority of research has focused on the grasses including winter wheat (Triticum aestivum L.) and cereal rye (Secale cereale L.), species that are relatively closely related to corn. Research in plant ecology has shown that increased diversity results in an increase in system productivity and stability (Tillman et al., 1996). The principle appears to apply to agro-ecosystems as both corn and soybean monocultures have reduced yields when compared to yields achieved in an annual corn-soybean rotation. Diversifying a rotation by including a winter cover crop may confer additional productivity benefits, but research has shown that not all alternative species confer the same benefits on system productivity. For corn, closely related grass species have been found to be relatively ineffective rotation crops (Porter et al., 1997) and cover crops (Raimbault et al., 1990), highlighting the need to move beyond current standard cereal cover crop species and evaluate novel, non-leguminous dicots for their efficacy as cover crops in corn based cropping systems.
The detrimental effects of deleterious soil microorganisms are well known but little has been documented regarding the beneficial members of the community (Nehl et al. 1996). Some bacteria have been shown to enhance plant growth. For example, when Bacillus polymyxa is co-inoculated with Rhizobium etli, it modifies the host plant growth (including increased lateral root formation and number of nodules) when compared to single inoculation with R. etli alone (Petersen et al., 1996). Some microorganisms act as antagonists toward others; for example, arbuscular mycorrhiza associates within plants and can aid in controlling infection by soil pathogens (Azcon-Aguilar and Barea, 1996). Changes in nutrient status resulting from different crop management practices can cause shifts in the microbial community. One illustration is that in reduced tillage cropping systems with plant litter localized near the soil surface, fungi have been found to dominate the microbial community (Newman, 1985). In ecosystems where litter is incorporated into the soil, the bacteria have a competitive advantage over fungi and dominate the community. Most studies have been conducted at a rudimentary level or by tracking specific species, but evidence is mounting that the soil microbial community as a whole is highly linked to agronomic practices and to crop performance.
To improve or design new cropping systems that will sustain the soil resource base while maintaining crop productivity, we must gain a better understanding of rhizosphere microbial community composition and function. Until recently, the methodology available only permitted us to indirectly examine the general ecology of these microorganisms at a very rudimentary level. Now, with the advent of molecular genetics methods, a number of tools are available to begin unraveling the complex ecology of microorganisms in the rhizosphere. Recently researchers have used profiles of differences in DNA fingerprints generated by PCR (polymerase chain reaction) to study microbial communities (Ferris et al., 1996; Muyzer et al., 1993). This method is now being used in a study by two of the team members (Nakatsu and Brouder) to determine the relationship of the rhizosphere microbial community and crop development under different agronomic treatments. Distinct and reproducible "fingerprint" patterns were generated from DNA extracted from the rhizosphere of growing corn and soybean plants in soils that had been either plowed or in no-till. Specifically, our results show that distinct and different populations of bacteria dominate the rhizospheres of corn and soybean when these crops are grown in rotation with each other (Wells et al., 1997). The dominant plant-specific populations also appear to change with different stages of development of their higher plant host, as well as with tillage system.
Ultimately, this research and its approach have the potential to revolutionize the way we select crop species for a rotation. "Fingerprinting" for beneficial and undesirable profiles in the rhizosphere may permit rapid screening for viable new or alternative crops to diversify current agro-ecosystems. Increasing the options for crop selection in rotational systems not only offers the inherent environmental quality benefits of increased biodiversity but it will improve the economic viability of farmers by allowing them to increase their commodity base and enter alternative and specialty markets.
1) Evaluate the potential for cover crop mixtures to improve soil structure, microbial biomass and diversity, and nutrient conservation and availability on four Indiana soils under no-till and conventional tillage systems and a corn/soybean rotation.
2) Determine the impact of cover crop mixtures on corn and soybean yields and weed suppression, on four Indiana soils under no-till and conventional tillage systems.
3) Evaluate and demonstrate cover crop mixtures and the resulting soil quality changes, on three producers' fields in Indiana.