Assessing soil quality changes in alternative and conventional cropping systems

Assessing soil quality changes in alternative and conventional cropping systems

Summary

Soil quality measurements in a four year crop rotation compared to a two year corn-soybean rotation managed under alternative and conventional management systems demonstrated that the four-year rotation is more biologically active and has better physical characteristics than the two-year rotation. Both two and four year rotation management systems that reduced tillage had greater aggregate stability and microbial biomass. In the 4-year rotation the adverse effects of increased tillage in the organic management system was ameliorated. In addition, the 4-year rotation reduces the buildup of manure derived phosphorus in the organic management system.

Objectives/Performance Targets

1) Characterize the effect of rotation and management systems on soil quality and biomass production in long-term experiments with previous high and low fertility management.
2) Evaluate the same soil quality characteristics for similar soils using paired-field or paired-farm comparisons of long-term alternative production systems with those in conventional or recently converted to alternative management.
3) Develop and implement an outreach component for dissemination of information concerning the impact on soil quality of alternative management systems.

Accomplishments/Milestones

PROGRESS
1) Experiment station component: The Variable Input and Crop Management Systems (VICMS) I and II plots at the Southwest Research and Outreach Center (SWROC) in Lamberton, MN were sampled in the fall of 2000 and 2001. Laboratory analysis for the fall of 2000 samples is complete. Analysis for the fall of 2001 samples is currently in progress. Additional in-field soil measures will be measured for a reduced set of plots in the spring of 2002. See table 1.

2) On-farm comparison component: The on-farm paired field comparisons were sampled in the fall of 2000 and 2001. This soil is being processed in the same time frame as the VICMS soil (Table 1).

3) Outreach and education component: Project results were presented at the SWROC organic field day on August 17, 2001 (over 200 attendees). The fall 2000 data from VICMS I was presented at a farmer meeting at the SWROC on January 19, 2002, and individual farmer soil analysis results were made available to the cooperating farmers.
We are planning at least two farmer meetings and/or field days during summer and fall 2002 to present additional results as well as to provide farmers with data on the status of their fields.

Table 1. Progress chart for soil quality parameters to be evaluated.
Soil Property Fall 2000 Fall 2001 Spring 2002
Aggregate stability & size distribution (wet) Complete (C) In progress (IP)
Bulk density Planned (P)
Penetration resistance P
Infiltration P
Microbial Biomass C&N C IP
Potentially mineralizable C&N — IP
Total organic matter C&N C IP
Particulate organic matter C IP
Earthworm populations P
P,K, nitrate, pH, Ca, Mg C IP

RESULTS
1) Reviewing the results from the fall 2000 sampling on the VICMS experimental plots, we are seeing trends that indicate rotation length significantly (alpha = 0.05) effects biological and physical soil quality indicators. Management system also significantly affects soil quality indicators although the 4-year rotation tends to ameliorate the negative impacts of some intense management practices.

Aggregate stability is a measure of the soil’s physical condition and is also related to biological activity. Higher aggregate stability values are associated with better water infiltration, water-holding capacity, and reduced runoff, while compaction is often associated with lower aggregate stability values. The 4-year rotation had a significantly higher percentage of aggregates greater than 1 mm in size, than the 2-year rotation in both the historically low (VICMS I) and high (VICMS II) fertility fields (table 4 and 8). Using the same measure to compare management systems we found that, in the 2-year rotation on VICMS I, the low purchased input (LPI) system had a significantly higher percentage of stable aggregates (>1mm) than the other management systems. But under a 4-year rotation the organic, minimum input (MIN), and LPI management systems were not significantly different from each other (table 2). In both fields samples taken after the alfalfa and oat years had significantly higher aggregate stability than the corn and soybean years of the rotation (table 5 and 9). This indicates that the small grain (oat) and alfalfa years of the rotation play a large role in rebuilding soil structure. It is speculated that the higher quality plant residue inputs from the legume and the fibrous root system of the oat play a large role in this ameliorative effect as well as the lack of tillage in alfalfa.

Organic and LPI management had higher microbial biomass carbon values than the other management systems (table 2 and 6).

VICMS I was historically a poorly managed site with low fertility and high weed densities and VICMS II was previously a well managed high fertility site. The results of chemical analysis show that the fertility of VICMS I has increased. Bray-P and K in the organic management plots, in both fields, is significantly higher than the other management systems, although Bray-P is not as high in the 4-year rotation as in the 2-year rotation (Table 3 and 7). Applying manure on a nitrogen requirement basis often leads to the over application of other nutrients. Phosphorus in the 2-year organic management system, in VICMS II, is in the very high range (Rehm et al, 2001) and the levels in VICMS I could reach that as well over time. Overloading of the soil with P leads to environmental pollution as the nutrient leaves the field and contaminates waterways, streams and lakes. Interestingly the organic 4-year rotation does not have the same P loading problem (Table 7). The values for P in the organic 4-year management system are medium (Rehm et al, 2001). Alfalfa has a higher P demand than other crops and so the inclusion of alfalfa in the 4-year rotation may decrease the excess amounts of P.

Overall nitrate levels did not indicate that nitrate levels were excessive. However the 4-year organic management system (0-8” depth) had significantly higher NO3-N levels than other rotation/management combinations (Table 3 and 7). This indicates that the application of manure in combination with the nitrogen produced by the alfalfa lead to higher N levels in this system over time.

2) The on-farm comparison data has not yet been fully reviewed. There is greater ambiguity in the results because of variability in soil type and landscape position, field histories, and differences in management practices. Initial review of the Fall 2000 data from the farmer comparisons indicates that the high amount of tillage utilized by organic growers to control weeds in corn may have a negative impact on aggregate stability and MBC in those fields.

Impacts and Contributions/Outcomes

This research (including the outreach component) will help farmers determine best management practices for their farms as well as how they can change their current practices to improve soil quality. The study is showing farmers how different management practices affect soils in the long-term. Most farmers do not have the freedom to experiment with management practices for the extended amount of time required to see significant changes in soil properties and so the long term experiment station data will be helpful in associating soil changes with various management practices.

Reducing tillage (LPI) and using organic management both resulted in improved soil quality. In addition the 4-year rotation increased aggregation and microbial biomass. This research along with evidence from long term yield, economic, and weed studies, demonstrates the benefits of a 4-year rotation for a sustainable and profitable agriculture system. This research also confirms that 2-year rotations are not a viable option for organic production, which is also reflected in the yield and weed data for this site (publications in review).

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