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Soil Health and Biodiversity
Topics:
Soil health
The WICST team interviewed 28 Wisconsin farmers in Walworth and Columbia
Counties and conducted a number of chemical, physical and biological tests
on the soils in the cropping systems trial. The goal was to evaluate
the changes in soil characteristics under the different systems between
1990 and 1997.
The major differences that emerged are as follows:
- Soil test levels of phosphorus and potassium
have been dropping in the low input CS3 system compared to the
high input CS1 system. However, after 10 years soil test levels are
still in the normal to very high range
- Soil pH was slightly lower — soil was
more acidic — under continuous corn than the other systems. Dropping
pH may result in irreversible changes in the nutrient holding capacity
(cation exchange capacity) of the soil.
- The pasture showed better soil aggregation
than the other systems.
- Earthworm numbers and types of earthworms
present in the soil varied by system.
- The most biologically complex cropping systems (CS3 and CS6) showed
an increase in microbial biomass over time,
while the least diverse system (CS1) showed a reduction.
- Total soil organic matter increased
in the pasture plots (CS6) at both sites over the course of the study,
so that it was significantly higher than in the other systems by 1997.
- The mean soil organic matter in the
forage systems was 12% higher than the mean for the cash crop systems
by 1998.
- The areas of highest surface compaction
occurred in CS3 due to intensive mechanical weed control.
Overall Conclusion: Few significant
differences in soil quality/health emerged under the different cropping
systems during these first years of the Trial. It must be pointed out
that the time frame of the study is very short compared to the 150-year
agricultural history of these soils. Furthermore, the soils on which
the WICST trial is conducted are among the finest in the world, buffering
changes in management and supporting a wide range of cropping systems.
Earthworms
The four species of earthworms found on the WICST plots can be broadly
classified into two different groups based on where they live in the soil.
Three species are topsoil-dwelling earthworms (technically called endogeics)
that create horizontal burrows in the top 25 cm of the soil, and one species
is a subsoil-dwelling earthworm (the nightcrawler or anecic) that creates
one or two burrows that can extend 3 meters into the soil profile. The
primary factors affecting earthworm populations appear to be tillage and
manure application but the two groups respond differently.
Topsoil-dwelling earthworms: These earthworms did not appear to be directly
affected by tillage intensity because their abundance did not fluctuate
during the different phases of the same rotation regardless of the amount
of tillage. However, the use of manure did result in more topsoil-dwellers.
The manure-receiving, forage-based systems had twice the number as the
cash-grain systems. Abundance ranged from 31 to 460 earthworms/m2.
Subsoil-dwelling earthworms: Populations of these earthworms correspond
directly to the amount of tillage in the system. Of the six systems, the
no-till corn-soybean system had the highest number and continuous corn
the lowest. Abundance ranged from 0 to 43 earthworms/m2.
Soil biodiversity
Extensive research has been conducted on the WICST plots to characterize
the communities of soil-dwelling organisms under the different cropping
systems. Because little similar work has been done elsewhere, efforts
have focused on how to characterize the abundance, diversity and overall
“community profile” of soil organisms, including microbes, nematodes,
arthropods (insects and spiders) and earthworms.
Our research shows that after 6-8 years in the different cropping systems,
the communities of soil organisms do diverge from each other, although
the implications of this for the health and long-term productivity of
the systems are not necessarily clear. In every group of organisms examined
the cropping systems differed from each other over time, sometimes in
terms of abundance, sometimes in terms of diversity, and sometimes in
terms of the specific species present. (Note that we used molecular techniques
so that unculturable microorganisms could be included in our sampling.)
To date, the project has identified 20 species of springtails (Collembolas)
new to Wisconsin (bringing the total to 52 for the state), and one species
“new to science,” underscoring how much remains to be discovered in groups
that are even less visible.
Soil erosion
The WICST project has not directly measured soil erosion on plots, largely
because our plots are located on land with little erosion potential.
However, we note that our work with cover crops (LINK) is of significant
interest to those concerned about erosion that might occur after the harvest
of small grains and short season crops. The recently Revised Universal
Soil Loss Equation reflects the benefits of cover crops. This has offered
producers a new incentive to consider systems that include a small grain
and cover crop (like CS3), and has given us renewed motivation to find
ways to adapt cover crop use to a variety of commercial farm settings.
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