There are many soil health tests offered by commercial laboratories, but there are several challenges associated with sampling effectively.
These challenges include selecting the right test for the information you want and then interpreting results to understand what it means for your farm. This is especially true when evaluating recently transitioned soils where soil health building practices are being used.
At the Soil Health and Agriculture Research Extension Farm in Mooreton, N.D., we have field scale, side-by-side comparisons for evaluating soil health indicators relevant for our region.
In 2017, we sampled 84 points across the SHARE Farm to run many of the soil health tests available (Figure 1) and relate it to soybean yield. We'll focus on the first year of no-till plus cover crops versus conventional tillage management comparison for the high clay soils typically found in the Red River Valley.
Relation to yield and crop
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Overall, none of the soil health indicators were strongly correlated with soybean yield. This isn't surprising since indicators tend to measure soil physical and biological properties that do not relate to plant growth over a short timeframe. We would expect, with time, soil health to improve under no till plus cover crops and that yield may reflect this change.
We have observed that soil health indicators that relate to carbon and nitrogen pools that turnover quickly (active pools, microbial food) are very sensitive to vegetation presence and identity. These measurements tended to be higher in the spring after a soybean crop, than after a corn crop. This means that nutrients will cycle faster when there are more nitrogen rich root-derived inputs into the soil.
No till plus cover crop versus conventional tillage
When we compared soil health indicators across conventional and no-till plus cover treatments (Figure 2), we saw that aggregate stability was sensitive to management, with higher aggregate stability in the no till plus cover crop soils. We also observed that soybean nodule count per gram of plant mass was also higher in the no-till plus cover crop soils. However, all of the microbial indicators (biomass and activity) were extremely variable across the field and across the treatments.
At this time, we recommend watching for development of soil structure (aggregates) as soils are transitioned into conservation practices because it tends to be more reliable than microbial indicators over time. If using microbial indicators to assess soil health, be sure to sample under similar conditions from year to year (soil moisture and temperature), to minimize variability in these indicators.
