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    The Depth of a Soil Sample affects the Soil Organic Matter Value

    Sample Depth Soil Organic Matter Variability
    Graph of Change in sample depth VS change in soil organic matter

    Microbial life and organic matter are greatest in the top couple of inches of soil.  When you collect a deeper soil sample, you add more soil from deeper down where there is much less organic matter.  That tends to “dilute” the amount of microbial life and organic matter that is in the top couple inches of soil.  That dilution from the deeper soil makes the lab’s reading of your soil organic matter drop.  The inverse is also true. If you collect a shallower sample it makes  the lab’s reading of your soil organic matter rise.differences-between-the-most-and-least-variable-sites-copy

    Each data point in this graph represents the change in soil sample depth on one site over 2 consecutive years, plotted against the corresponding change in SOM.  Most data points fall into the graph’s colored quadrants, where a later shallower soil sample has a higher SOM, and a later deeper soil sample has a lower SOM than the previous year’s sample.  The red trend line confirms that sample depth affects our SOM values. 

    However, the data points in this graph are widely scattered, and 30% of the data points fall outside of these 2 colored quadrants.  If sample depth were the only thing affecting SOM, you would expect to see all the data points much more closely clustered around the trend line, with very few data points in the two white quadrants.  That is not what we are seeing.  That means that other things besides sample depth (management, inputs, environmental factors, etc.) are also affecting changes in SOM.

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    Differences between the most and least variable sites.

    Grazing Animals Organic Matter Inputs Variability
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    There are significant differences between our sites with the most variable soil health scores (our Swingers) and our sites with the least variable scores (our Parked sites).  The most significant difference between the 2 groups is that Swinging sites have 3 times the number of average grazing days as Parked sites. While Swinging and Parked sites have the same numbers of sites with grazing animals, Swinging sites have animals grazing on-site for a much longer period of time.  2/3rds of Swinging sites are pastures, and the majority of Swinging sites use conventional growing methods.  Swinging sites also apply an average of 7.5 T/acre of organic matter inputs, almost 50% more than Parked Sites at 5.6 T/ac.  These findings support our 2023 findings that more grazing animals and organic matter inputs increase variability.
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    Past Practices can explain some variability.

    Compost Crop Groups Grazing Animals Manure Organic Matter Inputs Variability
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    We continue to see a great deal of variability in our lab results when we compare sites with themselves year-to-year.  We divided our sites into 4 crop categories (Commercial Veg/Flower/Fruit; Commodity Row Crops and Dryland Grains; Home Gardens; and Perennial Hay/Alfalfa/Pastures) and examined each category’s variability.  These 4 graphs show that an examination of past practices can often explain some of the exceptionally big jumps in variability which we see in every crop group.
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    Variability is highest in Home Gardens and Pastures

    Grazing Animals Organic Matter Inputs Variability
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    These 4 graphs show that Home Gardens and Perennial Hay/Alfalfa/Pastures have the most variable soil health scores year-to-year.  Each site’s 3-5 years of soil health scores are represented by a column of 3-5 colored data points connected by a vertical black line (a blue square for 2019, red circle for 2020, green triangle for 2021, yellow diamond for 2022 and aqua diamond for 2023). Each square-circle-triangle-diamond-Blackline combo represents the Soil Health Scores for one site for 3-5 years.  Sites with the most variability have the tallest black lines between their lowest and highest scores.
    Last year we showed that large amounts of organic matter inputs and more days of grazing animals increased variability.  This year we have shown that home gardeners apply the highest rates of organic matter to their sites, and we know that grazing animals are usually found in pastures.  It makes sense then that Home Gardens and Perennial Hay/Alfalfa/Pastures would experience the greatest variability in Soil Health Scores.
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    Our Growers have made progress!!

    Cover Crops Days Of Living Cover Organic Matter Inputs Progress Tillage Intensity Variability
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    The variability we continue to see in our lab results has thrown a great big monkey wrench into our ability to assess progress using only our lab results. However, we can still assess the progress our growers have made in increasing their use of tried-and-true soil health practices, including:
    • Increasing their Days of Living Cover
    • Increasing their use of Cover Crops
    • Decreasing their Tillage Intensity
    • Increasing their Organic Matter Inputs
    Our growers have made excellent progress incorporating 3 out of 4 of these soil health practices in the last 5 years.
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    Variability with 2 Consecutive Years of Grazing or Organic Matter Inputs

    Grazing Animals Organic Matter Inputs Variability
    Variability with 2 Consecutive Years of Grazing or Organic Matter Inputs
    We sorted our sites into 3 groups and calculated the average variability for each group.  This bar graph shows that the groups which grazed animals or added organic matter to their sites for 2 consecutive years have approximately three times as much variability in their lab results as the group with NO grazing animals and NO organic matter inputs.