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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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    Variability due to Grazing Animals & Organic Matter Inputs

    Grazing Animals Organic Matter Inputs Soil Organic Matter Variability
    Variability due to Grazing Animals & Organic Matter Inputs
    In the top graph above, 71 sites with both grazing animals and organic matter inputs (OMI) are each represented by a quadruplet of data points connected by a vertical black line (a blue square for 2019, red circle for 2020, green triangle for 2021, and yellow diamond for 2022).  Each square-circle-triangle-diamond-black-line combo represents the Soil Organic Matter (SOM) values for one site for 4 years. According to the literature, SOM is supposed to be quite stable and very difficult to change, and yet we are seeing large swings in individual sites’ SOM data, especially when grazing animals are present or organic matter is imported to the site, as is the case in the top graph above.
    We only have 12 sites in our study which have no grazing animals or imported organic matter for 3 or more years.  The second lower graph shows that the variability in SOM values for these 12 sites is much less than for sites with grazing animals or organic matter inputs.

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    Predicting the Soil Health of our 3 Main Crop Groups, PART 2

    Alkaline Soil Crop Groups Days Of Living Cover Grazing Animals Irrigation Water Organic Matter Inputs PH Soil Organic Matter Tillage Intensity
    Predicting the Soil Health of our 3 Main Crop Groups, PART 2
    Perennial Hay/Alfalfa/Pastures: The Pasture group has the highest average soil health scores of these three crop groups.  Although the Pasture group has lower supplemental water days and lower organic matter added, their very high days of living cover and very high grazing days, along with their very low tillage intensity and lower soil pH seem to more than make up for their water challenges, in terms of soil health.
    Commodity Row Crops: The Commodity crop group has the lowest average scores of these three groups.  Although they have done an excellent job of reducing their tillage intensity, that fact alone cannot make up for their high soil pH, lowest days of living cover and lowest organic matter added.  They have only 2/3rds of the water availability as the Commercial Veg/Flower/Fruit group, which explains their lower days of cover crops that often require fall seeding and fall water.  Inter-seeding cover crops aerially or when the main crop is still small are work-arounds but not always practical.  Low commodity prices mean the cost of additional organic matter inputs like compost and manure are hard to justify.
    Commercial Veg/Flower/Fruit: The Commercial Veg group has the highest tillage intensity by far, but also triple the organic matter inputs of the other 2 groups.  These huge organic matter inputs, along with their longer water season, greater use of cover crops, and lower soil pH overpower their intense tillage and boost their average soil health scores above the commodity crops’ averages.  Their longer water season means they can plant more fall cover crops and string together succession plantings for a longer growing season.  Their high value vegetables mean that they can afford organic matter input costs and hauling fees.