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.