University of Maryland Extension

Is soil organic matter holding you back?

Image Credit: 
Sara BhaduriHauck

Now that the growing season is in full swing, you’re hopefully seeing some explosive plant growth. If your fields seem less than fertile, however, you might be wondering what’s holding your plants back from their full potential.   

If a routine soil test shows that your pH and fertility are in line, consider checking out your soil’s organic matter. Depending on the soil testing lab you use, organic matter may or may not be included in the basic test package. For agronomic soils, strive for a soil organic matter (SOM) of higher than 3%. Manipulating SOM requires a careful balance of inputs and amendments, but with the right strategy a SOM of 4% or higher is possible in our area.

SOM is the proportion of the soil that is made of things that used to be living but have since decomposed into a stable form called humus. SOM is an essential part of soil structure as it acts like a glue that holds soil particles together. Organic matter helps soil particles aggregate, or clump together, creating the pore structure in soil that is crucial for root aeration, drainage, and water holding capacity. SOM also aids in fertility because it increases the soil’s cation exchange capacity, essentially holding more nutrients in reserve for when plants need them. Similarly, it impacts the soil’s ability to hold water but not tie it up so that it’s available for plants to use. SOM also helps the soil retain its natural capacity to buffer against changes in pH; in our area, soils tend to become acidic over time, but higher SOM helps to prevent this from occurring.      

There is no quick fix for improving SOM, but there are simple steps you can take to improve it over time.

  • Use less tillage. SOM is reduced by aeration, humidity, and high temperature. If you utilize a no-till or reduced tillage approach, you prevent these conditions from reaching the soil and thus preserve your SOM.
  • Increase the amount of organic material you add to the soil. Leaving crop residues on the field, using green manures, and fertilizing with animal manures are all ways to add organic material. Remember, however, that organic matter is organic material that has already decomposed into a stable state – so adding composted materials is a quicker way to increase SOM.
  • Reduce erosion. Most SOM is found in topsoil; when topsoil is lost, so is SOM.

If your SOM is adequate now, don’t forget that it can be depleted by tillage and erosion. Maintaining SOM is just as important as building it!                                    

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