|Clarification of DUFF_DEPTH Measurement for Training Purposes – 2004
Although the 2004 DWM field manual was updated to include more information of duff sampling techniques, the full description of training DUFF_DEPTH is below.
As defined, duff_depth is the depth in tenths of inches of the duff layer. The duff layer is the organic material layer between the A-horizon (or uppermost soil mineral horizon) and the litter layer. The duff layer is decomposing organic material, decomposed to the point at which there is no identifiable organic materials (pine straw, leaves, twigs, etc).
We anticipate that measuring the duff depth will pose no unusual problems in most situations and states. However, there are some locations where the duff layer is extremely deep, due to buildup of large amounts of sphagnum moss and other material. Problems arise when crews encounter deep, organic soils and/or soils where differentiation between mineral soils horizon, duff layer, and litter layer is difficult. We don't want crews expending massive amounts of time digging through 4 ft of organic matter trying to reach the end of a very deep duff layer, or what they perceive as the end of the duff layer. If the duff layer is in fact very deep (more than 8 inches, matches the duff depth in soil sampling at which estimation is allowed)) then crews are allowed to estimate the full depth of this layer. To start with, crews should dig down as deep as possible and measure this depth. If they determine that duff continues beyond this point they can estimate the remaining portion as follows:
Examine the subplot area and the area immediately around the sample point. If there is a cutbank or a windthrown tree nearby where the soil layers are exposed, obtain an estimate of the duff layer from this disturbed area. Compare that estimate with the measured depth already taken and estimate a value for the entire duff depth. The QA staff will have to be lenient when evaluating the tolerance and MQO’s.
In some situations, where its difficult to distinguish the duff, litter, and organic soil layers, crews will need to acquire a sense of duff density and texture. Not only is duff different from litter in that litter contains recognizable dead plant parts, but it is much denser. A handful of litter is much lighter than a handful of duff. In fact, when processing duff and litter layer measurements, duff can easily weigh 6X more than litter per cubic foot. Slight error in measuring duff, through inclusion of litter, will be compounded when per acre tonnage estimates are created. When differentiating between duff and the uppermost mineral horizon, your eyes and soil texture should serve as good guides. If unsure of the bottom of the duff layer, crews should feel the texture of the suspect soil in their hand. Crews may eliminate needless sampling of duff depths and reduce measurement error by making sure the soil from the bottom of the duff layer is organic and not a mineral horizon. Crews should be trained to avoid sampling A horizons which have some organic material in it. Rhetorical questions to ask are: is this duff layer dried out, would it burn? If someone wanted to calculate the total amount of carbon in this duff layer, would they find organic matter or mineral soil along the full measured depth at the sample point?
A light litter layer with deep, organic soils --> make sure the litter is scraped away revealing a soil horizon with no identifiable plant parts, use hands to keep feeling soil texture down the soil profile till the soil texture changes from that of a smooth organic feel till the texture changes to a more gritty, mineral feel... this is the bottom of the duff layer.
Additional info (information from soil sampling may aid in Duff identification:
Soil Indicator Tips for Telling the Difference between Mineral and Organic Soil Horizons
In some soils, telling the difference between the bottom of the O-Horizon and the top of an A-horizon (organic-rich mineral horizon) can be difficult. If uncertain:
Look for evidence of plant parts (e.g., leaves, needles). If you can see them decomposing in place, you’re still in organic.
Rub the soil between your finger. Does it crumble (organic) or feel more like modeling clay (try pinching into a ribbon).
Look for shiny flecks of mica or quartz (won’t help in all soils).
Wet a sample of the material and press it between your fingers. Note the color of the liquid that runs out. The blacker the color, the higher the organic content.
Check for a change in density (mineral soils are denser).
Definitions used in Soils Manual
Duff (Humus) A soil layer dominated by organic material derived from the decomposition of plant and animal litter and deposited on either an organic or a mineral surface. This layer is distinguished from the litter layer in that the original organic material has undergone sufficient decomposition that the source of this material (e.g., individual plant parts) can no longer be identified.
Litter Undecomposed or only partially decomposed organic material that can be readily identified (e.g., plant leaves, twigs, etc.)