Methods
Where and When are Soil Properties Measured?
FIA field personnel collect soil data during the Phase 3 field season, which begins in early June and ends in September. Soil samples are sent to the laboratory immediately after collection where they are stabilized by air drying. Laboratory analyses are conducted throughout the fall and winter following the field season.
What Types of Measurements are Made in the Field?
On-plot measurements include soil compaction and bare soil observations. Soil compaction, the percentage of the soil surface exhibiting evidence of soil compaction as well as the type of compaction, is measured by ocular estimation. The relative amount of bare soil is also estimated. Field measurements related to erosion and compaction estimates are made on all four subplots on the Phase 3 field plot.
Soil samples are collected on FIA sample plots along soil sampling lines adjacent to subplots 2, 3, and 4. Soils are collected if, and only if, the soil sampling location is in a forested condition. If the soil cannot be sampled at the designated sampling point due to trampling or an obstruction (e.g., boulder, tree, standing water), the sampling point is relocated to any location within a radius of 5 feet. A total of five samples are collected on each plot (three forest floor, two mineral).
The entire forest floor layer is sampled from a known area after measuring the thickness of the litter and duff layers at the north, south, east, and west edges of a 12 inch diameter sampling frame. Only organic material that is < ¼" inch diameter is collected; rocks and larger woody materials are discarded prior to collection.
Once the forest floor has been removed, mineral and organic soils are sampled volumetrically by collecting cores from two depths: 0 to 4 inches and 4 to 8 inches. The texture of each layer is estimated in the field and characterized as organic, loamy, clayey, sandy, or coarse sandy. Following soil sampling, the depth to any restrictive horizon within the top 20 inches is estimated using a soil probe.
What Types of Measurements are Made in the Laboratory?
Soil samples are mailed to the regional laboratory for additional physical and chemical analysis. Laboratory measurements include:
| Forest floor samples | Mineral and organic soil samples |
| Bulk density | Bulk density |
| Water content | Water content |
| Total carbon | Coarse fragment (>2-mm) content |
| Total nitrogen | pH (in water and 0.01 M CaCl2) |
| Total carbon | |
| Total organic carbon | |
| Total inorganic carbon | |
| Carbonates (only for soils where pH>7.5) | |
| Total nitrogen | |
| Exchangeable cations (Na, K, Mg, Ca, Al) | |
| Extractable sulfur and trace metals (Mn, Fe, Ni, Cu, Zn, Cd, Pb) | |
| Extractable phosphorus (pH-dependent method) |
Last Modified: 03/02/2007