Screening soil

Since the mineral soil layers are too big to be returned to the lab, they must be processed in the field. As the soil is removed from each layer, it is sieved through a 1.5 cm screen to separate the coarse fragment component (the rocks) and any roots. Both the rocks on the screen and the soil on the tarp are then weighed by the bucketful on a digital scale suspended from a tripod. Roots are collected in bags and returned to the lab for weighing. The bucketfuls are recorded and later added up to a) subtract rock volume from the total volume of the layer, and b) calculate a soil mass for each layer. With mass and volume measurements, we can calculate soil bulk density for each layer.

Since we are interested in the mass of the soil without any water (oven dry mass), a subsample from each layer (a handful from each bucketful) is collected and taken back to the lab. There it is weighed wet and dry to determine a moisture content representative of the entire layer. This percentage is subtracted from the field weight of the soil bucketfuls to get a total oven dry mass for the layer. The subsample is also used for chemical analysis to determine soil nutrient content for each layer.

As each bucketful of soil and rock is weighed, it is dumped on a second tarp, seen in the background of this photo. When the excavation is complete, the soil and rocks are dumped back into the hole and covered with litter to minimize the disturbance to the area.