2.3 - Density, Porosity, and Compaction

soil solid %

soil solid % = [soil bulk density/soil particle density] x 100%

bulk density factors

- soil texture - soil organic matter content - soil use and management - depth of the soil sample from the surface - density = mass/volume

soil bulk density calculations (Pb)

1. collecting a soil sample of known volume 2. drying in an oven to 105⁰ C 3. weighing it to find its oven dry mass Pb = dry soil mass/volume of soil sample *Volume cylinder = π×r²×l *Volume cube = l×w×h lbs/ft³ = 62.4×g/cm³

soil porosity %

Soil porosity % = 100% - soil solids %

particle density

The density of the solid particles only, the mass per unit volume of soil solids, excluding OM (volume does not include soil pores) - basic physical property that compares the mass of a substance to its volume - most mineral soils are relatively uniform in density, ranging from 2.6-2.8, avg 2.65 g/cm³ - not affected by pore space - it is a function of the mineral composition, not the arrangement of the particles - particle density = soil particle mass/volume of soil particle

bulk density

The mass of a dry soil sample per unit bulk volume (voids and all) as compared with the mass of an equal volume of water. - basic physical property that compares the mass of a substance to its volume - affected by amount of pore space - generally increases with depth - inverse relationship with OM content and clay content - an indirect measure of the amount of pore space in the soil. Greater porosity = less of this - comparisons of soil compaction in similar soils can be made on the basis of this

porosity

The volume percentage of the total bulk not occupied by solid particles. - ranges from about 30% in sands to 60% in clays - Most mineral soils in 'good' condition will have about 50% porosity and 50% solid