Soil Analyses Significant in Forest Soils Investigations and Methods of Determination: 2. Measurement of Certain Physical Properties of Forest Soils

OF DETERMINATIONS. MEASUREMENT OF CERTAIN PHYSICAL PROPERTIES OF FOREST SOILS H. J. LuTz PORE volume, air .capacity, water-holding capacity, and volume weight are important properties of forest soils. Various methods for measuring these characters have been developed, notably in Germany, Switzerland, Russia, and America. The purpose of the writer, however, is to describe the procedure which has been followed in his own laboratory and to point out certain problems which challenge solution rather than to review the different methods that have been used. The following remarks apply only to measurements in soils which are relatively free from rocks. Soils containing a high proportion of rocks present special problems which are as yet largely unsolved, although some work is being done in an attempt to develop appropriate techniques. However, the progress of this work has been so slow that presentation of results at this time is unwarranted. Soils which are relatively free from rocks are sampled with steel cylinders 10 cm high with a volume of i liter. No preliminary moistening of the soil has been practiced; samples are taken without regard to field moisture content. This practice appears fairly satisfactory when one is dealing with relatively coarsetextured soil materials. After removal from the soil body the sample may be trimmed in the field or this may be done later in the laboratory. When trimmed, a filter paper is placed on the lower end of the sample which is then set in a tank, and water flowed in until it stands at about one-third the cylinder height. After about an hour water is again flowed into the tank, bringing the level to about two-thirds the cylinder height. Finally, after waiting another hour the water level is brought to the top of the cylinder which is then allowed to stand for 24 to 48 hours. The purpose of the gradual addition of water to the tank is to obtain better replacement of the air in the soil mass. The next step in the procedure has been to apply thin gum rubber covers to both top and bottom of the cylinder while it was submerged. After removing the cylinder with its water-tight covers and drying the surface moisture, the system is weighed. Theoretically the cylinder should at this stage contain only the solid soil particles and water, the volume of the latter being assumed to equal the pore volume. The covers are then removed and the soil sample, still in the cylinder, is allowed to drain on sandy soil material for a period of at least one hour. During this time the top of the cylinder is covered -with a moist cloth to prevent evaporation loss from the sample. It has been found recently that the bothersome task of fitting rubber covers to the submerged cylinders can be eliminated from the preceding step. All that is necessary is equipment for weighing under water the cylinder with the contained sample. A heavy duty specific gravity balance is the principal item required. After determining the weight of the sample under water it is drained as indicated above. When drainage is complete the sample is reweighed and the gravitational water which has moved out is taken as a measure of the air-capacity, or the noncapillary pore space. Finally the sample is oven-dried. The weight of the sample after drainage less the oven-dry weight represents the water-holding capacity. Volume weight is of course obtained from the oven-dry weight. This technic has the advantage of being fairly rapid and of permitting measurement of the various properties in the same sample. Several problems arise, however, which merit consideration, and possibly investigation. In the first place various workers have observed that finetextured soils, if sampled when fairly dry, expand when they are saturated. I believe that in such cases the practice has been to trim off the soil material which rises above the top of the cylinder. Is this practice justifiable? When dealing with soils whose volume changes materially with variations in water content should not one saturate the soil as a preliminary to taking the cylinder sample? This question has not affected the writer directly since only once have cylinder samples been observed to show swelling. Another question which is quite perplexing relates to replacement of the air in the soil by water. Is one justified in assuming that cold water treatment without evacuation, regardless of how carried out, actually does result in full replacement of air in the soil? On