Mekaanisesta maa-analyysistä

Authors

• Reijo Heinonen Maatalouskoelaitos, maa-tutkimusosasto, Helsinki

DOI: https://doi.org/10.23986/afsci.71330

Abstract

The method of mechanical soil analysis so far used at the department of soil science is briefly described. The author then points out some imperfections of the method, discusses possible analysis procudures by which the errors could be avoided and introduces a new method which in the main follows the international method (7). The special features in which the new procedure, now adopted at the department, differs from the international method are as follows: After HCI treatment the soil paste is rubbed with the finger. In some clays this has proved to be the only way to destroy the small floes. Each international fraction is divided into two subfractions. Pipette samples are taken after 4 mins. 48 secs., 48 mins, and 8 hrs. After the pipette samples have been taken, the suspension is washed through the 0.6, 0.2 and 0.06 mm sieves. The residue is evaporated to dryness, dried at + 105° C, and weighed. The results are given on a dry matter basis, and the loss on treatment with H2O2 and with HCI, and the air-dry moisture are given separately. Calculations are given on p. 3. Sometimes the extension of analysis into the clay fraction (4 cm./32 hours for particles <0.0006 mm.) gives very valuable information on the nature of the soil. An example on p. 4 illustrates the fact that only this last fractionation distinguishes clearly between two soils, both of which are classified as heavy clays by the ordinary analysis (over 60 % clay). In the first sample the greater part of the clay is coilloidal, in the second the ratio is reversed. By visual examination the second sample was classified as clayey silt. Most difficulties arise in the determination of fine sand fractions, because the sieving and pipette methods must be tied up, and they never fit together exactly. The difference cannot be regarded as an experimental error of the whole analysis because it is inside the finesand fraction anyway. So it was decided to determine fraction 0.2—0.06 by sieving and fraction 0.06—0.02 by difference. Decantation would probably give the best approach to the problem, but it is too time-consuming to fit into the routine, in which one person carries out a series of 10 analyses in 6—7 days. As we use only 500 cc. settling cylinders, and the sieving greatly increases the volume of the suspension, it is best not to sieve until after the pipettings. Experience shows that this does not cause errors in the pipetted fractions. If the total amount of soil inorganic dry matter were determined by drying the soil paste immediately after HCI treatment (as is done in many laboratories) it would be very difficult, and sometimes impossible, to redisperse it properly. As the suspension is dried last, the weight of the peptisator, the free part of which is in the form of Na2CO3, must be taken into account. An average value of 175 mg is subtracted from the residue and 7 mg from the pipette samplings, respectively. Lastly the author gives examples of summation curves, in which the use of the logarithm of settling velocities is to be preferred to apparent size classes. The tieing-up of pipetted and sieved fractionations is thereby illustrated.