Yliopiston maanviljelyskemian laitos, Helsinki
When the importance of the texture of the topsoil became apparent (3, 4), it was recognised that more reliable information was necessary about the distribution of topsoils among the different textural classes, and especially about the occurrence of very heavy clay as a topsoil. Practically all earlier estimates are based on mechanical analyses of subsoils, as it has been assumed that there is not much textural difference between the subsoil and the topsoil, an assumption now proved to be wrong. The author made use of a statistically representative sample of the soils of the Helsinki and Turku hydrotechnical districts (S.W. Finland), made available by JUUSELA and WÄRE (5). One field per 1000 hectares of cultivated land was sampled, and from each field four samples were taken as follows: upper half of the field a1 topsoil, a2 subsoil ~ 30 cm depth, lower half of the field b1 topsoil, b2 subsoil ~ 30 cm depth. From the material the author picked all the fields where the clay content of the b2 sample had been shown to exceed 60 % (5). This made a total of 121 fields, but complete series of four samples were only available from 82 fields. A random omission of this extent does not, however, significantly reduce the representativeness of the material. For the present paper the author made a mechanical soil analysis of all samples by an accurate hydrometer method including H2O2 treatment (2), and an organic carbon determination of the topsoil samples. From the results, presented in Figs. 1 to 6, the following facts emerge: The clay content was generally smaller in the upper part of the field than in the lower part, and smaller in the topsoil than in the subsoil. From the earlier analyses of the b2 samples (5), the percentage of very heavy clay (over 60 % < 2 μ) may be estimated as 15 % of the cultivated land of the studied area (the distinction from the post-glacial muddy clay is not quite clear). Now, after all samples have been analysed, the estimate drops to 12 % if subsoils are considered, and to 5 % if topsoils are considered. The modal class of the total organic carbon content was 3.0—3.4 % (Fig. 5). The carbon content was positively correlated with the clay content of the topsoil (but not with that of the subsoil) in the upper parts of the fields (Fig. 6). This is in line with the theory that the higher the clay content, the slower is the decomposition of the organic matter. No such correlation was observed in the lower parts of the fields, where the original drainage conditions may decide the organic matter content.
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