Soil Structure/Soil Biota Interrelationships

Soil Structure/Soil Biota Interrelationships

International Workshop on Methods of Research on Soil Structure/Soil Biota Interrelationships, Held at the International Agricultural Centre, Wageningen, The Netherlands, 24–28 November 1991
1993, Pages 105-118
Soil Structure/Soil Biota Interrelationships

Water repellency of sieve fractions from sandy soils and relationships with organic material and soil structure

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ABSTRACT

Bisdom, E.B.A., Dekker, L.W. and Schoute, J.F.Th., 1993. Water repellency of sieve fractions from sandy soils and relationships with organic material and soil structure. In: L. Brussaard and M.J. Kooistra (Editors), Int. Workshop on Methods of Research on Soil Structure/Soil Biota Interrelationships. Geoderma, 56: 105–118.

In soil micromorphology most studies are done with light and electron microscopes on thin sections from selected samples. Information is obtained on nature, distribution, size, form, quantity, etc. of soil constituents and features. Significant data, not readily obtainable from thin sections, can be supplied by using sieve fractions. Amongst such data are specific soil structures (rounded macro-aggregates, micro-aggregates and coated plant fragments) which may, partly or wholly, have been made by soil biota.

During the study of water repellency of agricultural and uncultivated sandy soils in the southwestern Netherlands, each sample was sieved, washed and treated with hydrogen peroxide. By measuring the water drop penetration time (WDPT) of a soil sample and of its sieve fractions, and by microscopic examination of soil components and features in each of these, one can indicate which soil constituents are important to the building up of water repellency.

The appearance and degree of water repellency are closely related with organic matter (plant fragments, roots, etc.). This organic matter can form a part of soil structures (e.g. micro- and macro-aggregates) or be the principal component itself. Water repellency (WDPT> 5 s), caused by microaggregates, can already occur in wettable soils, in the finest (<53 μm) out of ten sieve fractions. Water repellency of the finest sieve fraction did not affect the wettability of the whole sample because the other nine sieve fractions (53 μm to > 2000 μm) were wettable.

With increasing water repellency, from slightly to extremely water-repellent soils, more sieve fractions are occupied by soil structures and components that contain water-repellent organic remains, e.g. macro-aggregates, plant fragments and coatings on sand grains. Most of these structures and micro-aggregates have presumably, partly or wholly, been made by soil biota. Conversely, the degree of water repellency of soils can be reduced by decomposition of organic matter and by coating of plant fragments with fine non-water-repellent soil materials; processes in which soil biota are also active.

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