EARTHWORM POPULATIONS AND STABILITY OF SOIL-STRUCTURE IN A SILT LOAM SOIL OF A RECENTLY RECLAIMED POLDER IN THE NETHERLANDS

Earthworms are believed to be beneficial to the quality of the soil st ructure. An important aspect of quality of soil. structure is the stab ility of aggregates. The influence of earthworms on soil structure sta bility was investigated in a silt loam soil under ''conventional'' (hi gh-input) and ''integrated'' (reduced-input) management. Aggregates fr om the top 5 cm of soil were sampled, air dried and analysed for their size distribution and water stability. Dry-sieved aggregate size dist ribution did not differ appreciably among fields and between years, so aggregates of a certain size represented a similar fraction of the so il for all fields. The wet-sieving procedure, used to measure aggregat e stability, yielded highly reproducible results. Within-field variabi lity of the stability was lower than between-field variability, so in further sampling only one mixed sample per field was analysed for soil structure stability. Analysing only a large and a small aggregate fra ction was enough to evaluate aggregate stability in this study, as the re was a gradual increase in stability from large to smaller aggregate s. A 2 year time series of samples was analysed for earthworm populati on density and aggregate stability. Aggregate stability showed a seaso nal pattern and was correlated significantly with earthworm density. O ther factors influencing aggregate stability were management history i n the conventional and integrated fields and, for a wider range of fie lds, organic matter content of the soil. The lower plastic limit, anot her stability parameter, was influenced mainly by organic matter and c lay content, and not by any of the seasonally changing factors.