THE EFFECT OF WATER TABLE DEPTH IN ORGANIC SOIL ON SUBSIDENCE AND SWELLING

Abstract

The effects of two water table depths (WTD), 0.6 and 0.9 m below the soil surface on subsidence, subsidence rate and swelling of an organic soil were observed in large undisturbed cores under greenhouse conditions. Measurements were made in two tiers, 0–0.3 m (top tier) and 0.3 m to WTD (bottom tier) during the growth of a carrot crop with WTD as above, and continued following a rise in the water table. The WTD of 0.9 m caused the top tier to subside twice as much as the same tier in the 0.6 m WTD. Top tier subsidence seemed irreversible in both WTD because minor swelling was observed following a rise in the water table. Most of the reversible subsidence occurred in the bottom tier. Raising the water table reduced the total profile subsidence by 36 and 24% for the 0.6 and 0.9 m WTD, respectively. After correction for oxidation, subsidence accounted for 3.2 and 5.9% of the 0.6- and 0.9-m profiles, respectively. The bulk density increase in the 0.6-m profile before and after the end of the experiment was not significant but a significant increase of 11% was measured in the 0.9-m profile. Subsidence rates decreased in both tiers during the growing period reaching a minimum in both WTD at harvest time. The subsidence rate in 0.9-m profile at 100 days after seeding was 2.5 times the rate in the 0.6-m profile. Key words: Organic soil, subsidence, swelling, water table