Soils Modified by Topsoil Dressing and Deep Tilling in Peaty Farmland

Abstract

In the Ishikari Plain in Hokkaido, where peat soils originally developed in a wetland setting, drainage of ground water and soil dressing were essential processes to develop new agricultural areas. Dressing soils were generally taken from the subsoil strata of adjacent areas, and include soils from plateaus or hills such as grey upland soils, brown forest soils, and occasionally dredged sediments from the bottom of dam lakes. Multiple rounds of soil dressing were required for compensation of ground subsidence and provision of extra nutrients. The thickness of the current surface mineral horizon averages 30 cm, and is often not classified as peat soil according to most soil classification systems. Multiple applications of soil dressings containing clayey soil reaching up to 38 cm thickness have been implemented since 1980s, supporting development of a gleyic horizon with positive dipyridyl soil reaction underlain by thick sapric peat horizons. The soil was classified as Transportic Sapric Histosols. This is one of the typical agricultural soils distributed on the Ishikari Plain, where multiple soil dressing processes are implemented with an effective drainage system to decrease ground water level. Fresh mineral soil dressing is preferable to reduce protein content in grown rice as compared to those grown directly on peat soils.