An organic rich, gleyed soil (Cumulic Haplaquoll) on the edge of a sag pond near San Marino High School illustrates the effects of poor drainage and tectonism on soil formation (pedogenesis) in a semiarid region during the Holocene. Combined with other characteristics of soil development, the organic matter masks the presence of six depositional strata in the granodioritic alluvium in which the soil formed. Two of these units occur only on the south side of a strand of the Raymond fault, three occur only on the north side, and one occurs on both sides. The dominant clay minerals are smectites that have formed from soil solution within the past 10,000 years. Beidellite formed in horizons with exchangeable aluminum (pH less than 6.5), and montmorillonite formed in horizons without exchangeable aluminum (pH greater than 6.5). A soil tongue produced by fault movement was relatively unweathered after it was emplaced by an earthquake that occurred less than 1,400 years ago. Compared to other Holocene soils, pedogenesis at this site is intermediate between the weakly developed Hanford soils formed in granitic alluvium in the San Joaquin Valley and the strongly developed Concepcion soils formed in continental terrace deposits along the coast.

Abstract Organic soil subsidence occurs mainly with drainage and development of peat for agriculture. Subsidence occurs either from densification (loss of buoyancy, shrinkage, and compaction) or from actual loss of mass (biological oxidation, burning, hydrolysis and leaching, erosion, and mining). Densification usully occurs soon after drainage is established. Slow, continuous loss of mass is due mainly to biological oxidation. Erosion is minor except in specific sites. Mining losses vary greatly and depend upon direct removal of the materials. Subsidence rates are determined mainly by type of peat, depth to water table, and temperature. Subsidence losses have been carefully measured in several locations (e.g., the Florida Everglades), and predictions of future subsidence developed in 1950 have proved reliable. Peat drainage and subsidence have several consequences: loss of plant rooting depth where the substrate is unfavorable (stony, acidic, saline), increased pumping for drainage, instability of roads and other structures, increase in nutrient outflows, colder surface temperature during winter nights, and increase of CO2 flux to the global atmosphere The water table for organic soils should be held as high as crop and field conditions allow to reduce subsidence. Computer models offer methods for refining oxidation rate processes and prediction of subsidence losses where adequate calibration data are available. Remote sensing offers a method of assessing organic soil area and drainage changes. These new technologies should improve our assessment, and guide our management, of organic soil resources.