The Structure of Soil Crumbs

Abstract

A model of a soil crumb is proposed in which the crumb consists of domains of orientated clay and quartz particles. A clay domain is defined as a group of clay crystals which are orientated and sufficiently close together to behave in water as a single unit. In normal agricultural soils, where calcium is the dominant exchangeable cation, the clay crystals in the crumbs are all aggregated into domains by drying. Drying to the wilting point is apparently sufficient. In the model the domains and the quartz particles may be linked to each other by organic matter or soil conditioners. The domains may also be held together by electrostatic forces.The model satisfies four conditions derived from experiment on the effect of organic matter on soil crumbs : I . The crystalline swelling of the clay in the crumb is unaltered-the individual crystals within the domains are unaffected by organic matter and are free to take up water. 2. Slaking is prevented-the strains set up in the crumb on wetting are shared between several domains through their common linkages by organic matter to quartz particles. 3. Dispersion of Na-saturated crumbs is prevented-the linkages of the domains by organic matter restrain the diffuse double layer swelling of the clay. 4. pF-moisture content curve is unaltered-the stabilization by organic matter does not require a rearrangement of the structure of the crumb.On the basis of the model, conventional wet-Sieving or dispersion tests assess the strength of the organic-matter linkages. Remoulding cNsc\hs byc&\m\ny t , $ thePTeyiQUs+Y S t & &qua'ft2-doma.m bonds, SO that on drying and re-testing the stability of the crumbs has decreased.The model has been shown to be consistent with present knowledge of the action of cement and h e in stabilizing soil, cement or lime being substituted for organic matter in the model. It also accounts qualitatively for the sensitivity of Soils, that is the decrease in shear strength after remoulding. On the basis of the model the amount of organic matter resistant to microbial attack should be proportional to the surface area of the clay domains. Remoulding soil (or tillage) by breaking the organic-matter bonds joining the clay domains to the quartz will free organic matter for microbial attack.This model does not apply to soil crumbs in which the crystalline swelling of the clay is prevented, for example by heat treatment.

Variabilidade espacial de atributos físicos e químicos relacionados com o estado de agregação de dois latossolos cultivados no sistema de semeadura direta

“…A matéria orgânica e as argilas, quando se combinam com os cátions apropriados, são os principais responsáveis pela formação dos agregados, atuando como agentes cimentantes (Emerson, 1959;Brady, 1979; Silva e Kato, 1997;Six et al, 2004). Contudo, também é preciso considerar que os cátions trocáveis do solo influem no processo de agregação (Arena, 1968) de maneira positiva (Série floculante) ou negativa (Série dispersante).…”

Partículas coloidais, dispersão e agregação em latossolos

“…Unidades compostas pela associação de filossilicatos foram originalmente chamadas de domínio (domain), isto é, "um grupo de cristais de argila orientados e suficientemente próximos para comportarem-se em água como uma única unidade" (EMERSON, 1959). O tamanho destas unidades varia entre 1 a 2 micrômetros.…”

Soil aggregation dynamics and carbon sequestration

“…The SOC protection is proportional to the specific external surface area of clay particles, and to the monolayer interfaces between clay and sand particles. Emerson (1959) found that organic matter unavailable to microbial processes is confined between the clay crystals. The stabilizing power of clay is high, thus clay soils contain more SOC than sandy soils (Wild, 1988).…”

“…Residence time of relatively labile organic matter can be about 7 years in both silt and clay particles, whereas residence time for stable organics can reach 400 years in silt and 1000 years in clay (Buyanovsky et al, 1994). Residence times of SOC in macro-and microaggregates may differ because of (1) differences in physiochemical attraction between mineral and organic particles and (2) location of organic binding agents within the aggregates (Emerson, 1959). The SOC residence time Fig.…”

“…Current conceptual models of soil aggregation are being directed to elucidate concerns about SOC sequestration in aggregates. Classical models: Emerson (1959) postulated a simplified model, which indicates that organic substances bridge sand and clay crystals to form and stabilize aggregates or what he called "soil crumbs." The model states that the number and nature of the bonds determine the aggregate stability.…”