Changes in the soil structural quality have caused alterations in root growth and agricultural yield. With the purpose of finding critical values of physical parameters and alternative, viable and suitable ways of minimize the soil degradation, this study aimed at quantifying aggregate tensile strength (TS), friability (F), bull density (Bd), total porosity (Tp), macroporosity (Ma), penetration resistance (PR) and total organic carbon (TOC). Relations were established to estimate the structural quality of some parameters of an Ultisol under different uses. The study was carried out on a farm, in the rural area of RS, inserted in the Arroio Moreira waterbasin, Brazil. Disturbed and undisturbed samples were collected from the 0.00 to 0.10 m layer. Regarding TS and soil use, 900 samples were selected, totalling 2700 aggregates. Undisturbed samples (135) were collected for the evaluation of Bd, porosity and PR. The TOC was quantified using 45 aggregates through the dry combustion method. All physical parameters with exception of F are important in monitoring the structural quality.The lowest values of TS, PR and Bd were verified in the soil under fallow, and the highest ones in the soil under pasture. An inverse relation between TS and TOC and between Bd and TOC was verified. Critical values of Bd and TS to crop development might be estimated from the PR.

RESUMO -O uso e manejo inadequado do solo resultam em efeitos negativos para a sua estrutura, afetando atributos físicos e químicos como densidade, estabilidade dos agregados em água e teor de carbono orgânico total. Assim, objetivou-se avaliar a estabilidade dos agregados e relacioná-la com a densidade e carbono orgânico do solo sob diferentes sistemas de uso e manejo. O trabalho foi desenvolvido em uma propriedade produtora de leite, localizada no sul do Rio Grande do Sul, no bioma Pampa. O solo predominante no local é classificado como Argissolo Vermelho-Amarelo. Foram coletadas amostras de solo com estrutura preservada e não preservada, na camada de 0,0 a 0,10 m, sob pousio, milho e pastagem, visando avaliar a densidade do solo (Ds), a distribuição de tamanho e o diâmetro médio ponderado (DMP) dos agregados e carbono orgânico total (COT). Os teores de COT diferenciaram-se estatisticamente entre os sistemas e seguem uma ordem decrescente: pousio > milho > pastagem. Os maiores valores de COT, entretanto, não se refletiram em maior DMP. Foi identificada uma relação significativa e inversa entre os valores médios de COT e de Ds, nos diferentes sistemas de uso em estudo, sendo que, quanto maior o teor de carbono orgânico, menor a densidade do solo. O solo sob pastagem apresentou maior densidade, menor teor de carbono orgânico total e menor concentração de agregados estáveis de maior tamanho, indicando uma qualidade estrutural inferior aos solos das áreas sob pousio e cultivo mínimo do milho.Palavras chave: carbono orgânico, estabilidade de agregados, pastagem, agropecuária, leite.ABSTRACT -The use and inadequate management of the soil result in negative effects on its structure, affecting physical and chemical attributes such as density, aggregate stability in water and total organic carbon content. Thus, the objective was to evaluate the stability of the aggregates and to relate it to the density and organic carbon of the soil under different systems of use and management. The work was developed in a milk producing property, located in the south of Rio Grande do Sul, in the Pampa biome. The predominant soil in the place is classified like Ultisol. Soil samples with a preserved and non preserved structure were collected in the 0.0 to 0.10 m layer under fallow, corn and pasture, aiming to evaluate soil density (Ds), size distribution and weighted average diameter of aggregates (WMD) and total organic carbon (TOC). The TOC levels were statistically different between the systems and follow a decreasing order: fallow> maize> pasture. The higher TOC values, however, were not reflected in higher WMD. A significant and inverse relationship between the mean values of TOC and Ds in the different systems of study was identified, and the higher the organic carbon content, the lower the soil density. Soil under pasture presented higher density, lower total organic carbon content and lower concentration of stable aggregates of larger size, indicating a lower structural quality than the soils under fallow and minimum maize cult...